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<h1>Improved Function Object Adapters</h1>
<p>The header <nobr><a href="../../boost/functional.hpp">functional.hpp</a></nobr>
provides enhancements to the function object adapters specified in the C++
Standard Library (sections 20.3.5, through to 20.3.8). The enhancements are
principally possible due to two changes:</p>
<ol>
<li>We use the Boost <nobr><tt><a href="../utility/call_traits.htm">call_traits</a></tt></nobr>
templates to avoid the problem of <a href="binders.html#refref">references
to references</a>, and to improve the efficiency of <a href="mem_fun.html#args">parameter
passing</a>.</li>
<li>We use two <a href="function_traits.html">function object traits</a> class
templates to avoid the need for <nobr><tt><a href="ptr_fun.html">ptr_fun</a></tt></nobr>
with the adapters in this library.</li>
</ol>
<h3>Contents</h3>
<p>The header contains the following function and class templates:</p>
<table border="1" cellpadding="5">
<tr>
<th align="left"><a href="function_traits.html">Function object traits</a>
<td valign="top"><tt><nobr>unary_traits</nobr><br>
<nobr>binary_traits</nobr></tt></td>
<td valign="top">Used to determine the types of function objects' and
functions' arguments. Eliminate the necessity for <nobr><tt>ptr_fun</tt></nobr>.</td>
</tr>
<tr>
<th align="left"><a href="negators.html">Negators</a></th>
<td valign="top"><tt><nobr>unary_negate</nobr><br>
<nobr>binary_negate</nobr><br>
<nobr>not1</nobr><br>
<nobr>not2</nobr></tt></td>
<td valign="top">Based on section 20.3.5 of the standard.</td>
</tr>
<tr>
<th align="left"><a href="binders.html">Binders</a></th>
<td valign="top"><tt><nobr>binder1st</nobr><br>
<nobr>binder2nd</nobr><br>
<nobr>bind1st</nobr><br>
<nobr>bind2nd</nobr></tt></td>
<td valign="top">Based on section 20.3.6 of the standard.</td>
</tr>
<tr>
<th align="left"><a href="ptr_fun.html">Adapters for pointers to functions</a></th>
<td valign="top"><tt><nobr>pointer_to_unary_function</nobr><br>
<nobr>pointer_to_binary_function</nobr><br>
<nobr>ptr_fun</nobr></tt></td>
<td valign="top">Based on section 20.3.7 of the standard. Not required for
use with this library since the binders and negators can adapt functions,
but may be needed with third party adapters.</td>
</tr>
<tr>
<th align="left"><a href="mem_fun.html">Adapters for pointers to member
functions</a></th>
<td valign="top"><tt><nobr>mem_fun_t</nobr><br>
<nobr>mem_fun1_t</nobr><br>
<nobr>const_mem_fun_t</nobr><br>
<nobr>const_mem_fun1_t</nobr><br>
<nobr>mem_fun_ref_t</nobr><br>
<nobr>mem_fun1_ref_t</nobr><br>
<nobr>const_mem_fun_ref_t</nobr><br>
<nobr>const_mem_fun1_ref_t</nobr><br>
<nobr>mem_fun</nobr><br>
<nobr>mem_fun_ref</nobr></tt></td>
<td valign="top">Based on section 20.3.8 of the standard.</td>
</tr>
</table>
<h3>Usage</h3>
<p>Using these adapters should be pretty much the same as using the standard
function object adapters; the only differences are that you need to write <nobr><tt>boost::</tt></nobr>
instead of <nobr><tt>std::</tt></nobr>, and that you will get fewer headaches.</p>
<p>For example, suppose you had a <tt>Person</tt> class that contained a <nobr><tt>set_name</tt></nobr>
function:
<blockquote>
<pre>
class Person
{
public:
void set_name(const std::string &name);
// ...
};
</pre>
</blockquote>
<p>You could rename a bunch of people in a collection, <tt>c</tt>, by writing</p>
<blockquote>
<pre>
std::for_each(c.begin(), c.end(),
boost::bind2nd(boost::mem_fun_ref(&Person::set_name), "Fred"));
</pre>
</blockquote>
<p>If the standard adapters had been used instead then this code would normally
fail to compile, because <tt><nobr>set_name</nobr></tt> takes a reference
argument. Refer to the comments in the <a href="binders.html#refref">binder
documentation</a> to explain why this is so.</p>
<h3>Compiler Compatibility</h3>
<p>The header and <a href="function_test.cpp">test program</a> have been
compiled with the following compilers:</p>
<table border="1" cellpadding="5">
<tr>
<th>Compiler</th>
<th>Comments</th>
</tr>
<tr>
<td valign="top">Borland C++Builder 4 Update 2</td>
<td valign="top">No known issues.</td>
</tr>
<tr>
<td valign="top">Borland C++ 5.5</td>
<td valign="top">No known issues.</td>
</tr>
<tr>
<td valign="top">g++ 2.95.2</td>
<td valign="top">No known issues.</td>
</tr>
<tr>
<td valign="top">Microsoft Visual C++ Service Pack 3</td>
<td valign="top">Compiler lacks partial specialisation, so this library
offers little more than is provided by the standard adapters:
<ul>
<li>The <nobr><tt>call_traits</tt></nobr> mechanism is unable to prevent
references to references, and so the adapters in this library will be
usable in fewer situations.</li>
<li>The <nobr><tt>function_traits</tt></nobr> mechanism is unable to
determine the argument and result types of functions, therefore <nobr><tt>ptr_fun</tt></nobr>
continues to be required to adapt functions.
</ul>
</td>
</tr>
</table>
<h3>Future Directions</h3>
<p>This library's primary focus is to solve the problem of references to
references while maintaining as much compatibility as possible with the standard
library. This allows you to use the techniques you read about in books and
magazines with many of today's compilers.</p>
<p>In the longer term, even better solutions are likely:</p>
<ol>
<li>Several Boost members are working on expression template libraries. These
will allow a more natural syntax for combining and adapting functions. As
this is a new technology, it may be some time before it has matured and is
widely supported by major compilers but shows great promise. In the
meantime, the functional.hpp library fills the gap.</li>
<li>The Standard Committee has recognised the problem of references to
references occurring during template instantiation and has moved to fix the
standard (see the <a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
standard core language active issues list</a>).</li>
</ol>
<h3>Author</h3>
<p><a href="../../people/mark_rodgers.htm">Mark Rodgers</a></p>
<h3>Acknowledgements</h3>
<p>Thanks to <a href="../../people/john_maddock.htm">John Maddock</a> for
suggesting the mechanism that allowed the function objects traits to work
correctly. <a href="../../people/jens_maurer.htm">Jens Maurer</a> provided
invaluable feedback during the <a href="../../more/formal_review_process.htm">formal
review process</a>.
<hr>
<p>Copyright 2000 Cadenza New Zealand Ltd. Permission to copy, use, modify,
sell and distribute this document is granted provided this copyright notice
appears in all copies. This document is provided "as is" without
express or implied warranty, and with no claim as to its suitability for any
purpose.</p>
<p>Revised 28 June 2000</p>
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