Re: Gamma's Singleton pattern

"Alf P. Steinbach" <>
Fri, 04 Sep 2009 08:38:25 +0200
* Alexander Dong Back Kim:

    (The following code is from

    class Log {

      static Log* Instance() {
        if (!m_pInstance)
          m_pInstance = new Log;
        return m_pInstance;
      void Write(char const *logline);
      bool SaveTo(char const *filename);
      Log(); // ctor is hidden
      Log(Log const&); // copy ctor is hidden

      static Log* m_pInstance;
      static std::list<std::string> m_data;

    // in log.cpp we have to add
    Log* Log::m_pInstance = NULL;

This is a typical Gamma's singleton pattern. Although I know this
works fine but still don't know why I must add the last line in the
CPP file instead of initializing it in the header. What's the reason
of doing this???

You can achieve the same effect with header-only code, but you can't use the
code above as pure header-only code.

The reason is that the definition of m_pInstance then can occur in more than one
compilation unit. And C++ forbids having more than one definition for an
ordinary external linkage variable or and routine, which is called the "ODR",
short for One Definition Rule. Mostly the ODR helps you catch errors where the
same name is inadvertently used for different purposes in different compilation
units, but unfortunately the C++ rules leave it to the implementation whether
any ODR breach will be detected.

There are two main ways of defining the singleton with all header file code.

They use different ways to tell the linker that multiple definitions of the
variable are OK, that they're the same, and that the linker should just pick one.

The first and most common is known as Meyer's singleton, and simply makes the
variable a local static variable:

     struct Log
         static Log& instance()
             static Log theInstance;
             return theInstance;

This Meyer's singleton will however be destroyed before static variables that
have been initialized before it, so those static variables can't use the logger.
Which presumably is the reason why your example code uses a pointer.

Using a pointer is however no problem with a Meyer's singleton:

     struct Log
         static Log& instance()
             static Log* theInstance = new Log;
             return *theInstance;

This dynamically allocated Meyer's singleton, however, has the opposite problem,
namely that the singleton will never be destroyed.

In order for outside code to be able to influence the time of destruction (or
whether that should occur at all) you need a pointer that's not just a local
static variable, a pointer with external linkage. Which presumably is the reason
why your example is not a Meyer's singleton. For all header code it can go like

     struct Log;

     namespace detail {
         template< typename Dummy >
         struct LogStatics_ { static Log* theInstance; }

         template< typename Dummy >
         Log* LogStatics_<Dummy> theInstance;

         typedef LogStatics_<void> LogStatics;
     } // namespace detail

     class Log: private detail::LogStatics
         static Log& instance()
             if( theInstance == 0 ) { theInstance = new Log; }
             return *theInstance;

         // Functionality to prolong or shorten lifetime or whatever.

The reason that this doesn't run afoul of the ODR is that the ODR has a special
exemption for templates, since templates in practice have to be defined in headers.

Cheers & hth.,

- Alf

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