count_if

Returns the number of elements within a sequence with a type for which a given unary function object evaluates to true.

template<
    typename Sequence,
    typename F
    >
typename result_of::count_if<Sequence, F>::type count_if(
    Sequence const& seq, F f);

Parameter	Requirement	Description
`seq`	A model of Forward Sequence, `f(e)` is a valid expression, convertible to `bool`, for each element `e` in `seq`	The sequence to search
`f`	A unary function object	The search predicate

count_if(seq, f)

Return type: int

Semantics: Returns the number of elements in seq where f evaluates to true.

Linear. At most result_of::size<Sequence>::value comparisons.

#include <boost/fusion/algorithm/query/count_if.hpp>

const vector<int,int,int> vec(1,2,3);
assert(count_if(vec,odd()) == 2);