template<class InputIterator, class T>
constexpr inline
typename iterator_traits<InputIterator>::difference_type
count(InputIterator first, InputIterator last, const T& value);

template <class ExecutionPolicy, class ForwardIterator, class T>
typename iterator_traits<ForwardIterator>::difference_type>
count(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, const T& value);

#include <iostream>
#include <algorithm>

int main()
{
    auto list = {1, 2, 3, 1, 2, 3, 3};
    auto count3 = std::count(std::cbegin(list), std::cend(list), 3);
    auto count7 = std::count(std::cbegin(list), std::cend(list), 7);

    std::cout << "count3: " << count3 << std::endl;
    std::cout << "count7: " << count7 << std::endl;
    return 0;
}

count3: 3
count7: 0