Qt and global variables

[Methedrine]

The singleton code looks nice. I'll try to use that. Thanks!

The Singleton implementation described is used widely but also contains a race-condition when it comes to use in a multithreaded program (see Alexandrescu's "Modern C++ Design" for further details).

class MyClass
{
  MyClass() {};
  virtual ~MyClass() {};
 
public:
  static MyClass* instance()
  {
    static QMutex mutex;
    if ( !m_pInstance )
    {
      mutex.lock();
      if (!m_pInstance)
        m_pInstance = new MyClass;
      mutex.unlock();
    }
 
    return m_pInstance;
  }
 
  void doSomething()
  {
    // ...
  }
 
protected:
  static MyClass* m_pInstance;
};
 
MyClass* MyClass::m_pInstance = NULL;

class MyClass
{
  MyClass() {};
  virtual ~MyClass() {};
 
public:
  static MyClass* instance()
  {
    static QMutex mutex;
    if ( !m_pInstance )
    {
      mutex.lock();
      if (!m_pInstance)
        m_pInstance = new MyClass;
      mutex.unlock();
    }
     
    return m_pInstance;
  }
 
  void doSomething()
  {
    // ...
  }
 
protected:
  static MyClass* m_pInstance;
};

MyClass* MyClass::m_pInstance = NULL;

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