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

From: Brian Crowder (crowder_at_[hidden])
Date: 2007-01-21 20:23:36

The class you want to inherit from is "enabled_shared_from_this" not 
"shared_from_this"

ethann.castell_at_[hidden] wrote:
>
> 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?
>
>
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