Re: Confused about a thread-safe singleton example.

From:

Noah Roberts <user@example.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 03 Dec 2008 09:02:47 -0800

Message-ID:

<gh6e3d$ei1$2@aioe.org>

anon wrote:

jason.cipriani@gmail.com wrote:

static Mutex mutex;
static TheClass *instance;

static TheClass * getInstance () {
   MutexLocker lock(mutex);
   if (!instance)
     instance = new TheClass();
   return instance;
}

Where do you unlock the mutex?

If the OP is following the same design as the future C++ api (seen in
boost::thread versions after 1.35 I believe) then it is unlocked when
scope is left.

The example then goes on to talk about how double-check locking is
broken, etc. My question is pretty much this: Is C++ static
initialization thread-safe? If not, then how does the above example
safely use "mutex"? If so, then what is wrong with this:

static TheClass instance; // not a pointer

static TheClass * getInstance () {
return &instance; // it's correctly initialized?
}

You do not have a compilable code, but if instance is global variable,
then it is initialized before entering main(). Therefore, before any
thread is created

Not if you create a thread globally. The future C++ api for instance
will create threads on construction of a thread variable.