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# pair

### Synopsis

    template<
          typename T1
        , typename T2
        >
    struct pair
    {
        typedef pair type;
        typedef T1 first;
        typedef T2 second;
    };

### Description

A transparent holder for two arbitrary types.

### Header

    #include <boost/mpl/pair.hpp>

### Example

Count a number of elements in the sequence together with a number of negative elements among these.

    typedef fold<
          vector_c<int,-1,0,5,-7,-2,4,5,7>
        , pair< int_<0>, int_<0> >
        , pair<
              next< first<_1> >
            , if_<
                  less< _2, int_<0> >
                , next< second<_1> >
                , second<_1>
                >
            >
        >::type p;

    BOOST_MPL_ASSERT_RELATION( p::first::value, ==, 8 );
    BOOST_MPL_ASSERT_RELATION( p::second::value, ==, 3 );

### See also

Data Types, Sequences, first, second