158 lines
5.5 KiB
HTML
Executable file
158 lines
5.5 KiB
HTML
Executable file
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
|
|
<html>
|
|
<head>
|
|
<meta http-equiv="Content-Language" content="en-us">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
|
|
|
|
<title>Boost Function Object Adapter Library</title>
|
|
</head>
|
|
|
|
<body bgcolor="#FFFFFF" text="#000000">
|
|
<table border="1" bgcolor="#007F7F" cellpadding="2" summary="">
|
|
<tr>
|
|
<td bgcolor="#FFFFFF"><img src="../../boost.png" alt=
|
|
"boost.png (6897 bytes)" width="277" height="86"></td>
|
|
|
|
<td><a href="../../index.htm"><font face="Arial" color=
|
|
"#FFFFFF"><big>Home</big></font></a></td>
|
|
|
|
<td><a href="../libraries.htm"><font face="Arial" color=
|
|
"#FFFFFF"><big>Libraries</big></font></a></td>
|
|
|
|
<td><a href="http://www.boost.org/people/people.htm"><font face="Arial" color=
|
|
"#FFFFFF"><big>People</big></font></a></td>
|
|
|
|
<td><a href="http://www.boost.org/more/faq.htm"><font face="Arial" color=
|
|
"#FFFFFF"><big>FAQ</big></font></a></td>
|
|
|
|
<td><a href="../../more/index.htm"><font face="Arial" color=
|
|
"#FFFFFF"><big>More</big></font></a></td>
|
|
</tr>
|
|
</table>
|
|
|
|
<h1>Function Pointer Adapters</h1>
|
|
|
|
<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
|
|
provides enhanced versions of both the function pointer adapters from the
|
|
C++ Standard Library (§20.3.7):</p>
|
|
|
|
<ul>
|
|
<li><tt>pointer_to_unary_function</tt></li>
|
|
|
|
<li><tt>pointer_to_binary_function</tt></li>
|
|
</ul>
|
|
|
|
<p>As well as the corresponding helper function template:</p>
|
|
|
|
<ul>
|
|
<li><tt>ptr_fun</tt></li>
|
|
</ul>
|
|
|
|
<p>However, you should not need to use the adapters in conjunction with the
|
|
adapters in this library due to our use of <a href=
|
|
"function_traits.html">function object traits</a>. You will however need to
|
|
use them if your implementation fails to work properly with our traits
|
|
classes (due to lack if partial specialisation), or if you wish to use a
|
|
function object adapter from a third party.</p>
|
|
|
|
<h3>Usage</h3>
|
|
|
|
<p>If you need to use these adapters, usage is identical to the standard
|
|
function pointer adapters. For example,</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
bool bad(std::string foo) { ... }
|
|
...
|
|
std::vector<std::string> c;
|
|
...
|
|
std::vector<std::string>::iterator it
|
|
= std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad)));
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>Note however that this library contains enhanced <a href=
|
|
"negators.html">negators</a> that support function object traits, so the
|
|
line above could equally be written</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
std::vector<std::string>::iterator it
|
|
= std::find_if(c.begin(), c.end(), boost::not1(bad));
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<h3>Argument Types</h3>
|
|
|
|
<p>The standard defines <tt>pointer_to_unary_function</tt> like this
|
|
(§20.3.8 ¶2):</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
template <class Arg, class Result>
|
|
class pointer_to_unary_function : public unary_function<Arg, Result> {
|
|
public:
|
|
explicit pointer_to_unary_function(Result (* f)(<strong>Arg</strong>));
|
|
Result operator()(<strong>Arg</strong> x) const;
|
|
};
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>Note that the argument to <tt>operator()</tt> is exactly the same type
|
|
as the argument to the wrapped function. If this is a value type, the
|
|
argument will be passed by value and copied twice.
|
|
<tt>pointer_to_binary_function</tt> has a similar problem.</p>
|
|
|
|
<p>However, if we were to try and eliminate this inefficiency by instead
|
|
declaring the argument as <tt>const Arg&</tt>, then if Arg were a
|
|
reference type, we would have a reference to a reference, which is
|
|
currently illegal (but see <a href=
|
|
"http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ core
|
|
language issue number 106)</a></p>
|
|
|
|
<p>So the way in which we want to declare the argument for
|
|
<tt>operator()</tt> depends on whether or not the wrapped function's
|
|
argument is a reference. If it is a reference, we want to declare it simply
|
|
as <tt>Arg</tt>; if it is a value we want to declare it as
|
|
<tt>const Arg&</tt>.</p>
|
|
|
|
<p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class
|
|
template contains a <tt>param_type</tt> typedef, which uses partial
|
|
specialisation to make precisely this decision. By declaring the
|
|
<tt>operator()</tt> as</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
Result operator()(typename call_traits<Arg>::param_type x) const
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>we achieve the desired result - we improve efficiency without generating
|
|
references to references.</p>
|
|
|
|
<h3>Limitations</h3>
|
|
|
|
<p>The call traits template used to realise this improvement relies on
|
|
partial specialisation, so this improvement is only available on compilers
|
|
that support that feature. With other compilers, the argument passed to the
|
|
function will always be passed by reference, thus generating the
|
|
possibility of references to references.</p>
|
|
<hr>
|
|
|
|
<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
|
|
"../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
|
|
height="31" width="88"></a></p>
|
|
|
|
<p>Revised
|
|
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
|
|
December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
|
|
|
|
<p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p>
|
|
|
|
<p><i>Distributed under the Boost Software License, Version 1.0. (See
|
|
accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
|
|
copy at <a href=
|
|
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
|
|
</body>
|
|
</html>
|