Re: Confused about a thread-safe singleton example.

Sam <>
Tue, 02 Dec 2008 20:17:35 -0600
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static Mutex mutex;
static TheClass *instance;

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

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?

The reason I ask is I almost never see it done like that, I always see
blog entries and articles that say the same thing "store instance in a
pointer, use a mutex to protect, and p.s. double-checked locking is
broken". It seems like doing it lock-free is made out to be a hard
problem, so *if* having a static instance works (but I don't know if
it does, that's my question), then why doesn't anybody ever suggest

Setting aside the fact that there's no such thing as threads or mutexes in
the C++ language (at least not yet), so you are using a platform specific
library here.

Your statically declared instance gets constructed at some unspecified point
before your main() function gets invoked. If you have other objects in
static scope, it is unspecified the order in which all your static instances
get initialized. This may be undesirable. It's possible that it is necessary
to construct your singleton in a more controlled fashion, after all your
other objects, in static scope or otherwise, get initialized. Using a
dynamically-allocated pointer to your singleton, and protecting it with a
mutex, gives you the means to accomplish that.
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