Re: using weak_ptr and shared_ptr for singleton

From: Nicola Musatti (yg-boost-users_at_[hidden])
Date: 2002-10-08 08:06:36

Stephan Born wrote:

> Hello!
> 
> I do some experiments on shared_ptr and weak_ptr. So I tried to use them 
> for a singleton-class, which does not need an explicit 
> singleton::release_instance method. The singleton-instance will be 
> destroyed everytime the last external shared_ptr holding the instance 
> goes out of scope. Is this a valid usage for weak_ptr? Are there any 
> traps I do not see using these smart_ptrs in this context?

Within the singleton class I hold the instance with a shared_ptr. I 
consider this the best option, as it guarantees the correct order of 
destruction when a singleton depends on another one.

> The drawback of using shared_ptr with singleton is the need for a public 
> destructor. But this is dangerous....it is possible to write
> 
> singleton::singleton_ptr first_ptr = singleton::instance();
> delete first_ptr.get();

You should rely on the constructor that takes an additional deleter 
argument, something like (not compiled and almost certainly incomplete 
and incorrect):

class SingletonDeleter {
   friend class shared_ptr<Singleton>;
   operator() (Singleton *p) { delete p; }
};

class Singleton {
     friend class SingletonDeleter;
   public:
     shared_ptr<Singleton,SingletonDeleter> instance() {
       if ( inst == 0 )
         inst = shared_ptr<Singleton,SingletonDeleter> (
                 new Singleton,SingletonDeleter());
       return inst;
     }
};

typedef shared_ptr<Singleton,SingletonDeleter> SingletonPtr;

> Declaring the destructor as private is only possible when defining
> boost::checked_delete as friend of the singleton-class.
[...]
> But then it is still possible to write the following code
> 
> singleton::singleton_ptr first_ptr = singleton::instance();
> boost:checked_delete( first_ptr.get() );
> 
> OK, it's very unlikely to do it by chance, but it is possible.

Moreover, you rely on what should be an implementation detail.

Cheers,
Nicola Musatti