Compile-time event hooks using CRTP in modern C++

hook.cpp

// Copyright (c) December 2025 Félix-Olivier Dumas. All rights reserved.
// Licensed under the terms described in the LICENSE file

template<typename Hooked>
struct IEventHookable {
protected:
    IEventHookable() {
        if constexpr (requires(Hooked h) { h.onCreated(); })
            static_cast<Hooked*>(this)->onCreated();
        else onCreatedDefault();
    }

    ~IEventHookable() {
        if constexpr (requires(Hooked h) { h.onDestroyed(); })
            static_cast<Hooked*>(this)->onDestroyed();
        else onDestroyedDefault();
    }

protected:
    void invokePreUpdate() {
        if constexpr (requires(Hooked h) { h.onPreUpdate(); })
            static_cast<Hooked*>(this)->onPreUpdate();
        else onPreUpdateDefault();
    }

    void invokeUpdate() {
        invokePreUpdate();
        if constexpr (requires(Hooked h) { h.onUpdate(); })
            static_cast<Hooked*>(this)->onUpdate();
        else onUpdateDefault();
        invokePostUpdate();
    }

    void invokePostUpdate() {
        if constexpr (requires(Hooked h) { h.onPostUpdate(); })
            static_cast<Hooked*>(this)->onPostUpdate();
        else onPostUpdateDefault();
    }

protected:
    void onCreatedDefault() { std::cout << "No 'onCreated' using default.\n"; }
    void onDestroyedDefault() { std::cout << "No 'onDestroyed' using default.\n"; }

    void onUpdateDefault() { std::cout << "No 'onUpdate' using default.\n"; }
    void onPreUpdateDefault() { std::cout << "No 'onPreUpdate' using default.\n"; }
    void onPostUpdateDefault() { std::cout << "No 'onPostUpdate' using default.\n"; }
};

template<typename Derived>
struct IUpdatable {
    void update()
        requires std::is_base_of_v<IEventHookable<Derived>, Derived>
    { static_cast<Derived*>(this)->invokeUpdate(); }
};

struct System : public IEventHookable<System>, public IUpdatable<System> {
public:         friend IEventHookable<System>; friend IUpdatable<System>;
private:
    void onCreated() {
        std::cout << "Creating new system...\n";
    }

    void onUpdate() {
        std::cout << "Updating System...\n";
    }

    void onDestroyed() {
        std::cout << "Destroying system...\n";
    }
};

int main() {
    System sys;
    sys.update();

    //only exposes .update()
}