struct functor_1 {
    void operator() (std::string arg1, std::string arg2)
    {
        std::cout << arg1 << arg2 " from first functor" << std::endl
    }
};
struct functor_2 {
    void operator() (std::string arg1, std::string arg2)
    {
        std::cout << arg1 << arg2 " from second functor" << std::endl
    }
};
join(functor_1(), functor_2())("Hello, ", "World!"); // will print two lines:
// "Hello, World! from first functor"
// .. and
// "Hello, World! from second functor"