Re: [Boost-users] Problem with weak_ptr exception

Subject: Re: [Boost-users] Problem with weak_ptr exception
From: Jacob Carlborg (doob_at_[hidden])
Date: 2009-10-28 10:06:31

On 10/28/09 15:56, Stefan Strasser wrote:
> I can't make sense of your code ("weakThis" in both A and B, even though B is
> derived from A?), but have a look at boost::shared_from_this

If I don't have a weak_ptr in B I get this error when trying to create a 
shared_ptr<B> from a weak_ptr<A>:
/shared_ptr.hpp:207: error: invalid conversion from 'A* const' to 'B*'

I've already tried boost::shared_from_this and it gives me the same result.

> Am Wednesday 28 October 2009 12:43:08 schrieb Jacob Carlborg:
>> I'm having some problems with bad_weak_ptr exceptions. I have reduced my
>> code into this test case:
>>
>> #include<boost/shared_ptr.hpp>
>> #include<boost/weak_ptr.hpp>
>>
>> using boost::shared_ptr;
>> using boost::weak_ptr;
>>
>> class A
>> {
>> 		weak_ptr<A>  weakThis;
>>
>> 	public:
>> 		virtual ~A(){}
>>
>> 		shared_ptr<A>  a;
>>
>> 	shared_ptr<A>  getThis ()
>> 	{
>> 		shared_ptr<A>  x(weakThis);
>> 		return x;
>> 	}
>>
>> 	void init ()
>> 	{
>> 		a = getThis();
>> 	}
>>
>> 	static shared_ptr<A>  create ()
>> 	{
>> 		shared_ptr<A>  x(new A);
>>
>> 		x->weakThis = x;
>> 		x->init();
>>
>> 		return x;
>> 	}
>> };
>>
>> class B : public A
>> {
>> 		weak_ptr<B>  weakThis;
>> 	public:
>> 		virtual ~B(){}
>>
>> 	shared_ptr<B>  getThis ()
>> 	{
>> 		shared_ptr<B>  b(weakThis);
>> 		return b;
>> 	}
>>
>> 	static shared_ptr<B>  create ()
>> 	{
>> 		shared_ptr<B>  x(new B);
>> 		x->weakThis = x;
>> 		x->init();
>>
>> 		return x;
>> 	}
>> };
>>
>> int main (char** argv, int argc)
>> {
>> 	shared_ptr<B>  b = B::create();
>>
>> 	return 0;
>> }
>>
>> If I create an instance of A instead of B or doesn't call x->init() in
>> the B's create function I don't get an exception. The problem is
>> creating a shared_ptr from the weak_ptr in getThis.
>>
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