singleton.cpp

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

template<typename Derived>
struct Singleton {
public:
    static Derived& instance() {
        static Derived instance(Token{});
        return instance;
    }

protected:
    struct Token {};

    Singleton() = default;

private:
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;
    Singleton(Singleton&&) = delete;
    Singleton& operator=(Singleton&&) = delete;
};

struct World final : Singleton<World> {
    friend Singleton<World>;

    void foo() const noexcept {}

private:
    explicit World(Token) {}
};

int main() {
    World::instance().foo(); // ✓

    World instance; // ✗
    World::foo() // ✗
}