The MPL Reference Manual: if_c

if_c

Synopsis

template<
      bool c
    , typename T1
    , typename T2
    >
struct if_c
{
    typedef unspecified type;
};

Description

Returns one of its two arguments, T1 or T2, depending on the value of integral constant c. if_c::type is a shorcut notation for if_< bool_,t1,t2 >::type.

Header

#include <boost/mpl/if.hpp>

Parameters

Parameter	Requirement	Description
`c`	An 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:

typedef if_c::type t;

Return type:	Any type.
Semantics:	Equivalent to `typedef if_< bool_,t1,t2 >::type t;`

Example

typedef if_c::type t1;
typedef if_c::type t2;

BOOST_MPL_ASSERT(( is_same ));
BOOST_MPL_ASSERT(( is_same ));

// allocates space for an object of class T on heap or "inplace"
// depending on its size
template< typename T > struct lightweight
{
    // ...
    typedef typename if_c<
          sizeof(T) <= sizeof(T*)
        , inplace_storage
        , heap_storage
        >::type impl_t;

    impl_t impl;
};

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