Re: singleton in a DLL loaded class

ajk <>
Wed, 27 Jun 2007 00:11:44 +0800
On Mon, 25 Jun 2007 17:57:36 -0000, PaulH <> 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.

The only funny thing I am doing is to load the DLL from two threads
simultaneously (thus the need for the singleton). But, according to
other posts I've read, the OS will point them both to the same
instance of the DLL so that only one thread actually loads the DLL.

Can anybody point out where I may be going wrong?


class SingletonClass : BaseClass
   static SingletonClass& Instance();
   //... Implement BaseClass functions
   static std::auto_ptr< SingletonClass > _theInstance;
   static Mutex* _m;

std::auto_ptr< SingletonClass > SingletonClass::_theInstance;
Mutex* SingletonClass::_m;

/*static*/ SingletonClass& SingletonClass::Instance()
   Mutex::Create( &_m );
   if( _theInstance.get() == NULL )
        _theInstance.reset( new SingletonClass() );
   return *_theInstance;

MY_API BaseClass* Create()
   return &SingletonClass::Instance();

auto_ptr in this context may be a problem. auto_ptr doesn't copy the
ptr so if you assign to an auto_ptr you are move the ownership from
one variable to another. is this what u want?

there are other ways to implement a singleton which are less resource
demanding, Dr Dobb had one method which seemed quite nice:

which instead of a mutex uses a state machine.


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