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Compile time type id with templates (C++)

Helio Nunes Santos on December 28, 2020

The issue: Another day while developing my game engine I had to handle events (which inherited from a common interface). The problem was...

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Pierre Gradot • Dec 29 '20

Hello!
I have almost skipped the "A solution" section. The code is impressive, I don't have time to understand it now, but it looks like a great template work :D

However, while reading the "Usage" section, I tend to believe you could have used std::variant from C++17. Here is an example:

#include <variant>
#include <iostream>

// Our events
class MouseEvent {
public:
    void do_mouse_event_stuff() const {
        std::cout << "Doing MouseEvent stuff\n";
    };
};

class KeyboardEvent {
public:
    void do_keyboard_event_stuff() const {
        std::cout << "Doing KeyboardEvent stuff\n";
    };
};

class JoystickEvent {
public:
    void do_joystick_event_stuff() const {
        std::cout << "Doing JoystickEvent stuff\n";
    };
};

// Another event, not recognized by the engine
class NotAnEngineEvent {
};

// Create a type to represent the events the engine can handle
using Event = std::variant<MouseEvent, KeyboardEvent, JoystickEvent>;

// To use with std::visit (see below)
struct EventHandler {
    void operator()(const MouseEvent& e) {
        e.do_mouse_event_stuff();
    }

    void operator()(const KeyboardEvent& e) {
        e.do_keyboard_event_stuff();
    }

    void operator()(const JoystickEvent& e) {
        e.do_joystick_event_stuff();
    }
};

int main() {
    // Create events
    Event m = MouseEvent{};
    Event k = KeyboardEvent{};
    Event j = JoystickEvent{};

    // Test the type of the event manually
    if (std::holds_alternative<MouseEvent>(m)) {
        auto e = std::get<MouseEvent>(m);
        e.do_mouse_event_stuff();
    }

    if (auto p = std::get_if<KeyboardEvent>(&k)) {
        p->do_keyboard_event_stuff();
    }

    // With std::visit
    EventHandler handler;

    std::visit(handler, m);
    std::visit(handler, k);
    std::visit(handler, j);

    // Event bad = NotAnEngineEvent{}; // don't compile
}

It is absolutely different from your solution: I don't create an ID that identifies a class. But I can determine the actual type of a generic event so that I can use its specific member functions.