[Boost-users] More on Newby question re (boost::shared_ptr) enable_shared_from_this

From: ethann.castell_at_[hidden]
Date: 2007-01-21 18:13:17

A big thanks to Phillip and Brian for their help so far. You've helped me 
increase my understanding of how shared_ptr's and enable_shared_from_this 
work.

I've taken on board what you've both said but I still feel like I am 
missing something. I'm getting bad_weak_ptr exceptions (resulting in 
memory leaks) which make no sense to me. I've debugged through the code 
and checked the use_count and weak_count.  The use_count looks ok but I'm 
not sure about the weak_count.

I have a base class which contains the shared_from_this and a hierarchy of 
extended classes that inherit from the base class.  The extended classes 
pass pointer's to themselves, to the other classes, so that linkages can 
be formed between them( at the base class level ).

class base_class:: public shared_from_this
prviate:
         boost::shared_ptr<base_class> m_Linked;                        // 
link to another base_class

base_class::base_class( boost::shared_ptr<base_class> pLinked)
{
        m_Linked = pLinked;                                             // 
store link to other base class
}

class B : public base_class
B::B(boost::shared_ptr<base_class> pLinked) : base_class( pLinked)

class B1 : public B
B1::B1(boost::shared_ptr<base_class> pLinked) : B( pLinked)

class B2 : public B1
B2::B2(boost::shared_ptr<base_class> pLinked) : B1( pLinked)

class A : public base_class

classA::Process()
{
        boost::shared_ptr<A> ptr_a = shared_from_this();                // 
get pointer to myself
 //afterwards - use_count = 2, weak_count = 2
        ...
        if (some_condition){
                boost::shared_ptr<B2> ptr_b2( ptr_a) ;                  // 
create new B2 and pass pointer to me.
        }else{                                                  // crashes 
with bad_weak_ptr error on this line.
        }
}

main()
{
        boost::shared_ptr<A> ptr_a( new A() );
        ptr_a->process
}

It looks a little like ptr_a is being destroyed at the }else{ statement 
(although I would have expected this at the end of Process().
At the else statement ptr_a.use_count = 3, which I believe to be correct.

I've tried passing the boost pointers by value, by reference and also as 
constants but none of these make a difference.

Any ideas?