Is there a better way to create custom error messages for perfect forwarding onto constructors?

Question

I have the following C++ code to test our perfect forwarding used in delegation of arguments to constructors for objects of different types:

#include <iostream>

// THIS CLASS TAKES 2 ARGUMENTS.
struct A {
    A(int i, int j) {
        i = i;
        j = j;
    }

    int i;
    int j;
};

// THIS CLASS TAKES 3 ARGUMENTS.
struct B {
    B(int i, int j, int k) {
        i = i;
        j = j;
        k = k;
    }

    int i;
    int j;
    int k;
};

// THIS CLASS IS A VARIANT-LIKE DELEGATOR
// WHOSE FIRST ARGUMENT IS THE TYPE TAG,
// AND THE REST ARE FORWARDED TO THE CORRESPONDING CLASS.
struct Delegator {
    enum class Type : int {
        CLASS_A,
        CLASS_B
    };

    union {
        A a;
        B b;
    };

    template <typename... Args>
    Delegator(Delegator::Type type, Args... args) {
        type_ = type;

        if (type_ == Delegator::Type::CLASS_A) {
            new (&a) A(std::forward(args...));
        } else if (type_ == Delegator::Type::CLASS_B) {
            new (&b) B(std::forward(args...));
        } else {
            throw std::invalid_argument("Delegator constructor: "
            "forwarded arguments does not match any of its member type constructors!");
        }
    }


private:
    Delegator::Type     type_;
};

int main() {
    // THIS IS GOOD.
    Delegator delegator(Delegator::Type::CLASS_A, 1, 2);

    // THIS IS BAD.
    Delegator delegator(Delegator::Type::CLASS_A, 1, 2, 3, 4);
}

I was wondering -- what is the current state of error messaging with C++ when dealing with these types of cases? Is there anything comparable to the (syntax-error) facility provided with Scheme so that users of an object can get a better error message upon improper usage? I don't care what standard of C++ is used -- I just want to know if it's currently possible to tweak the error diagnostic messages of C++.

Can the programmer do anything about these long templated-code error messages for future users? Or is it all in the hands of the compiler?

You can use other tools, see http://stackoverflow.com/q/47980/72178. — ks1322, Jan 14 '17 at 13:49
@ks1322 Thank you for the link. However, I am asking specifically for an in-language solution (which I don't think exists but I'm asking because I still want it). — CinchBlue, Jan 14 '17 at 13:57
Please post actual code. `Delegator delegator(Delegator::Type::CLASS_A, 1, 2);` this does not compile. Your problems are different than your question, basically, so answering your question is like answering a question about aging cheese brought back from the moon. The moon isn't made of cheese, and your problem isn't an error message when the arguments are wrong, but rather that your technique does not work. You appear to be wanting to write a homebrew `std::variant`? Your runtime arg dispatching to different compile time code (only some of which is valid) is your first problem. — Yakk - Adam Nevraumont, Jan 14 '17 at 14:02

Is there a better way to create custom error messages for perfect forwarding onto constructors?

0 Answers0