Re: Confused about a thread-safe singleton example.

anon <anon@no.invalid>
Wed, 03 Dec 2008 07:57:10 +0100
<gh5ak6$o77$> wrote:

I have a C++-specific question about thread-safe singleton instances.
There's a trivial example here:

That goes like this, very simple and straightforward:

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?

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

btw in this case, the getInstance function can be like this:
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


I think your suggestion should be like this:

static TheClass * getInstance () {
    MutexLocker lock(mutex);
    return &instance; // it's correctly initialized?
because in the first function, there is a mutex locked, so only one
thread can access the class instance at the time

Sorry I didn't answer your question - I do not know the answer :(

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