Re: Threadsafe singletons

"Matthias Hofmann" <>
31 Jul 2006 11:57:38 -0400
"David Barrett-Lennard" <> schrieb im Newsbeitrag

I've often used something similar; I've even posted about it
once or twice here. In my case, I use an explicit pointer and
new, but the principle is the same. Except that I know how an
explicit pointer and new work---I can only guess as to how the
compiler ensures creation on only the first call to GetInstance.
At least some compilers, in a threaded environment, use
something like pthread_once to initialize the variable, so
constructing it before entering main isn't necessary.

Good point. I didn't know some compilers would do that.

What exactly is the problem with creation of a local static object and

At the
other extreme, other compilers document nothing, and may use
some technique which isn't thread safe. In the absense of any
specific guarantees, I prefer to avoid counting too much on what
the compiler does here.

My own technique is basically:

    static MySingleton* ourInstance = &MySingleton::instance() ;

        if ( ourInstance == NULL ) {
            ourInstance = new MySingleton ;
        return *ourInstance ;

How is it deleted?

This interests me too.

Formally, there is no guarantee that static variables are
constructed before entering main, so you have no guaranteed that
your s_init object (or my ourInstance pointer) will be
initialized before entering main.

Really! Are you referring to the C++ standard?

It is described in 3.6.2/3. If I understand this part of the standard
correctly, it does, however, guarantee that the ourInstance pointer will be
initialized before it is first used.

Matthias Hofmann
Anvil-Soft, CEO - The Creators of Toilet Tycoon - Die Macher des Klomanagers

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