Re: Confused about a thread-safe singleton example.

James Kanze <>
Wed, 3 Dec 2008 01:53:56 -0800 (PST)
On Dec 3, 1:25 am, ""
<> 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;

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?


It's actually very hard to answer concretely here. Objects with
static lifetime in class or namespace scope are constructed
before main is entered, so if threads aren't started until then,
you're OK. Except that if getInstance is called before then,
you may have an order of initialization problem. And the
guarantee of construction before main is entered doesn't apply
to dynamically linked objects (which aren't addressed by the
standard, but then, neither is multi-threading).

My usual solution here is something like:

    namespace {
    TheClass* ourInstance = &TheClass::instance() ;

        if ( ourInstance == NULL ) {
            ourInstance = new TheClass ;
            // or
            static TheClass theOneAndOnly ;
            ourInstance = &theOneAndOnly ;
        return *ourInstance ;

The initializer for the static variable ensures that
TheClass::instance is called at least once before main is
entered, and once you've returned from a call to
TheClass::instance(), the function is thread safe without locks.

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 it?

Because it's open to order of initialization issues.

James Kanze (GABI Software)
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