Re: Writing Singleton Classes

From: Tim Clacy (nospamtcl_at_nospamphaseone.nospamdk)
Date: 05/13/04 

Date: Thu, 13 May 2004 21:08:40 +0200

cppaddict wrote:
> Hi,
>
> In this tutorial on singleton class in C++
> (http://gethelp.devx.com/techtips/cpp_pro/10min/10min0200.asp) the
> author gives two implementations of a simple singleton class, claiming
> that only the first is safe for multi-threaded appliactions. I want
> to know why this so.

cppaddict,

The first implementation needs a slight change to be thread-safe:

    Singleton* Singleton::Instance ()
    {
        // In the common case, simply return the existing instance (no
locking required)
        //
        if (pinstance != 0)
            return pinstance;

        // If there was no instance above, we need to lock now and
double-check whether
        // another thread created an instance in between the check above and
now
        //
        Lock();
        if (pinstance == 0)
            pinstance = new Singleton;
        Unlock();

        return pinstance;
    }

There are a dozen or so Singleton articles on CodeProject; the following is
very good and covers threading issues:

Singleton Pattern: A review and analysis of existing C++ implementations
http://www.codeproject.com/cpp/singleton.asp

Tim

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