Re: Gamma's Singleton pattern

Stuart Redmann <>
Thu, 3 Sep 2009 23:55:52 -0700 (PDT)
On 4 Sep., 07:31, Alexander Dong Back Kim <> wrote:

Hi all,

    (The following code is from



    // 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???

If you leave it in the header, each compilation unit (.cpp file) will
define a variable called Log::m_pInstance. When the linker tries to
link all translated compilation units (the .obj files), it will see a
symbol called Log::m_pInstance in each file, which will result in a
linker error: multiple definition of symbol Log::m_pInstance.

If you really want to avoid putting things into a .cpp file, you can
use a static object inside the Instance function:

#include <iostream>

class Log
  static Log* Instance()
    static Log Instance;
    return &Instance;
  void Write(char const *logline)
    std::cout << "Logging: " << logline;
  Log ()
  Log(Log const&)

int main(int argc, char* argv[])
  Log::Instance ()->Write ("Hello Log!");

The static object has the advantage that it won't leak memory. The
disadvantage is that initialization from multiple threads may lead to
race conditions (see
and also the premature destruction of the variable. To avoid this, you
could stick to the (leaking) pointer:

  static Log* Instance()
    // Now the life-time of the new'ed Log will be longer
    // than any other static variable.
    static Log* pInstance = NULL;
    if (!pInstance)
      pInstance = new Log ();
    return pInstance;


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