| .. 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) |