Re: using weak_ptr and shared_ptr for singleton

8 Oct 2002

      Nicola Musatti wrote:
...
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.
But I *want* it to be delete automagically...and I do not see the 
danger: if there is a dependent second_singleton it would hold a 
shared_ptr to my singleton, and singleton would not be destroyed before 
second_singleton is destroyed by its own. Maybe it should be made an 
option by using policies....but I really do not the danger.
...
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 instance() {
       if ( inst == 0 )
         inst = shared_ptr (
                 new Singleton,SingletonDeleter());
       return inst;
     }
};
typedef shared_ptr SingletonPtr;
Yes, I fear it *is* incorrect ;) look at the bottom of the posting for 
my implelentation
...
...
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.
OK, I hope that this will be the correct way:
* I do not rely on implementation-detail ( make boost::checked_delete a 
friend )
* I provide my own deleter-functor

<code>

#include 
#include 
#include 

class singleton : boost::noncopyable
{
public:
   typedef boost::shared_ptr< singleton >  singleton_ptr;

   static singleton_ptr instance()
   {
     singleton_ptr tmp;

     if( s_instance.expired() )
     {
       tmp.reset( new singleton, checked_deleter() );  // <-- NEW!!!
       s_instance = tmp;
     }
     else
     {
       tmp = boost::make_shared( s_instance );
     }

     return tmp;
   }

   void foo()
   {
     // do something
     i++;
   }

private:
   // NEW !!!!
   struct checked_deleter
   {
     operator() ( singleton * p_ )
     {
       // does the same as boost::checked_delete from BOOST 1.28.0
       typedef char type_must_be_complete[ sizeof( p_ ) ];
       delete p_;
     }
   };
   friend struct checked_deleter;

   typedef boost::weak_ptr< singleton >  internal_singleton_ptr;

   singleton(){};
   ~singleton(){};   // NOW IT'S PRIVATE !!!

   int i;

   static internal_singleton_ptr s_instance;
};

singleton::internal_singleton_ptr singleton::s_instance;

int main()
{
   {
     singleton::singleton_ptr first_ptr = singleton::instance();

     {
       singleton::singleton_ptr second_ptr = singleton::instance();
       second_ptr->foo();
     }

     first_ptr->foo();
   }

   singleton::singleton_ptr another_ptr = singleton::instance();

   // OK, these things do not work anymore.....
   //delete( another_ptr.get() );
   //boost::checked_delete( another_ptr.get() );
   //singleton::singleton_deleter deleter();
   //deleter( another_ptr.get() );

   another_ptr->foo();

   return 0;
}

</code>

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