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<td width="277"><A href="../../index.htm"> <img src="../../boost.png" alt="boost.png (6897 bytes)" width="277" height="86" border="0"></A>
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<h1>mem_fn.hpp</h1>
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<td colspan="2" height="64"> </td>
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<h2>Contents</h2>
<h3 style="MARGIN-LEFT: 20pt"><a href="#Purpose">Purpose</a></h3>
<h3 style="MARGIN-LEFT: 20pt"><a href="#FAQ">Frequently Asked Questions</a></h3>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Q1">Can <b>mem_fn</b> be used instead of the
standard <b>std::mem_fun[_ref]</b> adaptors?</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Q2">Should I replace every occurence of <b>std::mem_fun[_ref]</b>
with <b>mem_fn</b> in my existing code?</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Q3">Does <b>mem_fn</b> work with COM methods?</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Q4">Why isn't BOOST_MEM_FN_ENABLE_STDCALL
defined automatically?</a></h4>
<h3 style="MARGIN-LEFT: 20pt"><a href="#Interface">Interface</a></h3>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Synopsis">Synopsis</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#CommonRequirements">Common requirements</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#get_pointer">get_pointer</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#mem_fn">mem_fn</a></h4>
<h3 style="MARGIN-LEFT: 20pt"><a href="#Implementation">Implementation</a></h3>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Files">Files</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#Dependencies">Dependencies</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#NumberOfArguments">Number of Arguments</a></h4>
<h4 style="MARGIN-LEFT: 40pt"><a href="#stdcall">"__stdcall", "__cdecl" and
"__fastcall" Support</a></h4>
<h3 style="MARGIN-LEFT: 20pt"><a href="#Acknowledgements">Acknowledgements</a></h3>
<h2><a name="Purpose">Purpose</a></h2>
<p>
<b>boost::mem_fn</b> is a generalization of the standard functions <b>std::mem_fun</b>
and <b>std::mem_fun_ref</b>. It supports member function pointers with more
than one argument, and the returned function object can take a pointer, a
reference, or a smart pointer to an object instance as its first argument. <STRONG>mem_fn</STRONG>
also supports pointers to data members by treating them as functions taking no
arguments and returning a (const) reference to the member.
</p>
<p>
The purpose of <b>mem_fn</b> is twofold. First, it allows users to invoke a
member function on a container with the familiar
</p>
<pre>
std::for_each(v.begin(), v.end(), boost::mem_fn(&Shape::draw));
</pre>
<p>
syntax, even when the container stores smart pointers.
</p>
<p>
Second, it can be used as a building block by library developers that want to
treat a pointer to member function as a function object. A library might define
an enhanced <b>for_each</b> algorithm with an overload of the form:
</p>
<pre>
template<class It, class R, class T> void for_each(It first, It last, R (T::*pmf) ())
{
std::for_each(first, last, boost::mem_fn(pmf));
}
</pre>
<p>
that will allow the convenient syntax:
</p>
<pre>
for_each(v.begin(), v.end(), &Shape::draw);
</pre>
<p>
When documenting the feature, the library author will simply state:
</p>
<h4 style="MARGIN-LEFT: 20pt">template<class It, class R, class T> void
for_each(It first, It last, R (T::*pmf) ());</h4>
<p style="MARGIN-LEFT: 20pt">
<b>Effects:</b> equivalent to std::for_each(first, last, boost::mem_fn(pmf));
</p>
<p>
where <b>boost::mem_fn</b> can be a link to this page. See <a href="bind.html">the
documentation of <b>bind</b></a> for an example.
</p>
<p>
<b>mem_fn</b> takes one argument, a pointer to a member, and returns a function
object suitable for use with standard or user-defined algorithms:
</p>
<pre>
struct X
{
void f();
};
void g(std::vector<X> & v)
{
std::for_each(v.begin(), v.end(), boost::mem_fn(&X::f));
};
void h(std::vector<X *> const & v)
{
std::for_each(v.begin(), v.end(), boost::mem_fn(&X::f));
};
void k(std::vector<boost::shared_ptr<X> > const & v)
{
std::for_each(v.begin(), v.end(), boost::mem_fn(&X::f));
};
</pre>
<p>
The returned function object takes the same arguments as the input member
function plus a "flexible" first argument that represents the object instance.
</p>
<p>
When the function object is invoked with a first argument <b>x</b> that is
neither a pointer nor a reference to the appropriate class (<b>X</b> in the
example above), it uses <tt>get_pointer(x)</tt> to obtain a pointer from <b>x</b>.
Library authors can "register" their smart pointer classes by supplying an
appropriate <b>get_pointer</b> overload, allowing <b>mem_fn</b> to recognize
and support them.
</p>
<p>
[Note: <b>get_pointer</b> is not restricted to return a pointer. Any object
that can be used in a member function call expression <tt>(x->*pmf)(...)</tt>
will work.]
</p>
<p>
[Note: the library uses an unqualified call to <b>get_pointer</b>. Therefore,
it will find, through argument-dependent lookup, <b>get_pointer</b> overloads
that are defined in the same namespace as the corresponding smart pointer
class, in addition to any <b>boost::get_pointer</b> overloads.]
</p>
<p>
All function objects returned by <b>mem_fn</b> expose a <b>result_type</b> typedef
that represents the return type of the member function. For data members, <STRONG>result_type</STRONG>
is defined as the type of the member.
</p>
<h2><a name="FAQ">Frequently Asked Questions</a></h2>
<h3><a name="Q1">Can <b>mem_fn</b> be used instead of the standard <b>std::mem_fun[_ref]</b>
adaptors?</a></h3>
<p>
Yes. For simple uses, <b>mem_fn</b> provides additional functionality that the
standard adaptors do not. Complicated expressions that use <b>std::bind1st</b>, <b>std::bind2nd</b>
or <a href="../compose/index.htm"><b>Boost.Compose</b></a> along with the
standard adaptors can be rewritten using <a href="bind.html"><b>boost::bind</b></a>
that automatically takes advantage of <b>mem_fn</b>.
</p>
<h3><a name="Q2">Should I replace every occurence of <b>std::mem_fun[_ref]</b> with <b>mem_fn</b>
in my existing code?</a></h3>
<p>
No, unless you have good reasons to do so. <b>mem_fn</b> is not 100% compatible
with the standard adaptors, although it comes pretty close. In particular, <b>mem_fn</b>
does not return objects of type <b>std::[const_]mem_fun[1][_ref]_t</b>, as the
standard adaptors do, and it is not possible to fully describe the type of the
first argument using the standard <b>argument_type</b> and <b>first_argument_type</b>
nested typedefs. Libraries that need adaptable function objects in order to
function might not like <b>mem_fn</b>.
</p>
<h3><a name="Q3">Does <b>mem_fn</b> work with COM methods?</a></h3>
<p>
Yes, if you <a href="#stdcall">#define BOOST_MEM_FN_ENABLE_STDCALL</a>.
</p>
<h3><a name="Q4">Why isn't BOOST_MEM_FN_ENABLE_STDCALL defined automatically?</a></h3>
<p>
Non-portable extensions, in general, should default to off to prevent vendor
lock-in. Had BOOST_MEM_FN_ENABLE_STDCALL been defined automatically, you could
have accidentally taken advantage of it without realizing that your code is,
perhaps, no longer portable. In addition, it is possible for the default
calling convention to be __stdcall, in which case enabling __stdcall support
will result in duplicate definitions.
</p>
<h2><a name="Interface">Interface</a></h2>
<h3><a name="Synopsis">Synopsis</a></h3>
<pre>
namespace boost
{
template<class T> T * <a href="#get_pointer_1">get_pointer</a>(T * p);
template<class R, class T> <i>unspecified-1</i> <a href="#mem_fn_1">mem_fn</a>(R (T::*pmf) ());
template<class R, class T> <i>unspecified-2</i> <a href="#mem_fn_2">mem_fn</a>(R (T::*pmf) () const);
template<class R, class T> <i>unspecified-2-1</i> <a href="#mem_fn_2_1">mem_fn</a>(R T::*pm);
template<class R, class T, class A1> <i>unspecified-3</i> <a href="#mem_fn_3">mem_fn</a>(R (T::*pmf) (A1));
template<class R, class T, class A1> <i>unspecified-4</i> <a href="#mem_fn_4">mem_fn</a>(R (T::*pmf) (A1) const);
template<class R, class T, class A1, class A2> <i>unspecified-5</i> <a href="#mem_fn_5">mem_fn</a>(R (T::*pmf) (A1, A2));
template<class R, class T, class A1, class A2> <i>unspecified-6</i> <a href="#mem_fn_6">mem_fn</a>(R (T::*pmf) (A1, A2) const);
// implementation defined number of additional overloads for more arguments
}
</pre>
<h3><a name="CommonRequirements">Common requirements</a></h3>
<p>
All <tt><i>unspecified-N</i></tt> types mentioned in the Synopsis are <b>CopyConstructible</b>
and <b>Assignable</b>. Their copy constructors and assignment operators do not
throw exceptions. <tt><i>unspecified-N</i>::result_type</tt> is defined as the
return type of the member function pointer passed as an argument to <b>mem_fn</b>
(<b>R</b> in the Synopsis.) <tt><i>unspecified-2-1</i>::result_type</tt> is
defined as <tt>R</tt>.
</p>
<h3><a name="get_pointer">get_pointer</a></h3>
<h4><a name="get_pointer_1">template<class T> T * get_pointer(T * p)</a></h4>
<blockquote>
<p>
<b>Returns:</b> <tt>p</tt>.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h3><a name="mem_fn">mem_fn</a></h3>
<h4><a name="mem_fn_1">template<class R, class T> <i>unspecified-1</i> mem_fn(R
(T::*pmf) ())</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t)</i></tt>
is equivalent to <tt>(t.*pmf)()</tt> when <i>t</i> is an l-value of type <STRONG>T </STRONG>
or derived, <tt>(get_pointer(t)->*pmf)()</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="mem_fn_2">template<class R, class T> <i>unspecified-2</i> mem_fn(R
(T::*pmf) () const)</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t)</i></tt>
is equivalent to <tt>(t.*pmf)()</tt> when <i>t</i> is of type <STRONG>T</STRONG>
<EM>[const]<STRONG> </STRONG></EM>or derived, <tt>(get_pointer(t)->*pmf)()</tt>
otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="mem_fn_2_1">template<class R, class T> <i>unspecified-2-1</i> mem_fn(R
T::*pm)</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t)</i></tt>
is equivalent to <tt>t.*pm</tt> when <i>t</i> is of type <STRONG>T</STRONG> <EM>[const]<STRONG>
</STRONG></EM>or derived, <tt>get_pointer(t)->*pm</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="mem_fn_3">template<class R, class T, class A1> <i>unspecified-3</i> mem_fn(R
(T::*pmf) (A1))</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t, a1)</i></tt>
is equivalent to <tt>(t.*pmf)(a1)</tt> when <i>t</i> is an l-value of type <STRONG>T
</STRONG>or derived, <tt>(get_pointer(t)->*pmf)(a1)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="mem_fn_4">template<class R, class T, class A1> <i>unspecified-4</i> mem_fn(R
(T::*pmf) (A1) const)</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t, a1)</i></tt>
is equivalent to <tt>(t.*pmf)(a1)</tt> when <i>t</i> is of type <STRONG>T</STRONG>
<EM>[const]<STRONG> </STRONG></EM>or derived, <tt>(get_pointer(t)->*pmf)(a1)</tt>
otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="mem_fn_5">template<class R, class T, class A1, class A2> <i>unspecified-5</i>
mem_fn(R (T::*pmf) (A1, A2))</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t, a1, a2)</i></tt>
is equivalent to <tt>(t.*pmf)(a1, a2)</tt> when <i>t</i> is an l-value of type <STRONG>
T</STRONG> or derived, <tt>(get_pointer(t)->*pmf)(a1, a2)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="mem_fn_6">template<class R, class T, class A1, class A2> <i>unspecified-6</i>
mem_fn(R (T::*pmf) (A1, A2) const)</a></h4>
<blockquote>
<p>
<b>Returns:</b> a function object <i>f</i> such that the expression <tt><i>f(t, a1, a2)</i></tt>
is equivalent to <tt>(t.*pmf)(a1, a2)</tt> when <i>t</i> is of type <STRONG>T</STRONG>
<EM>[const]</EM> or derived, <tt>(get_pointer(t)->*pmf)(a1, a2)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h2><a name="Implementation">Implementation</a></h2>
<h3><a name="Files">Files</a></h3>
<ul>
<li>
<a href="../../boost/mem_fn.hpp">boost/mem_fn.hpp</a>
(main header)
<li>
<a href="../../boost/bind/mem_fn_cc.hpp">boost/bind/mem_fn_cc.hpp</a>
(used by mem_fn.hpp, do not include directly)
<li>
<a href="../../boost/bind/mem_fn_vw.hpp">boost/bind/mem_fn_vw.hpp</a>
(used by mem_fn.hpp, do not include directly)
<li>
<a href="../../boost/bind/mem_fn_template.hpp">boost/bind/mem_fn_template.hpp</a>
(used by mem_fn.hpp, do not include directly)
<li>
<a href="test/mem_fn_test.cpp">libs/bind/test/mem_fn_test.cpp</a>
(test)
<li>
<a href="test/mem_fn_derived_test.cpp">libs/bind/test/mem_fn_derived_test.cpp</a>
(test with derived objects)
<li>
<a href="test/mem_fn_fastcall_test.cpp">libs/bind/test/mem_fn_fastcall_test.cpp</a>
(test for __fastcall)
<li>
<a href="test/mem_fn_stdcall_test.cpp">libs/bind/test/mem_fn_stdcall_test.cpp</a>
(test for __stdcall)
<li>
<a href="test/mem_fn_void_test.cpp">libs/bind/test/mem_fn_void_test.cpp</a> (test
for void returns)</li>
</ul>
<h3><a name="Dependencies">Dependencies</a></h3>
<ul>
<li>
<a href="../config/config.htm">Boost.Config</a></li>
</ul>
<h3><a name="NumberOfArguments">Number of Arguments</a></h3>
<p>
This implementation supports member functions with up to eight arguments. This
is not an inherent limitation of the design, but an implementation detail.
</p>
<h3><a name="stdcall">"__stdcall", "__cdecl" and "__fastcall" Support</a></h3>
<p>
Some platforms allow several types of member functions that differ by their <b>calling
convention</b> (the rules by which the function is invoked: how are
arguments passed, how is the return value handled, and who cleans up the stack
- if any.)
</p>
<p>
For example, Windows API functions and COM interface member functions use a
calling convention known as <b>__stdcall</b>. Borland VCL components use <STRONG>__fastcall</STRONG>.
UDK, the component model of OpenOffice.org, uses <STRONG>__cdecl</STRONG>.
</p>
<p>
To use <b>mem_fn</b> with <b>__stdcall</b> member functions, <b>#define</b> the
macro <b>BOOST_MEM_FN_ENABLE_STDCALL</b> before including, directly or
indirectly, <b><boost/mem_fn.hpp></b>.
</p>
<P>To use <B>mem_fn</B> with <B>__fastcall</B> member functions, <B>#define</B> the
macro <B>BOOST_MEM_FN_ENABLE_FASTCALL</B> before including <B><boost/mem_fn.hpp></B>.
</P>
<P>To use <B>mem_fn</B> with <B>__cdecl</B> member functions, <B>#define</B> the
macro <B>BOOST_MEM_FN_ENABLE_CDECL</B> before including <B><boost/mem_fn.hpp></B>.
</P>
<P><STRONG>It is best to define these macros in the project options, via -D on the
command line, or as the first line in the translation unit (.cpp file) where
mem_fn is used.</STRONG> Not following this rule can lead to obscure errors
when a header includes mem_fn.hpp before the macro has been defined.</P>
<P>[Note: this is a non-portable extension. It is not part of the interface.]
</P>
<p>
[Note: Some compilers provide only minimal support for the <b>__stdcall</b> keyword.]
</p>
<h2><a name="Acknowledgements">Acknowledgements</a></h2>
<p>
Rene Jager's initial suggestion of using traits classes to make <b>mem_fn</b> adapt
to user-defined smart pointers inspired the <b>get_pointer</b>-based design.
</p>
<p>
Numerous improvements were suggested during the formal review period by Richard
Crossley, Jens Maurer, Ed Brey, and others. Review manager was Darin Adler.
</p>
<p>
Steve Anichini pointed out that COM interfaces use <b>__stdcall</b>.
</p>
<p>
Dave Abrahams modified <b>bind</b> and <b>mem_fn</b> to support void returns on
deficient compilers.
</p>
<p>Daniel Boelzle pointed out that UDK uses <STRONG>__cdecl</STRONG>.<br>
<br>
<br>
<small>Copyright 2001, 2002 by Peter Dimov and Multi Media Ltd. Copyright
2003-2005 Peter Dimov. 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>.</small></p>
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