[Bug libstdc++/57228] New: [c++11] constructing shared_ptr to enable_shared_from_this-derived class from raw pointers

[Bug libstdc++/57228] New: [c++11] constructing shared_ptr to enable_shared_from_this-derived class from raw pointers

[f.heckenbach@fh-soft.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57228

            Bug ID: 57228
           Summary: [c++11] constructing shared_ptr to
                    enable_shared_from_this-derived class from raw
                    pointers
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de

Created attachment 30076
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30076&action=edit
Test case

http://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr says:
"When constructing a shared_ptr from a raw pointer to an object of a
type derived from std::enable_shared_from_this, the constructors of
shared_ptr detect other owners of the object through the weak
reference stored in that base class. This allows ownership of the
object to be shared with existing shared_ptrs. Otherwise, it is
undefined behavior to construct a shared_ptr for an object that is
already managed by another shared_ptr."

Therefore, I gather the attached program should share ownership,
but it doesn't: It says "gp2.use_count() = 1;" and aborts due to
double deletion.

Using shared_from_this() works, though, so it's probably not a
serious problem.

[Bug libstdc++/57228] [c++11] constructing shared_ptr to enable_shared_from_this-derived class from raw pointers

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57228

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
cppreference is wrong.

This exact case has been discussed on the LWG reflector and everyone agreed
it's invalid.  The shared_ptr(Y*) constructor use_count() == 1 as a
postcondition, which is not compatible with the claim that you can share
ownership with existing shared_ptr objects.

[Bug libstdc++/57228] [c++11] constructing shared_ptr to enable_shared_from_this-derived class from raw pointers

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57228

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is http://cplusplus.github.io/LWG/lwg-active.html#2179 but if the
behaviour changes I'll eat my hat.