Re: newbie question on singleton class

James Kanze <>
13 May 2007 04:01:49 -0700
On May 13, 2:11 am, Gianni Mariani <> wrote:

On May 12, 11:53 pm, wrote:

I liked this article by Danny Kalev on the singleton design

However I was confused by this:

The class's implementation looks like this:

  Singleton* Singleton::pinstance = 0;// initialize pointer
  Singleton* Singleton::Instance ()
    if (pinstance == 0) // is it the first call?
      pinstance = new Singleton; // create sole instance
    return pinstance; // address of sole instance

A much easier way to write this is:

   Singleton* Singleton::Instance ()
      static Singleton* pinstance = new Singleton;
      return pinstance; // address of sole instance

This will also generate thread safe code on some compilers.

So it is claimed. In fact, I only know of one compiler that
even tries, and it fails on my most important target (Solaris on

In practice, what I usually do is to leave the instance variable
outside of the function (more often in an anonymous namespace
than as a static member), and write something like:

    Singleton* ourInstance = Singleton::instance() ;

This ensures (in practice, anyway) that the Singleton::instance
function is called before entering main, and thus before I've
created any threads. And once the pointer is non-null, it is
never modified, so (at least according to Posix), there is no
longer any need to protect accesses. (Note that this code is a
little tricky, since it counts on zero initialization.)

So I would have thought that Singleton* Singleton::pinstance = 0;
violates private access.

The initialization expression is in the scope of the class.

And doesn't access any private variables anyway:-).

James Kanze (Gabi Software) email:
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