| .. Metafunctions/Type Selection//if_ |10 |
| |
| if\_ |
| ==== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| template< |
| typename C |
| , typename T1 |
| , typename T2 |
| > |
| struct if\_ |
| { |
| typedef |unspecified| type; |
| }; |
| |
| |
| |
| Description |
| ----------- |
| |
| Returns one of its two arguments, ``T1`` or ``T2``, depending on the value ``C``. |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/if.hpp> |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===================================+===============================================+ |
| | ``C`` | |Integral Constant| | A selection condition. | |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| | ``T1``, ``T2``| Any type | Types to select from. | |
| +---------------+-----------------------------------+-----------------------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For any |Integral Constant| ``c`` and arbitrary types ``t1``, ``t2``: |
| |
| |
| .. parsed-literal:: |
| |
| typedef if_<c,t1,t2>::type t; |
| |
| :Return type: |
| Any type. |
| |
| :Semantics: |
| If ``c::value == true``, ``t`` is identical to ``t1``; otherwise ``t`` is |
| identical to ``t2``. |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| typedef if\_<true\_,char,long>::type t1; |
| typedef if\_<false\_,char,long>::type t2; |
| |
| BOOST_MPL_ASSERT(( is_same<t1, char> )); |
| BOOST_MPL_ASSERT(( is_same<t2, long> )); |
| |
| |
| .. parsed-literal:: |
| |
| // allocates space for an object of class T on heap or "inplace" |
| // depending on its size |
| template< typename T > struct lightweight |
| { |
| // ... |
| typedef typename if\_< |
| less_equal< sizeof\_<T>, sizeof\_<T*> > |
| , inplace_storage<T> |
| , heap_storage<T> |
| >::type impl_t; |
| |
| impl_t impl; |
| }; |
| |
| |
| See also |
| -------- |
| |
| |Metafunctions|, |Integral Constant|, |if_c|, |eval_if| |
| |
| |
| .. 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) |
