Double-checked Locking for Shared Pointers

Disclaimer: I come from a Java background, and, as such, I have no clue on how a lot of the internals of C++ (and associated libraries) work.

I have read enough to know that double-checked locking is evil, and a correct and safe implementation of a singleton pattern requires the appropriate tools.

I believe the following code may be unsafe, subject to compiler reordering and assignment of uninitialized objects, but I am not sure if I am missing something that I do not understand about the language.

typedef boost::shared_ptr<A> APtr;

APtr g_a;
boost::mutex g_a_mutex;
const APtr& A::instance()
{
  if (!g_a)
  {
    boost::mutex::scoped_lock lock(g_a_mutex);
    if (!g_a)
    {
      g_a = boost::make_shared<A>();
    }
  }

  return g_a;
}

I believe the actual code is compiled under C++03.

Is this implementation unsafe? If so, how?