Tag Archive for once_flag

Implementing a Thread-safe Singleton with C++11

21 Oct, 2012  15 Comments

Note: see my more recent article “Implementing a Thread-Safe Singleton with C++11 Using Magic Statics” for a more modern solution.

C++11 makes it easier to write a thread-safe singleton. Here is an example. The class definition of the singleton looks as follows:

#include <memory>
#include <mutex>

class CSingleton
{
public:
	virtual ~CSingleton() = default;
	static CSingleton& GetInstance();

private:
	static std::unique_ptr<CSingleton> m_instance;
	static std::once_flag m_onceFlag;
	CSingleton() = default;
	CSingleton(const CSingleton& src) = delete;
	CSingleton& operator=(const CSingleton& rhs) = delete;
};

The implementation of the GetInstance() method is very easy using C++11 std::call_once() and a lambda:

std::unique_ptr<CSingleton> CSingleton::m_instance;
std::once_flag CSingleton::m_onceFlag;

CSingleton& CSingleton::GetInstance()
{
	std::call_once(m_onceFlag,
		[] {
			m_instance.reset(new CSingleton);
	});
	return *m_instance.get();
}
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