Discoverable 'global vars'

registry.cpp

#include <memory>
#include <utility>
#include <cassert>
#include <iostream>

template<typename Service>
struct Registry
{
	Registry() = delete;
	~Registry() = delete;

	inline static bool empty() noexcept {
		return !bool{svc};
	}

	template<typename Implementation=Service, typename... Args>
	inline static void assign(Args&& ...args) {
		svc = std::make_shared<Implementation>(std::forward<Args>(args)...);
	}

	inline static void reset() noexcept {
		svc.reset();
	}

	inline static Service& get() noexcept {
		return *svc;
	}

private:
	inline static std::shared_ptr<Service> svc = nullptr;
};

/// Some Services

struct ITimer {
	virtual void foo() = 0;
};

struct QtTimer: public ITimer {
	virtual void foo() override {
		std::cout << "QtTimer::foo()\n";
	}
};

struct ILogger {
	virtual void log(const char*) const noexcept = 0;
};

struct CoutLogger: public ILogger {
	virtual void log(const char* txt) const noexcept override {
		std::cout << "Log: " << txt << "\n";
	}
};

/// Usage

struct AppRegistry {
	using Timer = Registry<ITimer>;
	using Logger = Registry<ILogger>;
};

int main() {
	// some initialization part
	AppRegistry::Timer::assign<QtTimer>();
	AppRegistry::Logger::assign<CoutLogger>();

	// some other, usafe
	AppRegistry::Timer::get().foo();
	AppRegistry::Logger::get().log("boo");

	return 0;
}