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- [/
- / 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]
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