The MPL Reference Manual: front

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# front

### Synopsis

    template<
          typename Sequence
        >
    struct front
    {
        typedef unspecified type;
    };

### Description

Returns the first element in the sequence.

### Header

    #include <boost/mpl/front.hpp>

### Model of

Tag Dispatched Metafunction

### Parameters

Parameter | Requirement | Description
--- | --- | ---
`Sequence` | Forward Sequence | A sequence to be examined.

### Expression semantics

For any Forward Sequence `s`:

    typedef front<s>::type t;

Return type: | A type.
Precondition: | `empty<s>::value == false`.
Semantics: | Equivalent to
    typedef deref< begin<s>::type >::type t;

### Complexity

Amortized constant time.

### Example

    typedef list<long>::type types1;
    typedef list<int,long>::type types2;
    typedef list<char,int,long>::type types3;

    BOOST_MPL_ASSERT(( is_same< front<types1>::type, long > ));
    BOOST_MPL_ASSERT(( is_same< front<types2>::type, int> ));
    BOOST_MPL_ASSERT(( is_same< front<types3>::type, char> ));

### See also

Forward Sequence, back, push_front, begin, deref, at