201 lines
6.8 KiB
HTML
Executable file
201 lines
6.8 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>Member Function Adapters</h1>
|
|
|
|
<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
|
|
includes improved versions of the full range of member function adapters
|
|
from the the C++ Standard Library (§20.3.8):</p>
|
|
|
|
<ul>
|
|
<li><tt>mem_fun_t</tt></li>
|
|
|
|
<li><tt>mem_fun1_t</tt></li>
|
|
|
|
<li><tt>const_mem_fun_t</tt></li>
|
|
|
|
<li><tt>const_mem_fun1_t</tt></li>
|
|
|
|
<li><tt>mem_fun_ref_t</tt></li>
|
|
|
|
<li><tt>mem_fun1_ref_t</tt></li>
|
|
|
|
<li><tt>const_mem_fun_ref_t</tt></li>
|
|
|
|
<li><tt>const_mem_fun1_ref_t</tt></li>
|
|
</ul>
|
|
|
|
<p>as well as the corresponding overloaded helper functions</p>
|
|
|
|
<ul>
|
|
<li><tt>mem_fun</tt></li>
|
|
|
|
<li><tt>mem_fun_ref</tt></li>
|
|
</ul>
|
|
|
|
<p>The following changes have been made to the adapters as specified in the
|
|
Standard:</p>
|
|
|
|
<ul>
|
|
<li>The <tt>first_argument_type</tt> typedef has been corrected for the
|
|
<tt>const_</tt> family of member function adapters (see <a href=
|
|
"#firstarg">below</a>).</li>
|
|
|
|
<li>The argument passed to <tt>mem_fun1_t</tt> and its variants is passed
|
|
using the <tt>call_traits::param_type</tt> for the member function's
|
|
argument type.</li>
|
|
</ul>
|
|
|
|
<h3 id="firstarg">first_argument_type</h3>
|
|
|
|
<p>The standard specifies <tt>const_mem_fun1_t</tt>, for example, like
|
|
this:</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
template <class S, class T, class A> class const_mem_fun1_t
|
|
: public binary_function<<strong>T*</strong>, A, S> {
|
|
public:
|
|
explicit const_mem_fun1_t(S (T::*p)(A) const);
|
|
S operator()(<strong>const T*</strong> p, A x) const;
|
|
};
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>Note that the first argument to <tt>binary_function</tt> is <tt>T*</tt>
|
|
despite the fact that the first argument to <tt>operator()</tt> is actually
|
|
of type <tt><em>const</em> T*</tt>.</p>
|
|
|
|
<p>Does this matter? Well, consider what happens when we write</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
struct Foo { void bar(int) const; };
|
|
const Foo *cp = new Foo;
|
|
std::bind1st(std::mem_fun(&Foo::bar), cp);
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>We have created a <tt>const_mem_fun1_t</tt> object which will
|
|
effectively contain the following</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
typedef Foo* first_argument_type;
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>The <tt>bind1st</tt> will then create a <tt>binder1st</tt> object that
|
|
will use this <tt>typedef</tt> as the type of a member which will be
|
|
initialised with <tt>cp</tt>. In other words, we will need to initialise a
|
|
<tt>Foo*</tt> member with a <tt>const Foo*</tt> pointer! Clearly this
|
|
is not possible, so to implement this your Standard Library vendor will
|
|
have had to cast away the constness of <tt>cp</tt>, probably within the
|
|
body of <tt>bind1st</tt>.</p>
|
|
|
|
<p>This hack will not suffice with the improved <a href=
|
|
"binders.html">binders</a> in this library, so we have had to provide
|
|
corrected versions of the member function adapters as well.</p>
|
|
|
|
<h3 id="args">Argument Types</h3>
|
|
|
|
<p>The standard defines <tt>mem_fun1_t</tt>, for example, like this
|
|
(§20.3.8 ¶2):</p>
|
|
|
|
<blockquote>
|
|
<pre>
|
|
template <class S, class T, class A> class mem_fun1_t
|
|
: public binary_function<T*, A, S> {
|
|
public:
|
|
explicit mem_fun1_t(S (T::*p)(<strong>A</strong>));
|
|
S operator()(T* p, <strong>A</strong> x) const;
|
|
};
|
|
</pre>
|
|
</blockquote>
|
|
|
|
<p>Note that the second argument to <tt>operator()</tt> is exactly the same
|
|
type as the argument to the member function. If this is a value type, the
|
|
argument will be passed by value and copied twice.</p>
|
|
|
|
<p>However, if we were to try and eliminate this inefficiency by instead
|
|
declaring the argument as <tt>const A&</tt>, then if A 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 second argument for
|
|
<tt>operator()</tt> depends on whether or not the member function's
|
|
argument is a reference. If it is a reference, we want to declare it simply
|
|
as <tt>A</tt>; if it is a value we want to declare it as
|
|
<tt>const A&</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>
|
|
S operator()(T* p, typename call_traits<A>::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 some improvements relies on
|
|
partial specialisation, so these improvements are only available on
|
|
compilers that support that feature. With other compilers, the argument
|
|
passed to the member function (in the <tt>mem_fun1_t</tt> family) 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>
|