Thread-safe Singleton class

Question

I have created this class to simulate a thread-safe Singleton.

Have I missed anything?

#include <boost/thread/mutex.hpp>

class Singleton
{
public:
  static Singleton& GetInstance()
  {
    boost::mutex::scoped_lock lock(m_mutex);
    static Singleton instance;
    return instance;
  }

private:
  static boost::mutex m_mutex;

  Singleton() {}
  ~Singleton() {}

  Singleton(const Singleton&);
  const Singleton& operator=(const Singleton&);
};

Answer 1

As usual, when dealing with singletons, consider that you probably shouldn't be making a singleton at all.

Read this, for example. A singleton is almost certainly not what you actually need, even if it is implemented correctly and in a thread-safe manner.

A plain old global might be a better option, or perhaps you should simply pass a reference around to those who need it.

Answer 2

Static m_mutex member isn't proper singleton itself. You can't use this singleton at static variables initialization time, because m_mutex isn't created lazily and may be not initialized.

Also, it's wrong to put actual logic to Singleton class, it should be template class.

#ifndef SINGLETON_H_689E0B11_E731_4555_AFE6_88598B582F1D
#define SINGLETON_H_689E0B11_E731_4555_AFE6_88598B582F1D

#include <cassert>
#include <boost/thread/mutex.hpp>

template<class T>
class Singleton
{
public:
  static T& GetInstance()
  {
    assert(!is_destructed);
    (void)is_destructed; // prevent removing is_destructed in Release configuration

    boost::mutex::scoped_lock lock(GetMutex());
    static T instance;
    return instance;
  }

private:
  static bool is_destructed;

  static boost::mutex& GetMutex()
  {
    static boost::mutex mutex;
    return mutex;
  }

  Singleton() {}
  ~Singleton() { is_destructed = true; }
};

// force creating mutex before main() is called
template<class T>
bool Singleton<T>::is_destructed = (Singleton<T>::GetMutex(), false);

#endif // SINGLETON_H_689E0B11_E731_4555_AFE6_88598B582F1D

As it have private destructor, it's noncopyable - no need to declare copy ctor and copy assignment operator.