Utility function that returns the negation of the result of the given function.

const std::ostream_iterator<int> ostream_it{ std::cout, " " };
const auto is_even = [](const auto n) { return n % 2 == 0; };
std::vector<int> v{ 0, 1, 2, 3, 4 };

// Print all even numbers.
std::copy_if(std::cbegin(v), std::cend(v), ostream_it, is_even); // 0 2 4
// Print all odd (not even) numbers.
std::copy_if(std::cbegin(v), std::cend(v), ostream_it, std::not_fn(is_even)); // 1 3