Re: [boost] [smart_ptr] enable_shared_from_this and shared_ptr to a base class

Subject: Re: [boost] [smart_ptr] enable_shared_from_this and shared_ptr to a base class
From: Adam Romanek (romanek.adam_at_[hidden])
Date: 2013-06-24 03:22:46

On 06/24/2013 08:52 AM, Andrey Semashev wrote:
> On Mon, Jun 24, 2013 at 10:28 AM, Adam Romanek <romanek.adam_at_[hidden]>wrote:
>>
>> class X
>> {
>> ...
>> };
>>
>> class Y: public X, public boost::enable_shared_from_**this<Y>
>> {
>> ...
>> };
>>
>> int main()
>> {
>>      X* x_ptr = new Y;
>>      boost::shared_ptr<X> sp(x_ptr);
>>      sp->foo();
>> }
>
> This is expected, given the way it's implemented and described.
> shared_from_this() description [1] mentions that there must be at least one
> shared_ptr that owns the object. I didn't find any formal definition of the
> "owns" relation but it doesn't look to me that it applies to your example,
> because there is no shared_ptr pointing to Y in your code.
>
> [1]
> http://www.boost.org/doc/libs/release/libs/smart_ptr/enable_shared_from_this.html

Andrey!

Thanks for such a quick response.

OK, this behavior may in fact be expected considering the 
implementation. However, I would not say the documentation reflects 
that. But let's look at the code first.

You wrote that there is no shared_ptr pointing to Y in my code. I can't 
agree. There is - sp. It points to Y through a pointer to X, which is 
perfectly valid. Moreover, in my opinion it "owns" an instance of Y and 
will attempt to destroy it when necessary.

Maybe it's a matter of using the right words but a colleague of mine 
spent an hour or two looking for a bug in his code, just to find out 
about this "feature" of enable_shared_from_this.

I appreciate your explanation but still, I think that the documentation 
is a bit imprecise and could be improved so that things were more clear.

WBR,
Adam Romanek