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:

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

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> ));