Re: Singletons and destructors

From:

Nicola Musatti <nicola.musatti@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 11 Jul 2007 14:05:21 CST

Message-ID:

<1184170837.272848.295990@22g2000hsm.googlegroups.com>

On Jul 10, 2:31 pm, Rune Allnor <all...@tele.ntnu.no> wrote:
[...]

Then I did a search on c.l.c++.m to learn about singletons, and found
the thread

http://groups.google.no/group/comp.lang.c++/browse_thread/thread/4fe4...

which makes me doubt if my strategy is a reasonable one. I can't
really
see the implications of all the details in that discussion, but it
sems to
be a sensitive point whether the destructor of a singleton class will
ever be called at all. If this is correct, I am not sure my idea
about
a singleton COM_handler class is as good as it seemed first time
around.

So, is there a way to implement a singleton class which guarantees
that the destructor is called?

Yes. Either use a local static variable of the singleton class or use
a referenced counted smart pointer like shared_ptr. I try to avoid
singletons as much as I can, but when I need one I use something like

class singleton {
  public:
    static boost::shared_ptr<singleton> instance() {
      if ( ptr == 0 )
        ptr.reset(new singleton);
      return ptr;
  private:
    static boost::shared_ptr<singleton> ptr;
};

The consistent use of shared_ptr ensures that you can have
interdependent singletons without incurring destruction order
problems, provided you avoid acquiring a singleton in another
singleton's destructor.

Cheers,
Nicola Musatti

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