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: Jonathan Wakely (jwakely.boost_at_[hidden])
Date: 2013-06-24 04:06:17

On 24 June 2013 08:22, Adam Romanek wrote:
> 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.

No it won't, the program has undefined behaviour because X::~X() is not virtual.

Don't upcast the Y* to X* before giving ownership to a shared_ptr,
that's a bug in your code. Everything works if you fix that.

Even if the destructor was virtual (so the code didn't have undefined
behaviour) your expectation of enable_shared_from_this is wrong.  For
it to work would require a dynamic_cast in every shared_ptr
constructor taking a raw pointer, to check if the X* points to a base
class of a Y*. That would add unacceptable overhead for many people.

The documentation says that there must be a shared_ptr that owns t,
where t  is an instance of T (in your case T is Y).  That is not true
for your program, for a shared_ptr to "own" a pointer t it must have
been constructed with a copy of _that_ pointer, not some other pointer
with a different type to some base class of the same object.  The
shared_ptr owns the pointer it was constructed with, and you do not
construct it with a Y*, so no shared_ptr owns a pointer to your Y
object.

I think there is a small documentation bug though,
enable_shared_from_this talks about owning an object of type T, but
the shared_ptr docs talk about owning a pointer.  The docs for
std::enable_shared_from_this correctly say "There shall be at least
one shared_ptr instance p that owns &t." (as opposed to "owns t").
The & should be added to the Boost docs.