.. Metafunctions/Miscellaneous//always |20

always

Synopsis

.. parsed-literal::

template< 
      typename X
    >
struct always
{
    // |unspecified|
    // |...|
};

Description

always<X> specialization is a variadic |Metafunction Class| always returning the same type, X, regardless of the number and types of passed arguments.

Header

.. parsed-literal::

#include <boost/mpl/always.hpp>

Model of

|Metafunction Class|

Parameters

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

Expression semantics

For an arbitrary type x:

.. parsed-literal::

typedef always<x> f;

:Return type: |Metafunction Class|.

:Semantics: Equivalent to

.. parsed-literal::

    struct f : bind< identity<_1>, x > {};

Example

.. parsed-literal::

typedef always<true\_> always_true;

BOOST_MPL_ASSERT(( apply< always_true,false\_> ));
BOOST_MPL_ASSERT(( apply< always_true,false\_,false\_ > ));
BOOST_MPL_ASSERT(( apply< always_true,false\_,false\_,false\_ > ));

See also

|Metafunctions|, |Metafunction Class|, |identity|, |bind|, |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)