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_ > ));