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The MPL Reference Manual: count_if
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count_if

Synopsis

template<
      typename Sequence
    , typename Pred
    >
struct count_if
{
    typedef unspecified type;
};

Description

Returns the number of elements in Sequence that satisfy the predicate Pred.

Parameters

Parameter Requirement Description
Sequence Forward Sequence A sequence to be examined.
Pred Unary Lambda Expression A count condition.

Expression semantics

For any Forward Sequence s and unary Lambda Expression pred:

typedef count_if::type n;
Return type:

Integral Constant.
Semantics:

Equivalent to

typedef lambda::type p;
typedef fold<
      s
    , long_<0>
    , if_< apply_wrap1_2>, next<_1>, _1 >
    >::type n;

Complexity

Linear. Exactly size::value applications of pred.

Example

typedef vector types;

BOOST_MPL_ASSERT_RELATION( (count_if< types, is_float<_> >::value), ==, 1 );
BOOST_MPL_ASSERT_RELATION( (count_if< types, is_same<_,char> >::value), ==, 2 );
BOOST_MPL_ASSERT_RELATION( (count_if< types, is_same<_,void> >::value), ==, 0 );
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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)