Re: Singleton_pattern and Thread Safety

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 10 Dec 2010 01:52:05 -0800 (PST)

Message-ID:

<53737d2a-c31e-42b6-98c6-e2cf1589b3c2@30g2000yql.googlegroups.com>

On Dec 9, 5:05 pm, Marcel M=FCller <news.5.ma...@spamgourmet.com> wrote:

Pallav singh wrote:

i have a query using given singleton that its not thread Safe ?

Since function getInstance() is returning the static object
singleton class AS far my knowlege, static object is
intialized only first time when control reaches there. The
second time control Thread reached there , compiler skipps
the initialization part.

That's right.

// Source file (.cpp)
Singleton& Singleton::getInstance()
{
  // Static Variables are initialized only first time Thread of
  // Execution reaches here first time.
  static Singleton instance;

This line is not guaranteed to be thread safe. In some implementation it
is safe.

In practice, it will be thread safe *if* the first call to
getInstance occurs before threading starts. If threading
doesn't start before entering main (normally an acceptable
restriction), then just declaring a variable with static
lifetime which is initialized by getInstance() is sufficient,
e.g. (at namespace scope):

Singleton* dummyForInitialization = &Singleton::getInstance();

return instance;
}

Note that the above still risks order of destruction issues;
it's more common to not destruct the singleton ever, with
something like:

    namespace {

    Singleton* ourInstance = &Singleton::instance();

    Singleton&
    Singleton::instance()
    {
        if (ourInstance == NULL)
            ourInstance = new Singleton;
        return *ourInstance;
    }
    }

(This solves both problems at once: initializing the variable
with a call to Singleton::instance and ensuring that the
singleton is never destructed.)

--
James Kanze