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## Chapter 1. Boost.Member Function

Copyright © 2001, 2002 Peter Dimov and Multi Media Ltd.

Copyright © 2003-2005 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

## 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 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.

## Frequently Asked Questions

### Can `mem_fn` be used instead of the standard `std::mem_fun[_ref]` adaptors?

Yes. For simple uses, `mem_fn` provides additional functionality that the standard adaptors do not. Complicated expressions that use `std::bind1st`, `std::bind2nd` or Boost.Compose along with the standard adaptors can be rewritten using `boost::bind` that automatically takes advantage of `mem_fn`.

### Should I replace every occurence of `std::mem_fun[_ref]` with `mem_fn` in my existing code?

No, unless you have good reasons to do so. `mem_fn` is not 100% compatible with the standard adaptors, although it comes pretty close. In particular, `mem_fn` does not return objects of type `std::[const_]mem_fun[1][_ref]_t`, as the standard adaptors do, and it is not possible to fully describe the type of the first argument using the standard `argument_type` and `first_argument_type` nested typedefs. Libraries that need adaptable function objects in order to function might not like `mem_fn`.

### Does `mem_fn` work with COM methods?

Yes, if you `#define BOOST_MEM_FN_ENABLE_STDCALL.

### Why isn't `BOOST_MEM_FN_ENABLE_STDCALL` defined automatically?

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.

## Interface

### Synopsis

    namespace boost
    {
        template<class T> T * get_pointer(T * p);

        template<class R, class T> unspecified-1 mem_fn(R (T::*pmf) ());

        template<class R, class T> unspecified-2 mem_fn(R (T::*pmf) () const);

        template<class R, class T> unspecified-2-1 mem_fn(R T::*pm);

        template<class R, class T, class A1> unspecified-3 mem_fn(R (T::*pmf) (A1));

        template<class R, class T, class A1> unspecified-4 mem_fn(R (T::*pmf) (A1) const);

        template<class R, class T, class A1, class A2> unspecified-5 mem_fn(R (T::*pmf) (A1, A2));

        template<class R, class T, class A1, class A2> unspecified-6 mem_fn(R (T::*pmf) (A1, A2) const);

        // implementation defined number of additional overloads for more arguments
    }

### Common requirements

All unspecified-N types mentioned in the Synopsis are CopyConstructible and Assignable. Their copy constructors and assignment operators do not throw exceptions. unspecified-N`::result_type` is defined as the return type of the member function pointer passed as an argument to `mem_fn` (`R` in the Synopsis.) unspecified-2-1`::result_type` is defined as `R`.

### `get_pointer`

    template<class T> T * get_pointer(T * p)

- Returns: `p`.
- Throws: Nothing.

### `mem_fn`

    template<class R, class T> unspecified-1 mem_fn(R (T::*pmf) ())

- Returns: a function object ϝ such that the expression ϝ`(t)` is equivalent to `(t.*pmf)()` when `t` is an l-value of type `T` or derived, `(get_pointer(t)->*pmf)()` otherwise.
- Throws: Nothing.

    template<class R, class T> unspecified-2 mem_fn(R (T::*pmf) () const)

- Returns: a function object ϝ such that the expression ϝ`(t)` is equivalent to `(t.*pmf)()` when `t` is of type `T` [`const`] or derived, `(get_pointer(t)->*pmf)()` otherwise.
- Throws: Nothing.

    template<class R, class T> unspecified-2-1 mem_fn(R T::*pm)

- Returns: a function object ϝ such that the expression ϝ`(t)` is equivalent to `t.*pm` when `t` is of type `T` [`const`] or derived, `get_pointer(t)->*pm` otherwise.
- Throws: Nothing.

    template<class R, class T, class A1> unspecified-3 mem_fn(R (T::*pmf) (A1))

- Returns: a function object ϝ such that the expression ϝ`(t, a1)` is equivalent to `(t.*pmf)(a1)` when `t` is an l-value of type `T` or derived, `(get_pointer(t)->*pmf)(a1)` otherwise.
- Throws: Nothing.

    template<class R, class T, class A1> unspecified-4 mem_fn(R (T::*pmf) (A1) const)

- Returns: a function object ϝ such that the expression ϝ`(t, a1)` is equivalent to `(t.*pmf)(a1)` when `t` is of type `T` [`const`] or derived, `(get_pointer(t)->*pmf)(a1)` otherwise.
- Throws: Nothing.

    template<class R, class T, class A1, class A2> unspecified-5 mem_fn(R (T::*pmf) (A1, A2))

- Returns: a function object ϝ such that the expression ϝ`(t, a1, a2)` is equivalent to `(t.*pmf)(a1, a2)` when `t` is an l-value of type `T` or derived, `(get_pointer(t)->*pmf)(a1, a2)` otherwise.
- Throws: Nothing.

    template<class R, class T, class A1, class A2> unspecified-6 mem_fn(R (T::*pmf) (A1, A2) const)

- Returns: a function object ϝ such that the expression ϝ`(t, a1, a2)` is equivalent to `(t.*pmf)(a1, a2)` when `t` is of type `T` [`const`] or derived, `(get_pointer(t)->*pmf)(a1, a2)` otherwise.
- Throws: Nothing.

## Implementation

### Files

- boost/mem_fn.hpp (main header)
- boost/bind/mem_fn_cc.hpp (used by `mem_fn.hpp`, do not include directly)
- boost/bind/mem_fn_vw.hpp (used by `mem_fn.hpp`, do not include directly)
- boost/bind/mem_fn_template.hpp (used by `mem_fn.hpp`, do not include directly)
- libs/bind/test/mem_fn_test.cpp (test)
- libs/bind/test/mem_fn_derived_test.cpp (test with derived objects)
- libs/bind/test/mem_fn_fastcall_test.cpp (test for `__fastcall`)
- libs/bind/test/mem_fn_stdcall_test.cpp (test for `__stdcall`)
- libs/bind/test/mem_fn_void_test.cpp (test for `void` returns)

### Dependencies

- Boost.Config

### Number of Arguments

This implementation supports member functions with up to eight arguments. This is not an inherent limitation of the design, but an implementation detail.

### `__stdcall`, `__cdecl`, and `__fastcall` Support

Some platforms allow several types of member functions that differ by their calling convention (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.)

For example, Windows API functions and COM interface member functions use a calling convention known as `__stdcall`. Borland VCL components use `__fastcall`. UDK, the component model of OpenOffice.org, uses `__cdecl`.

To use `mem_fn` with `__stdcall` member functions, `#define` the macro `BOOST_MEM_FN_ENABLE_STDCALL` before including `<boost/mem_fn.hpp>`.

To use `mem_fn` with `__fastcall` member functions, `#define` the macro `BOOST_MEM_FN_ENABLE_FASTCALL` before including `<boost/mem_fn.hpp>`.

To use `mem_fn` with `__cdecl` member functions, `#define` the macro `BOOST_MEM_FN_ENABLE_CDECL` before including `<boost/mem_fn.hpp>`.

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. Not following this rule can lead to obscure errors when a header includes `mem_fn.hpp` before the macro has been defined.

[Note: this is a non-portable extension. It is not part of the interface.]

[Note: Some compilers provide only minimal support for the `__stdcall` keyword.]

## Acknowledgements

- Rene Jager's initial suggestion of using traits classes to make `mem_fn` adapt to user-defined smart pointers inspired the `get_pointer`-based design.
- Numerous improvements were suggested during the formal review period by Richard Crossley, Jens Maurer, Ed Brey, and others. Review manager was Darin Adler.
- Steve Anichini pointed out that COM interfaces use `__stdcall`.
- Dave Abrahams modified `bind` and `mem_fn` to support `void` returns on deficient compilers.
- Daniel Boelzle pointed out that UDK uses `__cdecl`.

This documentation was ported to Quickbook by Agustín Bergé.

Last revised: December 13, 2019 at 18:24:22 GMT |