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

24 Jun 2013

      On Mon, Jun 24, 2013 at 11:22 AM, Adam Romanek wrote:
...
On 06/24/2013 08:52 AM, Andrey Semashev wrote:
...
On Mon, Jun 24, 2013 at 10:28 AM, Adam Romanek 
...
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.htmlhttp://www.boost.org/doc/libs/release/libs/smart_ptr/enable_shared_from_this...
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.
The shared_ptr has no knowledge of Y since it is not constructed with a
pointer to Y. Actually, without a virtual destructor (like in your code) Y
is never destructed. So from the pointer's standpoint, it doesn't own Y,
but it owns X.

I know, this may not be obvious and it deserves better documentation.
...
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.
Agreed.