The MPL Reference Manual: numeric_cast

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# numeric_cast

### Synopsis

    template<
          typename SourceTag
        , typename TargetTag
        >
    struct numeric_cast;

### Description

Each `numeric_cast` specialization is a user-specialized unary Metafunction Class providing a conversion between two numeric types.

### Header

    #include <boost/mpl/numeric_cast.hpp>

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`SourceTag` | Integral Constant | A tag for the conversion's source type.
`TargetTag` | Integral Constant | A tag for the conversion's destination type.

### Expression semantics

If `x` and `y` are two numeric types, `x` is convertible to `y`, and `x_tag` and `y_tag` are the types' corresponding Integral Constant tags:

    typedef apply_wrap2< numeric_cast<x_tag,y_tag>,x >::type  r;

Return type: | A type.
Semantics: | `r` is a value of `x` converted to the type of `y`.

### Complexity

Unspecified.

### Example

    struct complex_tag : int_<10> {};

    template< typename Re, typename Im > struct complex
    {
        typedef complex_tag tag;
        typedef complex type;
        typedef Re real;
        typedef Im imag;
    };

    template< typename C > struct real : C::real {};
    template< typename C > struct imag : C::imag {};

    namespace boost { namespace mpl {

    template<> struct numeric_cast< integral_c_tag,complex_tag >
    {
        template< typename N > struct apply
            : complex< N, integral_c< typename N::value_type, 0 > >
        {
        };
    };

    template<>
    struct plus_impl< complex_tag,complex_tag >
    {
        template< typename N1, typename N2 > struct apply
            : complex<
                  plus< typename N1::real, typename N2::real >
                , plus< typename N1::imag, typename N2::imag >
                >
        {
        };
    };

    }}

    typedef int_<2> i;
    typedef complex< int_<5>, int_<-1> > c1;
    typedef complex< int_<-5>, int_<1> > c2;

    typedef plus<c1,i> r4;
    BOOST_MPL_ASSERT_RELATION( real<r4>::value, ==, 7 );
    BOOST_MPL_ASSERT_RELATION( imag<r4>::value, ==, -1 );

    typedef plus<i,c2> r5;
    BOOST_MPL_ASSERT_RELATION( real<r5>::value, ==, -3 );
    BOOST_MPL_ASSERT_RELATION( imag<r5>::value, ==, 1 );

### See also

Metafunctions, Numeric Metafunction, `plus`, `minus`, `times`