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<html>
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</div>
<div class="section boost_localfunction_advanced_topics">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_localfunction.advanced_topics"></a><a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced Topics</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.default_parameters">Default
Parameters</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros">Commas
and Symbols in Macros</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns">Assignments
and Returns</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.nesting">Nesting</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_">Accessing
Types (concepts, etc)</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_">Specifying
Types (no Boost.Typeof)</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.inlining">Inlining</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.recursion">Recursion</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.overloading">Overloading</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications">Exception
Specifications</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers">Storage
Classifiers</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions">Same
Line Expansions</a></span></dt>
<dt><span class="section"><a href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_">Limitations
(operators, etc)</a></span></dt>
</dl></div>
<p>
This section illustrates advanced usage of this library. At the bottom there
is also a list of known limitations of this library.
</p>
<div class="section boost_localfunction_advanced_topics_default_parameters">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.default_parameters"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.default_parameters" title="Default Parameters">Default
Parameters</a>
</h3></div></div></div>
<p>
This library allows to specify default values for the local function parameters.
However, the usual C++ syntax for default parameters that uses the assignment
symbol <code class="computeroutput"><span class="special">=</span></code> cannot be used. <sup>[<a name="boost_localfunction.advanced_topics.default_parameters.f0" href="#ftn.boost_localfunction.advanced_topics.default_parameters.f0" class="footnote">17</a>]</sup> The keyword <code class="computeroutput"><span class="keyword">default</span></code>
is used instead:
</p>
<pre class="programlisting"><code class="literal"><span class="emphasis"><em>parameter-type parameter-name</em></span></code><span class="special">,</span> <span class="keyword">default</span> <code class="literal"><span class="emphasis"><em>parameter-default-value</em></span></code><span class="special">,</span> <span class="special">...</span>
</pre>
<p>
For example, let's program a local function <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span>
<span class="identifier">y</span><span class="special">)</span></code>
where the second parameter <code class="computeroutput"><span class="identifier">y</span></code>
is optional and has a default value of <code class="computeroutput"><span class="number">2</span></code>
(see also <a href="../../../test/add_default.cpp" target="_top"><code class="literal">add_default.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">2</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Default parameter.</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
Programmers can define a <code class="computeroutput"><span class="identifier">WITH_DEFAULT</span></code>
macro similar to the following if they think it improves readability over
the above syntax (see also <a href="../../../test/add_with_default.cpp" target="_top"><code class="literal">add_with_default.cpp</code></a>):
<sup>[<a name="boost_localfunction.advanced_topics.default_parameters.f1" href="#ftn.boost_localfunction.advanced_topics.default_parameters.f1" class="footnote">18</a>]</sup>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">WITH_DEFAULT</span> <span class="special">,</span> <span class="keyword">default</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span> <span class="identifier">WITH_DEFAULT</span> <span class="number">2</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Default.</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_commas_and_symbols_in_macros">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.commas_and_symbols_in_macros"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.commas_and_symbols_in_macros" title="Commas and Symbols in Macros">Commas
and Symbols in Macros</a>
</h3></div></div></div>
<p>
The C++ preprocessor does not allow commas <code class="computeroutput"><span class="special">,</span></code>
within macro parameters unless they are wrapped by round parenthesis <code class="computeroutput"><span class="special">()</span></code> (see the <a href="http://www.boost.org/libs/utility/identity_type" target="_top">Boost.Utility/IdentityType</a>
documentation for details). Therefore, using commas within local function
parameters and bindings will generate (cryptic) preprocessor errors unless
they are wrapped with an extra set of round parenthesis <code class="computeroutput"><span class="special">()</span></code>
as explained here.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Also macro parameters with commas wrapped by angular parenthesis <code class="computeroutput"><span class="special"><></span></code> (templates, etc) or square parenthesis
<code class="computeroutput"><span class="special">[]</span></code> (multidimensional array
access, etc) need to be wrapped by the extra round parenthesis <code class="computeroutput"><span class="special">()</span></code> as explained here (this is because the
preprocessor only recognizes the round parenthesis and it does not recognize
angular, square, or any other type of parenthesis). However, macro parameters
with commas which are already wrapped by round parenthesis <code class="computeroutput"><span class="special">()</span></code> are fine (function calls, some value
expressions, etc).
</p></td></tr>
</table></div>
<p>
In addition, local function parameter types cannot start with non-alphanumeric
symbols (alphanumeric symbols are <code class="computeroutput"><span class="identifier">A</span><span class="special">-</span><span class="identifier">Z</span></code>, <code class="computeroutput"><span class="identifier">a</span><span class="special">-</span><span class="identifier">z</span></code>,
and <code class="computeroutput"><span class="number">0</span><span class="special">-</span><span class="number">9</span></code>). <sup>[<a name="boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" href="#ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" class="footnote">19</a>]</sup> The library will generate (cryptic) preprocessor errors if a
parameter type starts with a non-alphanumeric symbol.
</p>
<p>
Let's consider the following example:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&</span> <span class="identifier">m</span><span class="special">,</span> <span class="comment">// (1) Error.</span>
<span class="special">::</span><span class="identifier">sign_t</span> <span class="identifier">sign</span><span class="special">,</span> <span class="comment">// (2) Error.</span>
<span class="keyword">const</span> <span class="identifier">size_t</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span>
<span class="keyword">default</span> <span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span><span class="special">,</span> <span class="comment">// (3) Error.</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">separator</span><span class="special">,</span> <span class="keyword">default</span> <span class="identifier">cat</span><span class="special">(</span><span class="string">":"</span><span class="special">,</span> <span class="string">" "</span><span class="special">)</span> <span class="comment">// (4) OK.</span>
<span class="special">)</span> <span class="special">{</span>
<span class="special">...</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
<span class="bold"><strong>(1)</strong></span> The parameter type <code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&</span></code> contains a comma <code class="computeroutput"><span class="special">,</span></code>
after the first template parameter <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
This comma is not wrapped by any round parenthesis <code class="computeroutput"><span class="special">()</span></code>
thus it will cause a preprocessor error. <sup>[<a name="boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" href="#ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" class="footnote">20</a>]</sup> The <a href="http://www.boost.org/libs/utility/identity_type" target="_top">Boost.Utility/IdentityType</a>
macro <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((</span></code><code class="literal"><span class="emphasis"><em>type-with-commas</em></span></code><code class="computeroutput"><span class="special">))</span></code> defined in the <code class="literal">boost/utility/identity_type.hpp</code>
header can be used to wrap a type within extra parenthesis <code class="computeroutput"><span class="special">()</span></code> so to overcome this problem:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">identity_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&))</span> <span class="identifier">m</span><span class="special">,</span> <span class="comment">// OK.</span>
<span class="special">...</span>
<span class="special">)</span> <span class="special">{</span>
<span class="special">...</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
This macro expands to an expression that evaluates (at compile-time) exactly
to the specified type (furthermore, this macro does not use variadic macros
so it works on any <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
compiler). Note that a total of two set of parenthesis <code class="computeroutput"><span class="special">()</span></code>
are needed: The parenthesis to invoke the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">(...)</span></code> macro plus the parenthesis to wrap the
type expression (and therefore any comma <code class="computeroutput"><span class="special">,</span></code>
that it contains) passed as parameter to the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((...))</span></code> macro. Finally, the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code> macro must be prefixed
by the <code class="computeroutput"><span class="keyword">typename</span></code> keyword <code class="computeroutput"><span class="keyword">typename</span> <span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">(</span></code><code class="literal"><span class="emphasis"><em>parenthesized-type</em></span></code><code class="computeroutput"><span class="special">)</span></code> when used together with the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_TPL">BOOST_LOCAL_FUNCTION_TPL</a></code>
macro within templates.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Often, there might be better ways to overcome this limitation that lead
to code which is more readable than the one using the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code>
macro.
</p></td></tr>
</table></div>
<p>
For example, in this case a <code class="computeroutput"><span class="keyword">typedef</span></code>
from the enclosing scope could have been used to obtain the following valid
and perhaps more readable code:
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">></span> <span class="identifier">map_type</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="keyword">const</span> <span class="identifier">map_type</span><span class="special">&</span> <span class="identifier">m</span><span class="special">,</span> <span class="comment">// OK (and more readable).</span>
<span class="special">...</span>
<span class="special">)</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
<span class="bold"><strong>(2)</strong></span> The parameter type <code class="computeroutput"><span class="special">::</span><span class="identifier">sign_t</span></code> starts with the non-alphanumeric
symbols <code class="computeroutput"><span class="special">::</span></code> thus it will generate
preprocessor errors if used as a local function parameter type. The <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code> macro can also be used
to overcome this issue:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="special">...</span>
<span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((::</span><span class="identifier">sign_t</span><span class="special">))</span> <span class="identifier">sign</span><span class="special">,</span> <span class="comment">// OK.</span>
<span class="special">...</span>
<span class="special">)</span> <span class="special">{</span>
<span class="special">...</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Often, there might be better ways to overcome this limitation that lead
to code which is more readable than the one using the <code class="computeroutput"><span class="identifier">BOOST_IDENTITY_TYPE</span></code>
macro.
</p></td></tr>
</table></div>
<p>
For example, in this case the symbols <code class="computeroutput"><span class="special">::</span></code>
could have been simply dropped to obtain the following valid and perhaps
more readable code:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="special">...</span>
<span class="identifier">sign_t</span> <span class="identifier">sign</span><span class="special">,</span> <span class="comment">// OK (and more readable).</span>
<span class="special">...</span>
<span class="special">)</span> <span class="special">{</span>
<span class="special">...</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
<span class="bold"><strong>(3)</strong></span> The default parameter value <code class="computeroutput"><span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span></code>
contains a comma <code class="computeroutput"><span class="special">,</span></code> after the
first template parameter <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
Again, this comma is not wrapped by any parenthesis <code class="computeroutput"><span class="special">()</span></code>
so it will cause a preprocessor error. Because this is a value expression
(and not a type expression), it can simply be wrapped within an extra set
of round parenthesis <code class="computeroutput"><span class="special">()</span></code>:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="special">...</span>
<span class="keyword">const</span> <span class="identifier">size_t</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span>
<span class="keyword">default</span> <span class="special">(</span><span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span><span class="special">),</span> <span class="comment">// OK.</span>
<span class="special">...</span>
<span class="special">)</span> <span class="special">{</span>
<span class="special">...</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
<span class="bold"><strong>(4)</strong></span> The default parameter value <code class="computeroutput"><span class="identifier">cat</span><span class="special">(</span><span class="string">":"</span><span class="special">,</span> <span class="string">" "</span><span class="special">)</span></code> is instead fine because it contains a comma
<code class="computeroutput"><span class="special">,</span></code> which is already wrapped by
the parenthesis <code class="computeroutput"><span class="special">()</span></code> of the function
call <code class="computeroutput"><span class="identifier">cat</span><span class="special">(...)</span></code>.
</p>
<p>
Consider the following complete example (see also <a href="../../../test/macro_commas.cpp" target="_top"><code class="literal">macro_commas.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&))</span> <span class="identifier">m</span><span class="special">,</span>
<span class="identifier">BOOST_IDENTITY_TYPE</span><span class="special">((::</span><span class="identifier">sign_t</span><span class="special">))</span> <span class="identifier">sign</span><span class="special">,</span>
<span class="keyword">const</span> <span class="identifier">size_t</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">,</span>
<span class="keyword">default</span> <span class="special">(</span><span class="identifier">key_sizeof</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>::</span><span class="identifier">value</span><span class="special">),</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">separator</span><span class="special">,</span> <span class="keyword">default</span> <span class="identifier">cat</span><span class="special">(</span><span class="string">":"</span><span class="special">,</span> <span class="string">" "</span><span class="special">)</span>
<span class="special">)</span> <span class="special">{</span>
<span class="comment">// Do something...</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_assignments_and_returns">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.assignments_and_returns"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.assignments_and_returns" title="Assignments and Returns">Assignments
and Returns</a>
</h3></div></div></div>
<p>
Local functions are function objects so it is possible to assign them to
other functors like <a href="http://www.boost.org/libs/function" target="_top">Boost.Function</a>'s
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span></code> in order to store the local function
into a variable, pass it as a parameter to another function, or return it
from the enclosing function.
</p>
<p>
For example (see also <a href="../../../test/return_assign.cpp" target="_top"><code class="literal">return_assign.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">call1</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">></span> <span class="identifier">f</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">==</span> <span class="number">5</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">call0</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">void</span><span class="special">)></span> <span class="identifier">f</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</span><span class="special">()</span> <span class="special">==</span> <span class="number">5</span><span class="special">);</span> <span class="special">}</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)></span> <span class="identifier">linear</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">slope</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">slope</span><span class="special">,</span>
<span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">2</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">slope</span> <span class="special">*</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">lin</span><span class="special">)</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)></span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">lin</span><span class="special">;</span> <span class="comment">// Assign to local variable.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="special">==</span> <span class="number">5</span><span class="special">);</span>
<span class="identifier">call1</span><span class="special">(</span><span class="identifier">lin</span><span class="special">);</span> <span class="comment">// Pass to other functions.</span>
<span class="identifier">call0</span><span class="special">(</span><span class="identifier">lin</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">lin</span><span class="special">;</span> <span class="comment">// Return.</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">call</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)></span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">linear</span><span class="special">(</span><span class="number">2</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">f</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="special">==</span> <span class="number">5</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
As with <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>, programmers are responsible to ensure that bound
variables are valid in any scope where the local function object is called.
Returning and calling a local function outside its declaration scope will
lead to undefined behaviour if any of the bound variable is no longer valid
in the scope where the local function is called (see the <a class="link" href="examples.html" title="Examples">Examples</a>
section for more examples on the extra care needed when returning a local
function as a closure). It is always safe instead to call a local function
within its enclosing scope.
</p></td></tr>
</table></div>
<p>
In addition, a local function can bind and call other local functions. Local
functions should always be bound by constant reference <code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">bind</span><span class="special">&</span></code>
to avoid unnecessary copies. For example, the following local function <code class="computeroutput"><span class="identifier">inc_sum</span></code> binds the local function <code class="computeroutput"><span class="identifier">inc</span></code> so <code class="computeroutput"><span class="identifier">inc_sum</span></code>
can call <code class="computeroutput"><span class="identifier">inc</span></code> (see aslo <a href="../../../test/transform.cpp" target="_top"><code class="literal">transform.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">offset</span> <span class="special">=</span> <span class="number">5</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">w</span><span class="special">;</span>
<span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><=</span> <span class="number">2</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">i</span> <span class="special">*</span> <span class="number">10</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">==</span> <span class="number">10</span><span class="special">);</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">==</span> <span class="number">20</span><span class="special">);</span>
<span class="identifier">w</span><span class="special">.</span><span class="identifier">resize</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="special">++</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">offset</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">inc</span><span class="special">)</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">w</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">inc</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">w</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">==</span> <span class="number">16</span><span class="special">);</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">w</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">==</span> <span class="number">26</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">inc</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">j</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">inc</span><span class="special">(</span><span class="identifier">i</span> <span class="special">+</span> <span class="identifier">j</span><span class="special">);</span> <span class="comment">// Call the other bound local function.</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">inc_sum</span><span class="special">)</span>
<span class="identifier">offset</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">w</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">inc_sum</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special">==</span> <span class="number">27</span><span class="special">);</span> <span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special">==</span> <span class="number">47</span><span class="special">);</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_nesting">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.nesting"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.nesting" title="Nesting">Nesting</a>
</h3></div></div></div>
<p>
It is possible to nest local functions into one another. For example (see
also <a href="../../../test/nesting.cpp" target="_top"><code class="literal">nesting.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">bind</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Nested.</span>
<span class="identifier">x</span><span class="special">++;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">g</span><span class="special">)</span>
<span class="identifier">x</span><span class="special">--;</span>
<span class="identifier">g</span><span class="special">();</span> <span class="comment">// Nested local function call.</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">f</span><span class="special">)</span>
<span class="identifier">f</span><span class="special">();</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_accessing_types__concepts__etc_">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.accessing_types__concepts__etc_"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.accessing_types__concepts__etc_" title="Accessing Types (concepts, etc)">Accessing
Types (concepts, etc)</a>
</h3></div></div></div>
<p>
This library never requires to explicitly specify the type of bound variables
(e.g., this reduces maintenance because the local function declaration and
definition do not have to change even if the bound variable types change
as long as the semantics of the local function remain valid). From within
local functions, programmers can access the type of a bound variable using
the following macro:
</p>
<pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span><span class="emphasis"><em>bound-variable-name</em></span><span class="special">)</span>
</pre>
<p>
The <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TYPEOF.html" title="Macro BOOST_LOCAL_FUNCTION_TYPEOF">BOOST_LOCAL_FUNCTION_TYPEOF</a></code>
macro expands to a type expression that evaluates (at compile-time) to the
fully qualified type of the bound variable with the specified name. This
type expression is fully qualified in the sense that it will be constant
if the variable is bound by constant <code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">bind</span><span class="special">[&]</span></code>
and it will also be a reference if the variable is bound by reference <code class="computeroutput"><span class="special">[</span><span class="keyword">const</span><span class="special">]</span>
<span class="identifier">bind</span><span class="special">&</span></code>
(if needed, programmers can remove the <code class="computeroutput"><span class="keyword">const</span></code>
and <code class="computeroutput"><span class="special">&</span></code> qualifiers using
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_const</span></code> and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span></code>,
see <a href="http://www.boost.org/libs/type_traits" target="_top">Boost.TypeTraits</a>).
</p>
<p>
The deduced bound type can be used within the body to check concepts, declare
local variables, etc. For example (see also <a href="../../../test/typeof.cpp" target="_top"><code class="literal">typeof.cpp</code></a>
and <a href="../../../test/addable.hpp" target="_top"><code class="literal">addable.hpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
<span class="comment">// Type-of for concept checking.</span>
<span class="identifier">BOOST_CONCEPT_ASSERT</span><span class="special">((</span><span class="identifier">Addable</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span><span class="identifier">sum</span><span class="special">)>::</span><span class="identifier">type</span><span class="special">>));</span>
<span class="comment">// Type-of for declarations.</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span>
<span class="identifier">factor</span><span class="special">)>::</span><span class="identifier">type</span> <span class="identifier">mult</span> <span class="special">=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
<span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">mult</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
<span class="identifier">add</span><span class="special">(</span><span class="number">6</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
Within templates, <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TYPEOF.html" title="Macro BOOST_LOCAL_FUNCTION_TYPEOF">BOOST_LOCAL_FUNCTION_TYPEOF</a></code>
should not be prefixed by the <code class="computeroutput"><span class="keyword">typename</span></code>
keyword but eventual type manipulations need the <code class="computeroutput"><span class="keyword">typename</span></code>
prefix as usual (see also <a href="../../../test/typeof_template.cpp" target="_top"><code class="literal">typeof_template.cpp</code></a>
and <a href="../../../test/addable.hpp" target="_top"><code class="literal">addable.hpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="identifier">T</span> <span class="identifier">calculate</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">factor</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">T</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION_TPL</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
<span class="comment">// Local function `TYPEOF` does not need `typename`.</span>
<span class="identifier">BOOST_CONCEPT_ASSERT</span><span class="special">((</span><span class="identifier">Addable</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_TYPEOF</span><span class="special">(</span><span class="identifier">sum</span><span class="special">)>::</span><span class="identifier">type</span><span class="special">>));</span>
<span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
<span class="identifier">add</span><span class="special">(</span><span class="number">6</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">sum</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
In this context, it is best to use the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TYPEOF.html" title="Macro BOOST_LOCAL_FUNCTION_TYPEOF">BOOST_LOCAL_FUNCTION_TYPEOF</a></code>
macro instead of using <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
to reduce the number of times that <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
is invoked (either the library already internally used <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
once, in which case using this macro will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
again, or the bound variable type is explicitly specified by programmers
as shown be below, in which case using this macro will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
at all).
</p>
<p>
Furthermore, within the local function body it possible to access the result
type using <code class="computeroutput"><span class="identifier">result_type</span></code>, the
type of the first parameter using <code class="computeroutput"><span class="identifier">arg1_type</span></code>,
the type of the second parameter using <code class="computeroutput"><span class="identifier">arg2_type</span></code>,
etc. <sup>[<a name="boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" href="#ftn.boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" class="footnote">21</a>]</sup>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_specifying_types__no_boost_typeof_">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_" title="Specifying Types (no Boost.Typeof)">Specifying
Types (no Boost.Typeof)</a>
</h3></div></div></div>
<p>
While not required, it is possible to explicitly specify the type of bound
variables so the library will not internally use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
to automatically deduce the types. When specified, the bound variable type
must follow the <code class="computeroutput"><span class="identifier">bind</span></code> "keyword"
and it must be wrapped within round parenthesis <code class="computeroutput"><span class="special">()</span></code>:
</p>
<pre class="programlisting"><span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by value with explicit type.</span>
<span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)&</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by reference with explicit type.</span>
<span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant value with explicit type.</span>
<span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>variable-type</em></span><span class="special">)&</span> <span class="emphasis"><em>variable-name</em></span> <span class="comment">// Bind by constant reference with explicit type.</span>
<span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>class-type</em></span><span class="special">*)</span> <span class="identifier">this_</span> <span class="comment">// Bind object `this` with explicit type.</span>
<span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="emphasis"><em>class-type</em></span><span class="special">*)</span> <span class="identifier">this_</span> <span class="comment">// Bind object `this` by constant with explicit type.</span>
</pre>
<p>
Note that within the local function body it is always possible to abstract
the access to the type of a bound variable using <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TYPEOF.html" title="Macro BOOST_LOCAL_FUNCTION_TYPEOF">BOOST_LOCAL_FUNCTION_TYPEOF</a></code>
(even when the bound variable type is explicitly specified in the local function
declaration).
</p>
<p>
The library also uses <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
to determine the local function result type (because this type is specified
outside the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
macro). Thus it is also possible to specify the local function result type
as one of the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
macro parameters prefixing it by <code class="computeroutput"><span class="keyword">return</span></code>
so the library will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>
to deduce the result type:
</p>
<pre class="programlisting"><span class="identifier">BOOST_LOCAL_FUNCTION_TYPE</span><span class="special">(</span><span class="keyword">return</span> <code class="literal"><span class="emphasis"><em>result-type</em></span></code><span class="special">,</span> <span class="special">...)</span>
</pre>
<p>
Note that the result type must be specified only once either before the macro
(without the <code class="computeroutput"><span class="keyword">return</span></code> prefix)
or as one of the macro parameters (with the <code class="computeroutput"><span class="keyword">return</span></code>
prefix). As always, the result type can be <code class="computeroutput"><span class="keyword">void</span></code>
to declare a function that returns nothing (so <code class="computeroutput"><span class="keyword">return</span>
<span class="keyword">void</span></code> is allowed when the result type
is specified as one of the macro parameters).
</p>
<p>
The following example specifies all bound variables and result types (see
also <a href="../../../test/add_typed.cpp" target="_top"><code class="literal">add_typed.cpp</code></a>):
<sup>[<a name="boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" href="#ftn.boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" class="footnote">22</a>]</sup>
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">adder</span> <span class="special">{</span>
<span class="identifier">adder</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">sum_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span>
<span class="keyword">int</span> <span class="identifier">sum</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">>&</span> <span class="identifier">nums</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">)</span> <span class="special">{</span>
<span class="comment">// Explicitly specify bound variable and return types (no type-of).</span>
<span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&)</span> <span class="identifier">factor</span><span class="special">,</span>
<span class="identifier">bind</span><span class="special">(</span><span class="identifier">adder</span><span class="special">*)</span> <span class="identifier">this_</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">,</span> <span class="keyword">return</span> <span class="keyword">int</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">this_</span><span class="special">-></span><span class="identifier">sum_</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">nums</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">add</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">sum_</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="keyword">int</span> <span class="identifier">sum_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Unless necessary, it is recommended to not specify the bound variable and
result types. Let the library deduce these types so the local function syntax
will be more concise and the local function declaration will not have to
change if a bound variable type changes (reducing maintenance).
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
When all bound variable and result types are explicitly specified, the
library implementation will not use <a href="http://www.boost.org/libs/typeof" target="_top">Boost.Typeof</a>.
</p></td></tr>
</table></div>
</div>
<div class="section boost_localfunction_advanced_topics_inlining">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.inlining"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.inlining" title="Inlining">Inlining</a>
</h3></div></div></div>
<p>
Local functions can be declared <a href="http://en.wikipedia.org/wiki/Inline_function" target="_top">inline</a>
to increase the chances that the compiler will be able to reduce the run-time
of the local function call by inlining the generated assembly code. A local
function is declared inline by prefixing its name with the keyword <code class="computeroutput"><span class="keyword">inline</span></code>:
</p>
<pre class="programlisting"><span class="emphasis"><em>result-type</em></span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="emphasis"><em>parameters</em></span><span class="special">)</span> <span class="special">{</span>
<span class="special">...</span> <span class="comment">// Body.</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="keyword">inline</span> <span class="emphasis"><em>name</em></span><span class="special">)</span> <span class="comment">// Inlining.</span>
</pre>
<p>
When inlining a local function, note the following:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
On <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
compliant compilers, inline local functions always have a run-time comparable
to their equivalent implementation that uses local functors (see the
<a class="link" href="alternatives.html" title="Annex: Alternatives">Alternatives</a>
section). However, inline local functions have the important limitation
that they cannot be assigned to other functors (like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span></code>)
and they cannot be passed as template parameters.
</li>
<li class="listitem">
On <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
compilers, <code class="computeroutput"><span class="keyword">inline</span></code> has no
effect because this library will automatically generate code that uses
<a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a> specific
features to inline the local function calls whenever possible even if
the local function is not declared inline. Furthermore, non <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
local functions can always be passes as template parameters even when
they are declared inline. <sup>[<a name="boost_localfunction.advanced_topics.inlining.f0" href="#ftn.boost_localfunction.advanced_topics.inlining.f0" class="footnote">23</a>]</sup>
</li>
</ul></div>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
It is recommended to not declare a local function inline unless it is strictly
necessary for optimizing pure <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
compliant code (because in all other cases this library will automatically
take advantage of <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
features to optimize the local function calls while always allowing to
pass the local function as a template parameter).
</p></td></tr>
</table></div>
<p>
For example, the following local function is declared inline (thus a for-loop
needs to be used for portability instead of passing the local function as
a template parameter to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>
algorithm, see also <a href="../../../test/add_inline.cpp" target="_top"><code class="literal">add_inline.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="keyword">inline</span> <span class="identifier">add</span><span class="special">)</span> <span class="comment">// Inlining.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">fill</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="number">1</span><span class="special">);</span>
<span class="keyword">for</span><span class="special">(</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">add</span><span class="special">(</span><span class="identifier">v</span><span class="special">[</span><span class="identifier">i</span><span class="special">]);</span> <span class="comment">// Cannot use for_each.</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_recursion">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.recursion"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.recursion" title="Recursion">Recursion</a>
</h3></div></div></div>
<p>
Local functions can be declared <a href="http://en.wikipedia.org/wiki/Recursion_(computer_science)#Recursive_procedures" target="_top">recursive</a>
so a local function can recursively call itself from its body (as usual with
C++ functions). A local function is declared recursive by prefixing its name
with the <code class="computeroutput"><span class="identifier">recursive</span></code> "keyword"
(thus <code class="computeroutput"><span class="identifier">recursive</span></code> cannot be
used as a local function name):
</p>
<pre class="programlisting"><span class="emphasis"><em>result-type</em></span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="emphasis"><em>parameters</em></span><span class="special">)</span> <span class="special">{</span>
<span class="special">...</span> <span class="comment">// Body.</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">recursive</span> <span class="emphasis"><em>name</em></span><span class="special">)</span> <span class="comment">// Recursive.</span>
</pre>
<p>
For example, the following local function is used to recursively calculate
the factorials of all the numbers in the specified vector (see also <a href="../../../test/factorial.cpp" target="_top"><code class="literal">factorial.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">calculator</span> <span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">results</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">factorials</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">>&</span> <span class="identifier">nums</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">num</span><span class="special">,</span>
<span class="keyword">bool</span> <span class="identifier">recursion</span><span class="special">,</span> <span class="keyword">default</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">result</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">num</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="identifier">result</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
<span class="keyword">else</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">num</span> <span class="special">*</span> <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">num</span> <span class="special">-</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// Recursive call.</span>
<span class="keyword">if</span><span class="special">(!</span><span class="identifier">recursion</span><span class="special">)</span> <span class="identifier">this_</span><span class="special">-></span><span class="identifier">results</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">result</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">recursive</span> <span class="identifier">factorial</span><span class="special">)</span> <span class="comment">// Recursive.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span><span class="special">(</span><span class="identifier">nums</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">nums</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">factorial</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Compilers have not been observed to be able to inline recursive local function
calls not even when the recursive local function is also declared inline:
</p>
<pre class="programlisting"><span class="special">...</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="keyword">inline</span> <span class="identifier">recursive</span> <span class="identifier">factorial</span><span class="special">)</span>
</pre>
<p>
Recursive local functions should never be called outside their declaration
scope. <sup>[<a name="boost_localfunction.advanced_topics.recursion.f0" href="#ftn.boost_localfunction.advanced_topics.recursion.f0" class="footnote">24</a>]</sup>
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
If a local function is returned from the enclosing function and called
in a different scope, the behaviour is undefined (and it will likely result
in a run-time error).
</p></td></tr>
</table></div>
<p>
This is not a limitation with respect to <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a> because lambdas can never call themselves recursively
(in other words, there is no recursive lambda function that can successfully
be called outside its declaration scope because there is no recursive lambda
function at all).
</p>
</div>
<div class="section boost_localfunction_advanced_topics_overloading">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.overloading"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.overloading" title="Overloading">Overloading</a>
</h3></div></div></div>
<p>
Because local functions are functors, it is possible to overload them using
the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">overloaded_function</span></code> functor of <a href="http://www.boost.org/libs/functional/overloaded_function" target="_top">Boost.Functional/OverloadedFunction</a>
from the <code class="literal">boost/functional/overloaded_function.hpp</code> header
(see the <a href="http://www.boost.org/libs/functional/overloaded_function" target="_top">Boost.Functional/OverloadedFunction</a>
documentation for details).
</p>
<p>
In the following example, the overloaded function object <code class="computeroutput"><span class="identifier">add</span></code>
can be called with signatures from either the local function <code class="computeroutput"><span class="identifier">add_s</span></code>, or the local function <code class="computeroutput"><span class="identifier">add_d</span></code>, or the local function <code class="computeroutput"><span class="identifier">add_d</span></code> with its extra default parameter,
or the function pointer <code class="computeroutput"><span class="identifier">add_i</span></code>
(see also <a href="../../../test/overload.cpp" target="_top"><code class="literal">overload.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">add_i</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"abc"</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span>
<span class="keyword">const</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">s</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">s</span> <span class="special">+</span> <span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add_s</span><span class="special">)</span>
<span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.23</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">d</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">y</span><span class="special">,</span> <span class="keyword">default</span> <span class="number">0</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">d</span> <span class="special">+</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add_d</span><span class="special">)</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">overloaded_function</span><span class="special"><</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&)</span>
<span class="special">,</span> <span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">)</span>
<span class="special">,</span> <span class="keyword">double</span> <span class="special">(</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">)</span> <span class="comment">// Overload giving default param.</span>
<span class="special">,</span> <span class="keyword">int</span> <span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span>
<span class="special">></span> <span class="identifier">add</span><span class="special">(</span><span class="identifier">add_s</span><span class="special">,</span> <span class="identifier">add_d</span><span class="special">,</span> <span class="identifier">add_d</span><span class="special">,</span> <span class="identifier">add_i</span><span class="special">);</span> <span class="comment">// Overloaded function object.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="string">"xyz"</span><span class="special">)</span> <span class="special">==</span> <span class="string">"abcxyz"</span><span class="special">);</span> <span class="comment">// Call `add_s`.</span>
<span class="identifier">BOOST_TEST</span><span class="special">((</span><span class="number">4.44</span> <span class="special">-</span> <span class="identifier">add</span><span class="special">(</span><span class="number">3.21</span><span class="special">))</span> <span class="special"><=</span> <span class="number">0.001</span><span class="special">);</span> <span class="comment">// Call `add_d` (no default).</span>
<span class="identifier">BOOST_TEST</span><span class="special">((</span><span class="number">44.44</span> <span class="special">-</span> <span class="identifier">add</span><span class="special">(</span><span class="number">3.21</span><span class="special">,</span> <span class="number">40.0</span><span class="special">))</span> <span class="special"><=</span> <span class="number">0.001</span><span class="special">);</span> <span class="comment">// Call `add_d`.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">add</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span> <span class="comment">// Call `add_i`.</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_exception_specifications">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.exception_specifications"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.exception_specifications" title="Exception Specifications">Exception
Specifications</a>
</h3></div></div></div>
<p>
It is possible to program exception specifications for local functions by
specifying them after the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
macro and before the body code block <code class="computeroutput"><span class="special">{</span>
<span class="special">...</span> <span class="special">}</span></code>.
</p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../doc/src/images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
Note that the exception specifications only apply to the body code specified
by programmers and they do not apply to the rest of the code automatically
generated by the macro expansions to implement local functions. For example,
even if the body code is specified to throw no exception using <code class="computeroutput"><span class="keyword">throw</span> <span class="special">()</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span></code>,
the execution of the library code automatically generated by the macros
could still throw (if there is no memory, etc).
</p></td></tr>
</table></div>
<p>
For example (see also <a href="../../../test/add_except.cpp" target="_top"><code class="literal">add_except.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">sum</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">factor</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">factor</span><span class="special">,</span> <span class="identifier">bind</span><span class="special">&</span> <span class="identifier">sum</span><span class="special">,</span>
<span class="keyword">double</span> <span class="identifier">num</span><span class="special">)</span> <span class="keyword">throw</span><span class="special">()</span> <span class="special">{</span> <span class="comment">// Throw nothing.</span>
<span class="identifier">sum</span> <span class="special">+=</span> <span class="identifier">factor</span> <span class="special">*</span> <span class="identifier">num</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
<span class="identifier">add</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_storage_classifiers">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.storage_classifiers"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.storage_classifiers" title="Storage Classifiers">Storage
Classifiers</a>
</h3></div></div></div>
<p>
Local function parameters support the storage classifiers as usual in <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>.
The <code class="computeroutput"><span class="keyword">auto</span></code> storage classifier
is specified as: <sup>[<a name="boost_localfunction.advanced_topics.storage_classifiers.f0" href="#ftn.boost_localfunction.advanced_topics.storage_classifiers.f0" class="footnote">25</a>]</sup>
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="emphasis"><em>parameter-type parameter-name</em></span>
</pre>
<p>
The <code class="computeroutput"><span class="keyword">register</span></code> storage classifier
is specified as:
</p>
<pre class="programlisting"><span class="keyword">register</span> <span class="emphasis"><em>parameter-type parameter-name</em></span>
</pre>
<p>
For example (see also <a href="../../../test/add_classifiers.cpp" target="_top"><code class="literal">add_classifiers.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">auto</span> <span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">register</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="comment">// Classifiers.</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">add</span><span class="special">)</span>
</pre>
<p>
</p>
</div>
<div class="section boost_localfunction_advanced_topics_same_line_expansions">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.same_line_expansions"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.same_line_expansions" title="Same Line Expansions">Same
Line Expansions</a>
</h3></div></div></div>
<p>
In general, it is not possible to expand the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>,
<code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_TPL">BOOST_LOCAL_FUNCTION_TPL</a></code>
macros multiple times on the same line. <sup>[<a name="boost_localfunction.advanced_topics.same_line_expansions.f0" href="#ftn.boost_localfunction.advanced_topics.same_line_expansions.f0" class="footnote">26</a>]</sup>
</p>
<p>
Therefore, this library provides additional macros <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_ID.html" title="Macro BOOST_LOCAL_FUNCTION_ID">BOOST_LOCAL_FUNCTION_ID</a></code>
and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_ID_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_ID_TPL">BOOST_LOCAL_FUNCTION_ID_TPL</a></code>
which can be expanded multiple times on the same line as long as programmers
specify unique identifiers as the macros' first parameters. The unique identifier
can be any token (not just numeric) that can be successfully concatenated
by the preprocessor (e.g., <code class="computeroutput"><span class="identifier">local_function_number_1_at_line_123</span></code>).
<sup>[<a name="boost_localfunction.advanced_topics.same_line_expansions.f1" href="#ftn.boost_localfunction.advanced_topics.same_line_expansions.f1" class="footnote">27</a>]</sup>
</p>
<p>
The <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_ID.html" title="Macro BOOST_LOCAL_FUNCTION_ID">BOOST_LOCAL_FUNCTION_ID</a></code>
and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_ID_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_ID_TPL">BOOST_LOCAL_FUNCTION_ID_TPL</a></code>
macros accept local function parameter declaration lists using the exact
same syntax as <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>.
For example (see also <a href="../../../test/same_line.cpp" target="_top"><code class="literal">same_line.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">LOCAL_INC_DEC</span><span class="special">(</span><span class="identifier">offset</span><span class="special">)</span> <span class="special">\</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">inc</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="comment">/* unique ID */</span> <span class="special">\</span>
<span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">offset</span><span class="special">;</span> <span class="special">\</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">inc</span><span class="special">)</span> <span class="special">\</span>
<span class="special">\</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">dec</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="special">\</span>
<span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">-</span> <span class="identifier">offset</span><span class="special">;</span> <span class="special">\</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">dec</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">LOCAL_INC_DEC_TPL</span><span class="special">(</span><span class="identifier">offset</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">T</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID_TPL</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">inc</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="special">\</span>
<span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">offset</span><span class="special">;</span> <span class="special">\</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="identifier">inc</span><span class="special">)</span> <span class="special">\</span>
<span class="special">\</span>
<span class="identifier">T</span> <span class="identifier">BOOST_LOCAL_FUNCTION_ID_TPL</span><span class="special">(</span><span class="identifier">BOOST_PP_CAT</span><span class="special">(</span><span class="identifier">dec</span><span class="special">,</span> <span class="identifier">__LINE__</span><span class="special">),</span> <span class="special">\</span>
<span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">offset</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="special">\</span>
<span class="keyword">return</span> <span class="identifier">x</span> <span class="special">-</span> <span class="identifier">offset</span><span class="special">;</span> <span class="special">\</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME_TPL</span><span class="special">(</span><span class="identifier">dec</span><span class="special">)</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">delta</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">LOCAL_INC_DEC_TPL</span><span class="special">(</span><span class="identifier">delta</span><span class="special">)</span> <span class="comment">// Multiple local functions on same line.</span>
<span class="comment">/** @todo back to BOOST_TEST(dec(inc(123)) == 123)) and doc that on vacpp compiler composition doesn't work */</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">inc</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span> <span class="special">+</span> <span class="identifier">delta</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">dec</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span> <span class="special">-</span> <span class="identifier">delta</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">void</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">delta</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">LOCAL_INC_DEC</span><span class="special">(</span><span class="identifier">delta</span><span class="special">)</span> <span class="comment">// Multiple local functions on same line.</span>
<span class="comment">/** @todo back to BOOST_TEST(dec(inc(123)) == 123)) and doc that on vacpp compiler composition doesn't work */</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">inc</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span> <span class="special">+</span> <span class="identifier">delta</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">dec</span><span class="special">(</span><span class="number">123</span><span class="special">)</span> <span class="special">==</span> <span class="number">123</span> <span class="special">-</span> <span class="identifier">delta</span><span class="special">);</span>
<span class="identifier">f</span><span class="special">(</span><span class="identifier">delta</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">report_errors</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
As shown by the example above, the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_ID.html" title="Macro BOOST_LOCAL_FUNCTION_ID">BOOST_LOCAL_FUNCTION_ID</a></code>
and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_ID_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_ID_TPL">BOOST_LOCAL_FUNCTION_ID_TPL</a></code>
macros are especially useful when it is necessary to invoke them multiple
times within a user-defined macro (because the preprocessor expands all nested
macros on the same line).
</p>
</div>
<div class="section boost_localfunction_advanced_topics_limitations__operators__etc_">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_localfunction.advanced_topics.limitations__operators__etc_"></a><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_" title="Limitations (operators, etc)">Limitations
(operators, etc)</a>
</h3></div></div></div>
<p>
The following table summarizes all C++ function features indicating those
features that are not supported by this library for local functions.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
C++ Function Feature
</p>
</th>
<th>
<p>
Local Function Support
</p>
</th>
<th>
<p>
Comment
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">export</span></code>
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions cannot be templates
(plus most C++ compilers do not implement <code class="computeroutput"><span class="keyword">export</span></code>
at all).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span></code><code class="literal"><span class="emphasis"><em>template-parameter-list</em></span></code><code class="computeroutput"><span class="special">></span></code>
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are implemented using
local classes and <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
local classes cannot be templates.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">explicit</span></code>
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are not constructors.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">inline</span></code>
</p>
</td>
<td>
<p>
Yes.
</p>
</td>
<td>
<p>
Local functions can be specified <code class="computeroutput"><span class="keyword">inline</span></code>
to improve the chances that <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
compilers can optimize the local function call run-time (but <code class="computeroutput"><span class="keyword">inline</span></code> local functions cannot be
passed as template parameters on <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
compilers, see the <a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced
Topics</a> section).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">extern</span></code>
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are always defined
locally within the enclosing scope and together with their declarations.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">static</span></code>
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are not member functions.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="keyword">virtual</span></code>
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are not member functions.
<sup>[<a name="boost_localfunction.advanced_topics.limitations__operators__etc_.f0" href="#ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f0" class="footnote">a</a>]</sup>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal"><span class="emphasis"><em>result-type</em></span></code>
</p>
</td>
<td>
<p>
Yes.
</p>
</td>
<td>
<p>
This is supported (see the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
section).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal"><span class="emphasis"><em>function-name</em></span></code>
</p>
</td>
<td>
<p>
Yes.
</p>
</td>
<td>
<p>
Local functions are named and they can call themselves recursively
but they cannot be operators (see the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
and <a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced
Topics</a> sections).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal"><span class="emphasis"><em>parameter-list</em></span></code>
</p>
</td>
<td>
<p>
Yes.
</p>
</td>
<td>
<p>
This is supported and it also supports the <code class="computeroutput"><span class="keyword">auto</span></code>
and <code class="computeroutput"><span class="keyword">register</span></code> storage
classifiers, default parameters, and binding of variables in scope
(see the <a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
and <a class="link" href="advanced_topics.html" title="Advanced Topics">Advanced
Topics</a> sections).
</p>
</td>
</tr>
<tr>
<td>
<p>
Trailing <code class="computeroutput"><span class="keyword">const</span></code> qualifier
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are not member functions.
</p>
</td>
</tr>
<tr>
<td>
<p>
Trailing <code class="computeroutput"><span class="keyword">volatile</span></code>
qualifier
</p>
</td>
<td>
<p>
No.
</p>
</td>
<td>
<p>
This is not supported because local functions are not member functions.
</p>
</td>
</tr>
</tbody>
<tbody class="footnotes"><tr><td colspan="3"><div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f0" href="#boost_localfunction.advanced_topics.limitations__operators__etc_.f0" class="para">a</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> It would be possible
to make a local function class inherit from another local function
class. However, this "inheritance" feature is not implemented
because it seemed of <a href="http://lists.boost.org/Archives/boost/2010/09/170895.php" target="_top">no
use</a> given that local functions can be bound to one another
thus they can simply call each other directly without recurring
to dynamic binding or base function calls.
</p></div></td></tr></tbody>
</table></div>
<h5>
<a name="boost_localfunction.advanced_topics.limitations__operators__etc_.h0"></a>
<span><a name="boost_localfunction.advanced_topics.limitations__operators__etc_.operators"></a></span><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_.operators">Operators</a>
</h5>
<p>
Local functions cannot be operators. Naming a local function <code class="computeroutput"><span class="keyword">operator</span><span class="special">...</span></code>
will generate a compile-time error. <sup>[<a name="boost_localfunction.advanced_topics.limitations__operators__etc_.f1" href="#ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f1" class="footnote">28</a>]</sup>
</p>
<p>
For example, the following code does not compile (see also <a href="../../../test/operator_error.cpp" target="_top"><code class="literal">operator_error.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">bool</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">point</span><span class="special">&</span> <span class="identifier">p</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">point</span><span class="special">&</span> <span class="identifier">q</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">q</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&&</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">y</span> <span class="special">==</span> <span class="identifier">q</span><span class="special">.</span><span class="identifier">y</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="keyword">operator</span><span class="special">==)</span> <span class="comment">// Error: Cannot use `operator...`.</span>
</pre>
<p>
</p>
<h5>
<a name="boost_localfunction.advanced_topics.limitations__operators__etc_.h1"></a>
<span><a name="boost_localfunction.advanced_topics.limitations__operators__etc_.goto"></a></span><a class="link" href="advanced_topics.html#boost_localfunction.advanced_topics.limitations__operators__etc_.goto">Goto</a>
</h5>
<p>
It is possible to jump with a <code class="computeroutput"><span class="keyword">goto</span></code>
within the local function body. For example, the following compiles (see
also <a href="../../../test/goto.cpp" target="_top"><code class="literal">goto.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">error</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">z</span> <span class="special">></span> <span class="number">0</span><span class="special">)</span> <span class="keyword">goto</span> <span class="identifier">success</span><span class="special">;</span> <span class="comment">// OK: Can jump within local function.</span>
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
<span class="identifier">success</span><span class="special">:</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">validate</span><span class="special">)</span>
<span class="keyword">return</span> <span class="identifier">validate</span><span class="special">(</span><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
However, it is not possible to jump with a <code class="computeroutput"><span class="keyword">goto</span></code>
from within the local function body to to a label defined in the enclosing
scope. For example, the following does not compile (see also <a href="../../../test/goto_error.cpp" target="_top"><code class="literal">goto_error.cpp</code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">error</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">BOOST_LOCAL_FUNCTION</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">z</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">z</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="keyword">goto</span> <span class="identifier">failure</span><span class="special">;</span> <span class="comment">// Error: Cannot jump to enclosing scope.</span>
<span class="keyword">else</span> <span class="keyword">goto</span> <span class="identifier">success</span><span class="special">;</span> <span class="comment">// OK: Can jump within local function.</span>
<span class="identifier">success</span><span class="special">:</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">BOOST_LOCAL_FUNCTION_NAME</span><span class="special">(</span><span class="identifier">validate</span><span class="special">)</span>
<span class="keyword">return</span> <span class="identifier">validate</span><span class="special">(</span><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">);</span>
<span class="identifier">failure</span><span class="special">:</span>
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.default_parameters.f0" href="#boost_localfunction.advanced_topics.default_parameters.f0" class="para">17</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> The assignment symbol <code class="computeroutput"><span class="special">=</span></code> cannot be used to specify default parameter
values because default values are not part of the parameter type so they
cannot be handled using template meta-programming. Default parameter values
need to be separated from the rest of the parameter declaration using the
preprocessor. Specifically, this library needs to use preprocessor meta-programming
to remove default values when constructing the local function type and
also to count the number of default values to provide the correct set of
call operators for the local functor. Therefore, the symbol <code class="computeroutput"><span class="special">=</span></code> cannot be used because it cannot be handled
by preprocessor meta-programming (non-alphanumeric symbols cannot be detected
by preprocessor meta-programming because they cannot be concatenated by
the preprocessor).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.default_parameters.f1" href="#boost_localfunction.advanced_topics.default_parameters.f1" class="para">18</a>] </sup>
The authors do not personally find the use of the <code class="computeroutput"><span class="identifier">WITH_DEFAULT</span></code>
macro more readable and they prefer to use the <code class="computeroutput"><span class="keyword">default</span></code>
keyword directly. Furthermore, <code class="computeroutput"><span class="identifier">WITH_DEFAULT</span></code>
needs to be defined differently for compilers without variadic macros
<code class="computeroutput"><span class="preprocessor">#define</span> <span class="identifier">WITH_DEFAULT</span>
<span class="special">(</span><span class="keyword">default</span><span class="special">)</span></code> so it can only be defined by programmers
based on the syntax they decide to use (see the <a class="link" href="no_variadic_macros.html" title="Annex: No Variadic Macros">No
Variadic Macros</a> section).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" href="#boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f0" class="para">19</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> This limitation is because
this library uses preprocessor token concatenation <code class="literal">##</code>
to inspect the macro parameters (to distinguish between function parameters,
bound variables, etc) and the C++ preprocessor does not allow to concatenate
non-alphanumeric tokens.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" href="#boost_localfunction.advanced_topics.commas_and_symbols_in_macros.f1" class="para">20</a>] </sup>
The preprocessor always interprets unwrapped commas as separating macro
parameters. Thus in this case the comma will indicate to the preprocessor
that the first macro parameter is <code class="computeroutput"><span class="keyword">const</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">tring</span></code>, the second macro parameter is
<code class="computeroutput"><span class="identifier">size_t</span><span class="special">>&</span>
<span class="identifier">m</span></code>, etc instead of passing <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">size_t</span><span class="special">>&</span> <span class="identifier">m</span></code>
as a single macro parameter.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" href="#boost_localfunction.advanced_topics.accessing_types__concepts__etc_.f0" class="para">21</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> The type names <code class="computeroutput"><span class="identifier">result_type</span></code> and <code class="computeroutput"><span class="identifier">arg</span></code><code class="literal"><span class="emphasis"><em>N</em></span></code><code class="computeroutput"><span class="identifier">_type</span></code> follow the <a href="http://www.boost.org/libs/type_traits" target="_top">Boost.TypeTraits</a>
naming conventions for function traits.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" href="#boost_localfunction.advanced_topics.specifying_types__no_boost_typeof_.f0" class="para">22</a>] </sup>
In the examples of this documentation, bound variables, function parameters,
and the result type are specified in this order because this is the order
used by <a href="http://en.wikipedia.org/wiki/C%2B%2B0x#Lambda_functions_and_expressions" target="_top">C++11
lambda functions</a>. However, the library accepts bound variables,
function parameters, and the result type in any order.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.inlining.f0" href="#boost_localfunction.advanced_topics.inlining.f0" class="para">23</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> This library uses an indirect
function call via a function pointer in order to pass the local function
as a template parameter (see the <a class="link" href="implementation.html" title="Annex: Implementation">Implementation</a>
section). No compiler has yet been observed to be able to inline function
calls when they use such indirect function pointer calls. Therefore,
inline local functions do not use such indirect function pointer call
(so they are more likely to be optimized) but because of that they
cannot be passed as template parameters. The indirect function pointer
call is needed on <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
but it is not needed on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
(see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm" target="_top">[N2657]</a>
and <a href="http://www.boost.org/libs/chrono" target="_top">Boost.Config</a>'s
<code class="computeroutput"><span class="identifier">BOOST_NO_LOCAL_CLASS_TEMPLATE_PARAMETERS</span></code>)
thus this library automatically generates local function calls that
can be inline on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>
compilers (even when the local function is not declared inline).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.recursion.f0" href="#boost_localfunction.advanced_topics.recursion.f0" class="para">24</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> This limitation comes from
the fact that the global functor used to pass the local function as a template
parameter (and eventually returned outside the declarations scope) does
not know the local function name so the local function name used for recursive
call cannot be set in the global functor. This limitation together with
preventing the possibility for inlining are the reasons why local functions
are not recursive unless programmers explicitly declare them <code class="computeroutput"><span class="identifier">recursive</span></code>.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.storage_classifiers.f0" href="#boost_localfunction.advanced_topics.storage_classifiers.f0" class="para">25</a>] </sup>
The <code class="computeroutput"><span class="keyword">auto</span></code> storage classifier
is part of the <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
standard and therefore supported by this library. However, the meaning
and usage of the <code class="computeroutput"><span class="keyword">auto</span></code> keyword
changed in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>.
Therefore, use the <code class="computeroutput"><span class="keyword">auto</span></code> storage
classifier with the usual care in order to avoid writing <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
code that might not work on <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.same_line_expansions.f0" href="#boost_localfunction.advanced_topics.same_line_expansions.f0" class="para">26</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> The <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION.html" title="Macro BOOST_LOCAL_FUNCTION">BOOST_LOCAL_FUNCTION</a></code>
and <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_TPL.html" title="Macro BOOST_LOCAL_FUNCTION_TPL">BOOST_LOCAL_FUNCTION_TPL</a></code>
macros internally use <code class="computeroutput"><span class="identifier">__LINE__</span></code>
to generate unique identifiers. Therefore, if these macros are expanded
more than on time on the same line, the generated identifiers will no longer
be unique and the code will not compile. (This restriction does not apply
to MSVC and other compilers that provide the non-standard <code class="computeroutput"><span class="identifier">__COUNTER__</span></code> macro.) Note that the <code class="computeroutput"><a class="link" href="../BOOST_LOCAL_FUNCTION_NAME.html" title="Macro BOOST_LOCAL_FUNCTION_NAME">BOOST_LOCAL_FUNCTION_NAME</a></code> macro
can always be expanded multiple times on the same line because the unique
local function name (and not <code class="computeroutput"><span class="identifier">__LINE__</span></code>)
is used by this macro to generate unique identifiers (so there is no need
for a <code class="computeroutput"><span class="identifier">BOOST_LOCAL_FUNCTION_NAME_ID</span></code>
macro).
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.same_line_expansions.f1" href="#boost_localfunction.advanced_topics.same_line_expansions.f1" class="para">27</a>] </sup>
Because there are restrictions on the set of tokens that the preprocessor
can concatenate and because not all compilers correctly implement these
restrictions, it is in general recommended to specify unique identifiers
as a combination of alphanumeric tokens.
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.boost_localfunction.advanced_topics.limitations__operators__etc_.f1" href="#boost_localfunction.advanced_topics.limitations__operators__etc_.f1" class="para">28</a>] </sup>
<span class="bold"><strong>Rationale.</strong></span> This is the because a local
function name must be a valid local variable name (the local variable used
to hold the local functor) and operators cannot be used as local variable
names.
</p></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2009-2012 Lorenzo
Caminiti<p>
Distributed under the Boost Software License, Version 1.0 (see accompanying
file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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