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// 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() // ✗

}