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Front Page / Sequences / Intrinsic Metafunctions / begin

begin

Synopsis

template<
      typename X
    >
struct begin
{
    typedef unspecified type;
};

Description

Returns an iterator that points to the first element of the sequence. If the argument is not a Forward Sequence, returns void_.

Header

#include <boost/mpl/begin_end.hpp>

Model of

Tag Dispatched Metafunction

Parameters

Parameter Requirement Description
X Any type A type whose begin iterator, if any, will be returned.

Expression semantics

For any arbitrary type x:

typedef begin<x>::type first;
Return type:Forward Iterator or void_.
Semantics:If x is a Forward Sequence, first is an iterator pointing to the first element of s; otherwise first is void_.
Postcondition:If first is an iterator, it is either dereferenceable or past-the-end; it is past-the-end if and only if size<x>::value == 0.

Complexity

Amortized constant time.

Example

typedef vector< unsigned char,unsigned short,
    unsigned int,unsigned long > unsigned_types;

typedef begin<unsigned_types>::type iter;
BOOST_MPL_ASSERT(( is_same< deref<iter>::type, unsigned char > ));

BOOST_MPL_ASSERT(( is_same< begin<int>::type, void_ > ));

See also

Iterators, Forward Sequence, end, size, empty