Re: [Boost-users] shared_ptr vs ordinary pointers

From: Jonathan Biggar (jon_at_[hidden])
Date: 2005-11-26 14:18:09

Merrill Cornish wrote:
> With an ordinary C++ pointer, if I have a BaseClass and a DerivedClass derived
> from BaseClass, then I can do
> 
>     BaseClass* objectPtr;
>     objectPtr = new DerivedClass();
> 
> Later, if BaseClass has a virtual destructor, I can call delete(objectPtr) and the
> DerivedClass destructor will be called.
> 
> Now, does this work with shared_ptr?
> 
>     shared_ptr<BaseClass> objectPtr;
>     objectPtr = shared_ptr<DerivedClass>(new DerivedClass());
> 
> and later, delete(objectPtr).

No, you shouldn't call delete directly.  It's done for you automatically 
when the last shared_ptr that points to your object is destroyed.

> Does any of the magic going on behind the scenes prevent shared_ptrs from
> being used like this?

shared_ptr would be close to useless if it didn't.  In fact, shared 
pointer does *more* to remember the dynamic type than ordinary pointers. 
  Forex, you can do this:

     {
        shared_ptr<void>	voidPtr;
        voidPtr = shared_ptr<Object(new Object());
      } # here your object is destroyed with the correct destructor

-- 
Jonathan Biggar
jon_at_[hidden]