Re: implementation of "Double-Checked Locking" Pattern in C++

From:

red floyd <no.spam@here.dude>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 25 Apr 2007 19:44:56 CST

Message-ID:

<zrQXh.544$HX7.19@newssvr19.news.prodigy.net>

Pedro Lamar?o wrote:

On 24 abr, 09:28, sk.sma...@gmail.com wrote:

What will happen when thread A is suspended just after it finishes
step two and thread B enters the ?Instance()? function?

It will detect that pInstance is not 0 anymore and may exit the
function, returning the address to a section of memory that has not
been initialized. Thus the code that was calling the Instance function
could dereference a Singleton object that has not been constructed
yet! Chaos ?

Singleton* Singleton::Instance() {

  if (0 == pInstance) {

    Guard lock(m_mutex);

    if (0 == pInstance) {
      Singleton* tmp = new Singleton(); // pInstance still zero
      pInstance = tmp; // pointer assignment is atomic (right?)
   }

}

  return pInstance;

}

Do we still need the inner if-clause?

Wrong question, it's the outer if that becomes redundant. The idea is
to avoid an unnecessary lock it the instance has already been allocated.

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