Re: Confused about a thread-safe singleton example.

James Kanze <>
Wed, 3 Dec 2008 01:43:34 -0800 (PST)
On Dec 3, 7:47 am, Alan Johnson <> wrote: 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? 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

1. Don't use singletons. Ever. Pretty much all of the value
of the GoF Design Patterns book is negated by the fact that
they chose to legitimize Singleton as a design pattern.
Singleton is just a fancy name for global variable. We should
call it the Global Variable (anti)Pattern.

And there are valid reasons for using it. Some things must be
inherently unique in a process.

3. Double checked locking works without memory fencing on

The author says that it works without fences, but he doesn't
give any reference to anything which guarantees it. (From a
quick glance at the AMD document, it looks like it might be
safe, but I'd need more analysis to be sure.)

Thus, your main opponent in the optimizer.

4. Whether or not the construction of a static variable is
threadsafe depends mostly on the implementation (the standard
says nothing about it). gcc has an option
=fno-threadsafe-statics to turn off the extra code emitted to
make local statics thread safe. I guess one could extrapolate
that by default local statics are thread safe in gcc (though I
have no idea if this is actually true).

Not that the g++ implementation of this is broken on some

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