Category: algorithms | Component type: function |
template <class InputIterator, class Predicate> iterator_traits<InputIterator>::difference_type count_if(InputIterator first, InputIterator last, Predicate pred); template <class InputIterator, class Predicate, class Size> void count_if(InputIterator first, InputIterator last, Predicate pred, Size& n);
The second version of count_if was the one defined in the original STL, and the first version is the one defined in the draft C++ standard; the definition was changed because the older interface was clumsy and error-prone. The older interface required the use of a temporary variable, which had to be initialized to 0 before the call to count_if.
Both interfaces are currently supported [1], for reasons of backward compatibility, but eventually the older version will be removed.
int main() { int A[] = { 2, 0, 4, 6, 0, 3, 1, -7 }; const int N = sizeof(A) / sizeof(int); cout << "Number of even elements: " << count_if(A, A + N, compose1(bind2nd(equal_to<int>(), 0), bind2nd(modulus<int>(), 2))) << endl; }
[1] The new count interface uses the iterator_traits class, which relies on a C++ feature known as partial specialization. Many of today's compilers don't implement the complete standard; in particular, many compilers do not support partial specialization. If your compiler does not support partial specialization, then you will not be able to use the newer version of count, or any other STL components that involve iterator_traits.