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The MPL Reference Manual: size
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size

Synopsis

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

Description

size returns the number of elements in the sequence, that is, the number of elements in the range [begin::type, end::type).

Parameters

Parameter Requirement Description
Sequence Forward Sequence A sequence to query.

Expression semantics

For any Forward Sequence s:

typedef size::type n;
Return type:

Integral Constant.
Semantics:

Equivalent to

typedef distance< begin::type,end::type >::type n;
Postcondition:

n::value >= 0.

Complexity

The complexity of the size metafunction directly depends on the implementation of the particular sequence it is applied to. In the worst case, size guarantees a linear complexity.

If the s is a Random Access Sequence, size::type is an O(1) operation. The opposite is not necessarily true — for example, a sequence class that models Forward Sequence might still give us an O(1) size implementation.

Example

typedef list0<> empty_list;
typedef vector_c numbers;
typedef range_c more_numbers;

BOOST_MPL_ASSERT_RELATION( size<list>::value, ==, 0 );
BOOST_MPL_ASSERT_RELATION( size::value, ==, 5 );
BOOST_MPL_ASSERT_RELATION( size::value, ==, 100 );
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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)