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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.

Parameters

Parameter Requirement Description
Sequence Forward Sequence A sequence to be examined.

Expression semantics

For any Forward Sequence s:

typedef front::type t;
Return type:

A type.
Precondition:

empty::value == false.
Semantics:

Equivalent to

typedef deref< begin::type >::type t;

Complexity

Amortized constant time.

Example

typedef list::type types1;
typedef list::type types2;
typedef list::type types3;

BOOST_MPL_ASSERT(( is_same< front::type, long > ));
BOOST_MPL_ASSERT(( is_same< front::type, int> ));
BOOST_MPL_ASSERT(( is_same< front::type, char> ));
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Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams
Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)