empty

Synopsis

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

Description

Returns an Integral Constant c such that c::value == true if and only if the sequence is empty.

Header

#include <boost/mpl/empty.hpp>

Model of

Tag Dispatched Metafunction

Parameters

Parameter	Requirement	Description
`Sequence`	Forward Sequence	A sequence to test.

Expression semantics

For any Forward Sequence s:

typedef empty<s>::type c;

Return type:	Boolean Integral Constant.
Semantics:	Equivalent to `typedef is_same< begin<s>::type,end<s>::type >::type c;`.
Postcondition:	`empty<s>::value == ( size<s>::value == 0 )`.

Complexity

Amortized constant time.

Example

typedef range_c<int,0,0> empty_range;
typedef vector<long,float,double> types;

BOOST_MPL_ASSERT( empty<empty_range> );
BOOST_MPL_ASSERT_NOT( empty<types> );