.. Sequences/Intrinsic Metafunctions//empty

empty

Synopsis

.. parsed-literal::

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

.. parsed-literal::

#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:

.. parsed-literal::

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

.. parsed-literal::

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> );

See also

|Forward Sequence|, |Integral Constant|, |size|, |begin| / |end|

.. copyright:: 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)