LazySingletonWithArgs.cpp

// C++11 singleton with lazy initialization & construction arguments
// Requires arguments to be initialized before usage (otherwise default values will be passed)

using namespace std;

// singleton with lazy initialization & construction arguments
template <typename Derived, typename Args>
class SingletonWithArgs
{
public:
	static Derived *getInstance() {
		static std::unique_ptr<Derived> instance(new Derived(*Args::getInstance()));
		return instance.get();
	}
protected:
	SingletonWithArgs() = default;
	SingletonWithArgs(const SingletonWithArgs&) = delete;
	SingletonWithArgs& operator= (const SingletonWithArgs&) = delete;
};

// singleton with lazy initialization & no arguments
template <typename Derived>
class SingletonNoArgs
{
public:
	static Derived* getInstance()
	{
		static std::unique_ptr<Derived> instance(new Derived());
		return instance.get();
	}
protected:
	SingletonNoArgs() = default;
	SingletonNoArgs(const SingletonNoArgs&) = delete;
	SingletonNoArgs& operator= (const SingletonNoArgs&) = delete;
};

// --- derived class 

// arguments to pass to derived class
class ConsoleLogArgs : public SingletonNoArgs<ConsoleLogArgs> {
public:
	string prefix;
};

// derived class
class ConsoleLog : public SingletonWithArgs<ConsoleLog, ConsoleLogArgs> {
	typedef SingletonWithArgs<ConsoleLog, ConsoleLogArgs> inherited;
public:
	void addText(const std::string &a_text) {
		cout << m_prefix << a_text;
	}
protected:
	ConsoleLog(const ConsoleLogArgs &args) {
		m_prefix = args.prefix;
	}
	friend class inherited;
protected:
	std::string m_prefix;
};

// -- usage

void foo() {
	ConsoleLog::getInstance()->addText("Test");
}

int main(int argc, char* argv[])
{
	//not possible:
	//  ConsoleLog log2; 
	//not possible:
	//  ConsoleLog log(*ConsoleLogArgs::getInstance());
	//not possible:
	//  std::unique_ptr<ConsoleLog> log(new ConsoleLog(*ConsoleLogArgs::getInstance()));
	ConsoleLogArgs::getInstance()->prefix = "log: ";
	foo();
}