Re: [boost] shared_ptr and passing myself to someone else

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2001-12-10 21:25:22

> I have run across a problem in some code that I am trying to convert
> to using shared_ptrs.
> 
> Basically it looks something like...
> 
> class A
> {
> public:
>    init()
>    {
>       // Pass a ptr to myself to B so it can call me
>       b_obj = new B(shared_ptr<A>(this));
>    }
> 
>    B* b_obj;
> }
> 
> class B
> {
> public:
>    B(shared_ptr<A> aPtr)
>    {
>       a_obj = aPtr;
>    }
> 
>    shared_ptr<A> a_obj;
> }
> 
> Now in main...
> 
> int main()
> {
>    shared_ptr<A> a = new A;
>    a->init();
> 
>    // Problem a and B::a_obj are 
>    // different shared_ptrs, pointing
>    // to same object

Which of course would be a bad thing.  Maybe this should be a
FAQ, but it is almost never right to traffic in raw pointers
in a shared_ptr design,  and a trick like shared_ptr<A>(this)
is very dangerous when you don't know for certain that 'this'
is the only extant pointer to the object.  It's a pity that
'this' isn't a reference, but that's another story.  As for
shared_ptr, I think that shared_ptr<T>(new T) should always
be hidden away in a factory function, preferably with no
other way to contruct a T,  and code should never otherwise
pass a raw pointer to a shared_ptr constructor.

Anyway, try something like this instead (warning, uncompiled
code off the top of my head): 

class A
{
public:
   shared_ptr<A> init()
   {
      shared_ptr<A> a_ptr = new A;

      // Pass a ptr to myself to B so it can call me
      b_obj = new B(a_ptr);

      return a_ptr;
   }

   B* b_obj;
}

class B
{
public:
   B(shared_ptr<A> aPtr)
   {
      a_obj = aPtr;
   }

   shared_ptr<A> a_obj;
}

int main()
{
   shared_ptr<A> a = A::init();
}