Re: singleton in a DLL loaded class

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 25 Jun 2007 15:45:34 -0400

Message-ID:

<#WjImF2tHHA.3468@TK2MSFTNGP02.phx.gbl>

PaulH <paul.heil@gmail.com> wrote:

On Jun 25, 2:00 pm, "Igor Tandetnik" <itandet...@mvps.org> wrote:

PaulH <paul.h...@gmail.com> wrote:

On Jun 25, 1:29 pm, "Igor Tandetnik" <itandet...@mvps.org> wrote:

PaulH <paul.h...@gmail.com> wrote:

I have a class loaded from a DLL that I would like to implement
as a multi-threadable singleton. Unfortunately, I seem to still be
getting two distinct instances of this class. It is implemented as
below.

//singletonclass.h
class SingletonClass : BaseClass
{
public:
   static SingletonClass& Instance();
   ~SingletonClass();
   //... Implement BaseClass functions
private:
   static std::auto_ptr< SingletonClass > _theInstance;
   static Mutex* _m;
}

//singletonclass.cpp
std::auto_ptr< SingletonClass > SingletonClass::_theInstance;
Mutex* SingletonClass::_m;

/*static*/ SingletonClass& SingletonClass::Instance()
{
   Mutex::Create( &_m );

What does Mutex::Create do? It appears you are creating a new mutex
every time Instance() is called.

I am.

But that defeats the whole point of having the mutex in the first
place.

So how do I make them use the same mutex?

Initialize it when the DLL loads. Just have a global object with a
constructor specifically for this purpose:

struct MutexCreator {
    MutexCreator() {
        Mutex::Create( &SingletonClass::_m );
    }
    // Perhaps a ~MutexCreator to destroy the mutex
};

// Runs the constructor during DLL initialization. Not used otherwise
MutexCreator mutex_creator;

--
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925