.. Metafunctions/Miscellaneous//identity |10

identity

Synopsis

.. parsed-literal::

template< 
      typename X
    >
struct identity
{
    typedef X type;
};

Description

The identity__ metafunction. Returns X unchanged.

__ http://mathworld.wolfram.com/IdentityFunction.html

Header

.. parsed-literal::

#include <boost/mpl/identity.hpp>

Model of

|Metafunction|

Parameters

+---------------+-------------------+-----------------------------------+ | Parameter | Requirement | Description | +===============+===================+===================================+ | X | Any type | An argument to be returned. | +---------------+-------------------+-----------------------------------+

Expression semantics

For an arbitrary type x:

.. parsed-literal::

typedef identity<x>::type r;

:Return type: A type.

:Semantics: Equivalent to

.. parsed-literal::

    typedef x r;

:Postcondition: is_same<r,x>::value == true.

Example

.. parsed-literal::

typedef apply< identity<_1>, char >::type t1;
typedef apply< identity<_2>, char,int >::type t2;

BOOST_MPL_ASSERT(( is_same< t1, char > ));
BOOST_MPL_ASSERT(( is_same< t2, int > ));

See also

|Metafunctions|, |Placeholders|, |Trivial Metafunctions|, |always|, |apply|

.. copyright:: Copyright 2001-2009 Aleksey Gurtovoy and David Abrahams 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)