doc/mem_fn/purpose.qbk

[/
 /  Copyright (c) 2001, 2002 Peter Dimov and Multi Media Ltd.
 /  Copyright (c) 2003-2005 Peter Dimov
 /
 / Distributed under the Boost Software License, Version 1.0. (See
 / accompanying file LICENSE_1_0.txt or copy at
 / http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:purpose Purpose]

`boost::mem_fn` is a generalization of the standard functions `std::mem_fun`
and `std::mem_fun_ref`. 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.
`mem_fn` also supports pointers to data members by treating them as functions
taking no arguments and returning a (const) reference to the member.

The purpose of `mem_fn` is twofold. First, it allows users to invoke a member
function on a container with the familiar

    std::for_each(v.begin(), v.end(), boost::mem_fn(&Shape::draw));

syntax, even when the container stores smart pointers.

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 `for_each` algorithm with an overload of the form:

    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));
    }

that will allow the convenient syntax:

    for_each(v.begin(), v.end(), &Shape::draw);

When documenting the feature, the library author will simply state:

    template<class It, class R, class T> void for_each(It first, It last, R (T::*pmf) ());

* /Effects:/ Equivalent to `std::for_each(first, last, boost::mem_fn(pmf))`.

where `boost::mem_fn` can be a link to this page. See the
[@boost:/libs/bind/bind.html documentation of `bind`] for an example.

`mem_fn` takes one argument, a pointer to a member, and returns a function
object suitable for use with standard or user-defined algorithms:

    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));
    };

The returned function object takes the same arguments as the input member
function plus a "flexible" first argument that represents the object instance.

When the function object is invoked with a first argument `x` that is neither
a pointer nor a reference to the appropriate class (`X` in the example above),
it uses `get_pointer(x)` to obtain a pointer from `x`. Library authors can
"register" their smart pointer classes by supplying an appropriate
`get_pointer` overload, allowing `mem_fn` to recognize and support them.


/[Note:/ `get_pointer` is not restricted to return a pointer. Any object that
can be used in a member function call expression `(x->*pmf)(...)` will work./]/

/[Note:/ the library uses an unqualified call to `get_pointer`. Therefore, it
will find, through argument-dependent lookup, `get_pointer` overloads that are
defined in the same namespace as the corresponding smart pointer class, in
addition to any `boost::get_pointer` overloads./]/

All function objects returned by `mem_fn` expose a `result_type` typedef that
represents the return type of the member function. For data members,
`result_type` is defined as the type of the member.

[endsect]