.. Metafunctions/Composition and Argument Binding//lambda |20

lambda

Synopsis

.. parsed-literal::

template< 
      typename X
    , typename Tag = |unspecified|
    >
struct lambda
{
    typedef |unspecified| type;
};

Description

If X is a |placeholder expression|, transforms X into a corresponding |Metafunction Class|, otherwise X is returned unchanged.

Header

.. parsed-literal::

#include <boost/mpl/lambda.hpp>

Parameters

+---------------+-----------------------+-----------------------------------------------+ | Parameter | Requirement | Description | +===============+=======================+===============================================+ | X | Any type | An expression to transform. | +---------------+-----------------------+-----------------------------------------------+ | Tag | Any type | A tag determining transform semantics. | +---------------+-----------------------+-----------------------------------------------+

Expression semantics

For arbitrary types x and tag:

.. parsed-literal::

typedef lambda<x>::type f;

:Return type: |Metafunction Class|.

:Semantics: If x is a |placeholder expression| in a general form X<a1,...an>, where X is a class template and a1,... an are arbitrary types, equivalent to

.. parsed-literal::

    typedef protect< bind<
          quote\ *n*\ <X>
        , lambda<a1>::type,\ |...| lambda<a\ *n*\ >::type
        > > f;

otherwise, ``f`` is identical to ``x``.

.. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. parsed-literal::

typedef lambda<x,tag>::type f;

:Return type: |Metafunction Class|.

:Semantics: If x is a |placeholder expression| in a general form X<a1,...an>, where X is a class template and a1,... an are arbitrary types, equivalent to

.. parsed-literal::

    typedef protect< bind<
          quote\ *n*\ <X,tag>
        , lambda<a1,tag>::type,\ |...| lambda<a\ *n*\ ,tag>::type
        > > f;

otherwise, ``f`` is identical to ``x``.

Example

.. parsed-literal::

template< typename N1, typename N2 > struct int_plus
    : int_<( N1::value + N2::value )>
{
};

typedef lambda< int_plus<_1, int_<42> > >::type f1;
typedef bind< quote\ ``2``\ <int_plus>, _1, int_<42> > f2;

typedef f1::apply<42>::type r1;
typedef f2::apply<42>::type r2;

BOOST_MPL_ASSERT_RELATION( r1::value, ==, 84 );
BOOST_MPL_ASSERT_RELATION( r2::value, ==, 84 );

See also

|Composition and Argument Binding|, |Invocation|, |Placeholders|, |bind|, |quote|, |protect|, |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)