template <typename A, typename B, typename U = std::less<int>> bool f(A a, B b, U u = U()) { return u(a, b); } int main() { std::cout << std::boolalpha; std::cout << f(5, 20) << '\n'; std::cout << f(100, 10) << '\n'; } 输出: true false 感谢各位...