Re: Confused about a thread-safe singleton example.

"Chris M. Thomasson" <no@spam.invalid>
Wed, 3 Dec 2008 20:45:19 -0800
<> wrote in message

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? 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

You can "easily" use `pthread_once()' and compiler specific directives to
construct a fairly "efficient" singleton pattern; something like this

namespace posix {

  template<typename T>
  class once {
    __thread T* g_tls;
    static T* g_obj;
    static pthread_once_t g_once;

    __cdecl void g_init() {
      assert(! g_obj);
      static T the_instance;
      g_obj = &the_instance;

    static T* instance() {
      if (! g_tls) {
        pthread_once(&g_once, g_init);
        g_tls = g_obj;
      return g_tls;

  template<typename T>
  T* once<T>::g_obj = NULL;

  template<typename T>
  pthread_once_t once<T>::g_once = PTHREAD_ONCE_INIT;

} // namespace posix

There are some important rules to be followed for Meyers singleton objects
behavior wrt dtors:

1. Don't call ANY unknown code whatsoever!

2a. Don't even look at any singleton!!

2b. Don't call ANY _known_ code which might want to look at _any_

3. Accessing another singleton from the dtor is BAD!!!


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