Re: question re. usage of "static" within static member functions of a class

From:

"Chris M. Thomasson" <no@spam.invalid>

Newsgroups:

comp.lang.c++

Date:

Thu, 10 Sep 2009 01:50:14 -0700

Message-ID:

<h8aei5$2ecq$1@news.ett.com.ua>

"Joshua Maurice" <joshuamaurice@gmail.com> wrote in message
news:aac0ea5e-c259-4177-9781-d94931593069@j9g2000prh.googlegroups.com...
On Sep 9, 5:15 pm, "Chris M. Thomasson" <n...@spam.invalid> wrote:
[...]

You can get around static initialization and destruction ordering issues
by
using a strongly thread-safe smart pointer to manage the singleton. The
pseudo-code would be something like the following pseudo-code:
_____________________________________________________________________

[...]

_____________________________________________________________________

This is strongly thread-safe and will always work no matter how the
static
ctor/dtor ordering comes out. The caveat, well, it's definitely not as
efficient as using a raw pointer and explicitly leaking the singleton.

There are so many things wrong with that code sample, I don't even
know where to start. (Exaggeration. I do know where to start.)

Actually, well... How much experience do you have wrt multi-threading
issues?

Firstly and most importantly, you're using double checked locking,
which is broken in effectively all C++ implementations.

I explicitly stated that one can:

"get around static initialization and destruction ordering issues by using a
strongly thread-safe smart pointer to manage the singleton"

Do you have any idea what I am writing about here? Go ahead and put it on
the self because this discussion can get advanced rather quickly!

Don't do that.

;^)

Na. Anyway, double-checked locking IS 100% workable, period. You just don't
know it yet. That is NOT my problem. Oh well, shi% happens.

Please read, continue to re-read if you don't get it, this excellent
paper:
http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf

What about it? Anyway, that paper is WELL known to me. Been there, done
that. Yawn.

Next, you also have a race condition on the construction of the mutex
itself,

;^/

Learn about POSIX:

http://www.opengroup.org/onlinepubs/7990989775/xsh/pthread_mutex_init.html

in addition to the double checked locking of the singleton
instance construction.

This is PERFECTLY fine. You just need to learn how to do it properly.

PTHREAD_MUTEX_INITIALIZER is entirely
equivalent to calling pthread_mutex_init, and thus is not thread-safe.

:^O

Learn about POSIX:

http://www.opengroup.org/onlinepubs/7990989775/xsh/pthread_mutex_init.html

Repeat:

lptr.reset(new T());

is not properly guarded (double checked locking) and

Yawn.

static pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;

is not properly guarded (simple race condition). Both are race
conditions when the first call of this function may be concurrent.

I am getting tired.

Also, users may want multiple singletons of the same type. One unit of
code makes a singleton of type T, and another piece of code entirely
separate will make another singleton of type T. Your code does not
support that. Instead, you will get exactly 1 instance of T across the
entire program. This is a significant limitation.

FINALLY! You actually make sense; congratulations!

:^D

You could
potentially get it around it with something like:
  //header code
  template <typename singleton_t, typename once_type>
  singleton_t & getSingleton();

  //code using the header to make a singeton in a hpp
  class singleton_x_type { /* ... */ };
  class once_type_for_singleton_X {};
  singleton_x_type & getSingletonX()
  {
    return getSingleton<singleton_x_type, once_type_for_singleton_X>
();
  }
If another piece of code wanted their own singleton of type X, then
they could define their own once_type and instantiate getSingleton on
that new once_type.

Sure.

Lastly, RAII is your friend. Don't ever explicitly call
pthread_mutex_lock or pthread_mutex_unlock except in your portability
layer mutex wrapper class.

Sure. Although, using try/catch is 100% perfectly fine. Whatever, this was
pseudo-code; lol. Anyway, here is example of RAII:

http://groups.google.com/group/comp.lang.c++/msg/63c3aecb9d0fbaa6

Blah, blah.

This is not a correctness issue per se, but
vastly improves the chances that the code will be correct.

Yes; RAII is very convenient.

Your solution could be made correct by eager initializing it by adding
namespace { bool force_init = (::once<your_type>(), true); }

My pseudo-code IS correct. IMVHO, you obviously need some education, that's
all; there is absolutely NOTHING to be ashamed about. I personally LOVE to
earn __all__ about new things!!!

However, at that point, your fixed code is pretty much equivalent to
several examples already posted, aka:
  T& getSingleton()
  { static T* x = new T;
     return *x;
  }
  namespace { bool force_init = (getSingleton(), true); }

NO!

;^/

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