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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<title>Boost: mem_fn.hpp documentation</title>
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<h1>mem_fn.hpp</h1>
</td>
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<tr>
<td colspan="2" height="64"> </td>
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</table>
<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" 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.
</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 function, 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>
A <b>get_pointer</b> overload for <b>boost::shared_ptr</b> is supplied.
</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.
</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.
</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 T> T * <a href="#get_pointer_2">get_pointer</a>(shared_ptr<T> const & p);
template<class R, class T> <i>implementation-defined-1</i> <a href="#mem_fn_1">mem_fn</a>(R (T::*pmf) ());
template<class R, class T> <i>implementation-defined-2</i> <a href="#mem_fn_2">mem_fn</a>(R (T::*pmf) () const);
template<class R, class T, class A1> <i>implementation-defined-3</i> <a href="#mem_fn_3">mem_fn</a>(R (T::*pmf) (A1));
template<class R, class T, class A1> <i>implementation-defined-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>implementation-defined-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>implementation-defined-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>implementation-defined-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>implementation-defined-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.)
</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>
<p>
<b>Returns:</b> <tt>p</tt>.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h4><a name="get_pointer_2">template<class T> T * get_pointer(shared_ptr<T> const & p)</a></h4>
<p>
<b>Returns:</b> <tt>p.get()</tt>.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h3><a name="mem_fn">mem_fn</a></h3>
<h4><a name="mem_fn_1">template<class R, class T> <i>implementation-defined-1</i> mem_fn(R (T::*pmf) ())</a></h4>
<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 <b>T</b>, <tt>(get_pointer(t)->*pmf)()</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h4><a name="mem_fn_2">template<class R, class T> <i>implementation-defined-2</i> mem_fn(R (T::*pmf) () const)</a></h4>
<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 <b>T <i>[</i>const<i>]</i></b>, <tt>(get_pointer(t)->*pmf)()</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h4><a name="mem_fn_3">template<class R, class T, class A1> <i>implementation-defined-3</i> mem_fn(R (T::*pmf) (A1))</a></h4>
<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 <b>T</b>, <tt>(get_pointer(t)->*pmf)(a1)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h4><a name="mem_fn_4">template<class R, class T, class A1> <i>implementation-defined-4</i> mem_fn(R (T::*pmf) (A1) const)</a></h4>
<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 <b>T <i>[</i>const<i>]</i></b>, <tt>(get_pointer(t)->*pmf)(a1)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h4><a name="mem_fn_5">template<class R, class T, class A1, class A2> <i>implementation-defined-5</i> mem_fn(R (T::*pmf) (A1, A2))</a></h4>
<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 <b>T</b>, <tt>(get_pointer(t)->*pmf)(a1, a2)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<h4><a name="mem_fn_6">template<class R, class T, class A1, class A2> <i>implementation-defined-6</i> mem_fn(R (T::*pmf) (A1, A2) const)</a></h4>
<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 <b>T <i>[</i>const<i>]</i></b>, <tt>(get_pointer(t)->*pmf)(a1, a2)</tt> otherwise.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
<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="mem_fn_test.cpp">libs/bind/mem_fn_test.cpp</a> (test)
<li><a href="mem_fn_stdcall_test.cpp">libs/bind/mem_fn_stdcall_test.cpp</a> (test for __stdcall)
<li><a href="mem_fn_void_test.cpp">libs/bind/mem_fn_void_test.cpp</a> (test for void returns)
</ul>
<h3><a name="Dependencies">Dependencies</a></h3>
<ul>
<li><a href="../config/config.htm">Boost.Config</a>
</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" 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>.
</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>
[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><br><br><br><small>Copyright © 2001 by Peter Dimov and Multi Media
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.</small></p>
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