Singleton notes

simgleton.cpp

// http://en.wikipedia.org/wiki/Double-checked_locking


Singleton& Singleton::getInstance() {
    static Singleton instance;
    return instance;
}

// More info
// http://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/

std::atomic<Singleton*> Singleton::m_instance;
std::mutex Singleton::m_mutex;

Singleton* Singleton::getInstance() {
    Singleton* tmp = m_instance.load(std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_acquire);
    if (tmp == nullptr) {
        std::lock_guard<std::mutex> lock(m_mutex);
        tmp = m_instance.load(std::memory_order_relaxed);
        if (tmp == nullptr) {
            tmp = new Singleton;
            std::atomic_thread_fence(std::memory_order_release);
            m_instance.store(tmp, std::memory_order_relaxed);
        }
    }
    return tmp;
}