| Front Page / Metafunctions / Type Selection / eval_if | 
template<
      typename C
    , typename F1
    , typename F2
    >
struct eval_if
{
    typedef unspecified type;
};
Evaluates one of its two nullary-metafunction arguments, F1 or F2, depending on the value C.
#include <boost/mpl/eval_if.hpp>
| Parameter | Requirement | Description | 
|---|---|---|
| C | Integral Constant | An evaluation condition. | 
| F1, F2 | Nullary Metafunction | Metafunctions to select for evaluation from. | 
For any Integral Constant c and nullary Metafunctions f1, f2:
typedef eval_if<c,f1,f2>::type t;
| Return type: | Any type. | 
|---|---|
| Semantics: | If c::value == true, t is identical to f1::type; otherwise t is identical to f2::type. |