What is the reason for `std::result_of` deprecated in C++17?

Question

I saw std::result_of is being deprecated in C++17.

• What is the reason for std::result_of deprecated in C++17?
• Also I would

Answer 1

@haelix:

I'm totaly with you concerning the lack of example on the cppreference page. Here's my take:

auto add_auto_fn(int a, int b) {
    return a + b;
}

template<typename U, typename V>
auto add_auto_template_fn(U a, V b) {
    return a + b;
}

int fortytwo(int a, int b) { return a + 42; }

struct my_functor{
    auto operator() (int a) { return a + 42; }
};

void test_invoke_result()
{
    {
        // For functions and auto function: use < decltype(&f), Args... >
        using T = std::invoke_result< decltype(&fortytwo), int, int>::type;
        static_assert(std::is_same<T, int>::value, "");
    }
    {
        // For templated auto functions: use < decltype(&f)<Args...>, Args... >
        using T = std::invoke_result< decltype(&add_auto_template_fn<int, double>), int, double>::type;
        static_assert(std::is_same<T, double>::value, "");
    }
    {
        // For simple lambdas: use < decltype(lambda), Args... >
        auto simple_lambda = [](int a) {  return a + 42; };
        using T = std::invoke_result< decltype(simple_lambda), int>::type;
        static_assert(std::is_same<T, int>::value, "");
    }
    {
        // For generic lambdas: use < decltype(lambda), Args... >
        auto generic_lambda = [](auto a) {  return a + 42; };
        using T = std::invoke_result< decltype(generic_lambda), double>::type;
        static_assert(std::is_same<T, double>::value, "");
    }
    {
        // For functors: use < functor, Args... >
        using T = std::invoke_result< my_functor, int>::type;
        static_assert(std::is_same<T, int>::value, "");
    }

}

void test_result_of()
{
    {
        // For functions and auto function: use < decltype(&f)(Args...) >
        using T = std::result_of< decltype(&fortytwo)(int, int)>::type;
        static_assert(std::is_same<T, int>::value, "");
    }
    {
        // For templated auto functions: use < decltype(&f<Args...>)(Args...) >
        using T = std::result_of< decltype(&add_auto_template_fn<int, double>)(int, double)>::type;
        static_assert(std::is_same<T, double>::value, "");
    }
    {
        // For simple lambdas: use < decltype(lambda)(Args...) >
        auto simple_lambda = [](int a) {  return a + 42; };
        using T = std::result_of< decltype(simple_lambda)(int)>::type;
        static_assert(std::is_same<T, int>::value, "");
    }
    {
        // For generic lambdas: use < decltype(lambda)(Args...) >
        auto generic_lambda = [](auto a) {  return a + 42; };
        using T = std::result_of< decltype(generic_lambda)(double)>::type;
        static_assert(std::is_same<T, double>::value, "");
    }
    {
        // For functors: use < functor(Args...) >
        using T = std::result_of< my_functor(int)>::type;
        static_assert(std::is_same<T, int>::value, "");
    }

}

Answer 2

T.C. already provided the obvious link, but perhaps the most horrific reason bears repeating: result_of involved forming the type F(Arg1, Arg2, ...) not for a function of those types returning F but for a function of type F accepting those types. (After all, the return type is the result of, well, result_of, not the input!)

Aside from the adjustments associated with forming the function type, the only difference between the two is syntactic.