double checked locking singleton

singleton_dcl.cpp

#include <cstdlib>
#include "Mutex.h"
#include "NonCopyable.h"

template <typename T>
class Singleton : NonCopyable {
   public:
    static T& instance() {
        if (val_ == NULL) {
            MutexLockGuard lock(mutex_);
            if (val_ == NULL) {
                val_ = new T();
                std::atexit(destroy);
            }
        }
        return *val_;
    }

   private:
    Singleton();
    ~Singleton();
    static void destroy() { delete val_; }

    Mutex mutex_;
    static T* val_;
};