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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_localfunction.implementation"></a><a class="link" href="implementation.html" title="Annex: Implementation">Annex: Implementation</a>
</h2></div></div></div>
<p>
This section gives an overview of the key programming techniques used to implement
this library.
</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>
The code listed here can be used by curious readers and library maintainers
as a reference in trying to understand the library source code. There is
absolutely no guarantee that the library implementation uses the exact code
listed here.
</p></td></tr>
</table></div>
<h4>
<a name="boost_localfunction.implementation.h0"></a>
<span class="phrase"><a name="boost_localfunction.implementation.local_classes_as_template_parameters"></a></span><a class="link" href="implementation.html#boost_localfunction.implementation.local_classes_as_template_parameters">Local
Classes as Template Parameters</a>
</h4>
<p>
This library uses a local class to implement the local function object. However,
in <a href="http://www.open-std.org/JTC1/SC22/WG21/docs/standards" target="_top">C++03</a>
local classes (and therefore the local function objects they implement) cannot
be passed as template parameters (e.g., to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</span></code>
algorithm), this is instead possible in <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>,
MSVC, and some other compilers (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_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS</span></code>).
To work around this limitation, this library investigated the following two
"tricks" (both tricks can be extended to support function default
parameters):
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
The <span class="emphasis"><em>casting functor trick</em></span> uses a non-local functor
that calls a static member function of the local class via a function pointer.
The static member function then calls the correct local function body after
type casting the object from a <code class="computeroutput"><span class="keyword">void</span><span class="special">*</span></code> pointer (local classes can always be used
for type casting via <code class="computeroutput"><span class="keyword">static_cast</span></code>
or similar).
</li>
<li class="listitem">
The <span class="emphasis"><em>virtual functor trick</em></span> derives the local functor
class from a non-local base class. The correct overridden implementation
of the virtual <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
is then called via dynamic binding.
</li>
</ol></div>
<p>
For example (see also <a href="../../../example/impl_tparam_tricks.cpp" target="_top"><code class="literal">impl_tparam_tricks.cpp</code></a>):
</p>
<p>
</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">detail</span><span class="special">/</span><span class="identifier">lightweight_test</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">algorithm</span><span class="special">></span>
<span class="comment">// Casting functor trick.</span>
<span class="keyword">struct</span> <span class="identifier">casting_func</span> <span class="special">{</span>
<span class="keyword">explicit</span> <span class="identifier">casting_func</span><span class="special">(</span><span class="keyword">void</span><span class="special">*</span> <span class="identifier">obj</span><span class="special">,</span> <span class="keyword">void</span> <span class="special">(*</span><span class="identifier">call</span><span class="special">)(</span><span class="keyword">void</span><span class="special">*,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&))</span>
<span class="special">:</span> <span class="identifier">obj_</span><span class="special">(</span><span class="identifier">obj</span><span class="special">),</span> <span class="identifier">call_</span><span class="special">(</span><span class="identifier">call</span><span class="special">)</span> <span class="special">{}</span>
<span class="comment">// Unfortunately, function pointer call is not inlined.</span>
<span class="keyword">inline</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">call_</span><span class="special">(</span><span class="identifier">obj_</span><span class="special">,</span> <span class="identifier">num</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="keyword">void</span><span class="special">*</span> <span class="identifier">obj_</span><span class="special">;</span>
<span class="keyword">void</span> <span class="special">(*</span><span class="identifier">call_</span><span class="special">)(</span><span class="keyword">void</span><span class="special">*,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&);</span>
<span class="special">};</span>
<span class="comment">// Virtual functor trick.</span>
<span class="keyword">struct</span> <span class="identifier">virtual_func</span> <span class="special">{</span>
<span class="keyword">struct</span> <span class="identifier">interface</span> <span class="special">{</span>
<span class="comment">// Unfortunately, virtual function call is not inlined.</span>
<span class="keyword">inline</span> <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&)</span> <span class="special">{}</span>
<span class="special">};</span>
<span class="keyword">explicit</span> <span class="identifier">virtual_func</span><span class="special">(</span><span class="identifier">interface</span><span class="special">&</span> <span class="identifier">func</span><span class="special">):</span> <span class="identifier">func_</span><span class="special">(&</span><span class="identifier">func</span><span class="special">)</span> <span class="special">{}</span>
<span class="keyword">inline</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span> <span class="special">(*</span><span class="identifier">func_</span><span class="special">)(</span><span class="identifier">num</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="identifier">interface</span><span class="special">*</span> <span class="identifier">func_</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">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="comment">// Local class for local function.</span>
<span class="keyword">struct</span> <span class="identifier">local_add</span> <span class="special">:</span> <span class="identifier">virtual_func</span><span class="special">::</span><span class="identifier">interface</span> <span class="special">{</span>
<span class="keyword">explicit</span> <span class="identifier">local_add</span><span class="special">(</span><span class="keyword">int</span><span class="special">&</span> <span class="identifier">_sum</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="special">:</span> <span class="identifier">sum_</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">_factor</span><span class="special">)</span> <span class="special">{}</span>
<span class="keyword">inline</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">body</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="keyword">inline</span> <span class="keyword">static</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="identifier">obj</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">num</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">local_add</span><span class="special">*</span> <span class="identifier">self</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">local_add</span><span class="special">*>(</span><span class="identifier">obj</span><span class="special">);</span>
<span class="identifier">self</span><span class="special">-></span><span class="identifier">body</span><span class="special">(</span><span class="identifier">self</span><span class="special">-></span><span class="identifier">sum_</span><span class="special">,</span> <span class="identifier">self</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="keyword">private</span><span class="special">:</span>
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">sum_</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="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">body</span><span class="special">(</span><span class="keyword">int</span><span class="special">&</span> <span class="identifier">sum</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="keyword">const</span> <span class="keyword">int</span><span class="special">&</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="special">}</span> <span class="identifier">add_local</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">casting_func</span> <span class="identifier">add_casting</span><span class="special">(&</span><span class="identifier">add_local</span><span class="special">,</span> <span class="special">&</span><span class="identifier">local_add</span><span class="special">::</span><span class="identifier">call</span><span class="special">);</span>
<span class="identifier">virtual_func</span> <span class="identifier">add_virtual</span><span class="special">(</span><span class="identifier">add_local</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="number">10</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="comment">// std::for_each(v.begin(), v.end(), add_local); // Error but OK on C++11.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</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">add_casting</span><span class="special">);</span> <span class="comment">// OK.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">for_each</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">add_virtual</span><span class="special">);</span> <span class="comment">// OK.</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">sum</span> <span class="special">==</span> <span class="number">200</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>
The casting functor trick measured slightly better run-time performances than
the virtual functor trick so the current implementation of this library uses
the casting functor trick (probably because in addition to the indirect function
call, the virtual functor trick also requires accessing the <a href="http://en.wikipedia.org/wiki/Virtual_method_table" target="_top">virtual
function table</a>). However, neither one of the two tricks was observed
to allow for compiler optimizations that inline the local function calls (because
they rely on one indirect function call via either a function pointer or a
virtual function respectively). Therefore, on compilers that accept local classes
as template parameters (MSVC, <a href="http://www.open-std.org/JTC1/SC22/WG21/" target="_top">C++11</a>,
etc, 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_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS</span></code>),
this library automatically generates code that passes the local class type
directly as template parameter without using neither one of these two tricks
in order to take full advantage of compiler optimizations that inline the local
function calls.
</p>
<h4>
<a name="boost_localfunction.implementation.h1"></a>
<span class="phrase"><a name="boost_localfunction.implementation.parsing_macros"></a></span><a class="link" href="implementation.html#boost_localfunction.implementation.parsing_macros">Parsing
Macros</a>
</h4>
<p>
This library macros can parse the list of specified parameters and detect if
any of the bound variable names matches the token <code class="computeroutput"><span class="identifier">this_</span></code>
(to generate special code to bind the object in scope), or if the variable
is bound by <code class="computeroutput"><span class="keyword">const</span></code> (to generate
special code to bind by constant), etc. The parameter tokens are inspected
using preprocessor meta-programming and specifically using the macros defined
by the files in the <code class="literal">boost/local_function/detail/preprocessor/keyword/</code>
directory. <a href="#ftn.boost_localfunction.implementation.f0" class="footnote" name="boost_localfunction.implementation.f0"><sup class="footnote">[31]</sup></a>
</p>
<p>
For example, the following code defines a macro that allows the preprocessor
to detect if a set of space-separated tokens ends with <code class="computeroutput"><span class="identifier">this_</span></code>
or not (see also <a href="../../../example/impl_pp_keyword.cpp" target="_top"><code class="literal">impl_pp_keyword.cpp</code></a>):
</p>
<p>
</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">local_function</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">keyword</span><span class="special">/</span><span class="identifier">thisunderscore</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">keyword</span><span class="special">/</span><span class="keyword">const</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">local_function</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">preprocessor</span><span class="special">/</span><span class="identifier">keyword</span><span class="special">/</span><span class="identifier">bind</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">lightweight_test</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="comment">// Expand to 1 if space-separated tokens end with `this_`, 0 otherwise.</span>
<span class="preprocessor">#define</span> <span class="identifier">IS_THIS_BACK</span><span class="special">(</span><span class="identifier">tokens</span><span class="special">)</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_IS_THISUNDERSCORE_BACK</span><span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_BIND_REMOVE_FRONT</span><span class="special">(</span> <span class="special">\</span>
<span class="identifier">BOOST_LOCAL_FUNCTION_DETAIL_PP_KEYWORD_CONST_REMOVE_FRONT</span><span class="special">(</span> <span class="special">\</span>
<span class="identifier">tokens</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="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">IS_THIS_BACK</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bind</span> <span class="identifier">this_</span><span class="special">)</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">IS_THIS_BACK</span><span class="special">(</span><span class="keyword">const</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="number">0</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>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.boost_localfunction.implementation.f0" class="footnote"><p><a href="#boost_localfunction.implementation.f0" class="para"><sup class="para">[31] </sup></a>
This technique is at the core of even more complex preprocessor parsing macros
like the ones that parse the <a href="http://sourceforge.net/projects/contractpp" target="_top">Contract++</a>
syntax.
</p></div>
</div>
</div>
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