Re: Does std::unique_ptr support self-reset?

From:

SG <sgesemann@googlemail.invalid>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 12 Aug 2013 07:30:12 -0700 (PDT)

Message-ID:

<kua9s5$jl0$1@news.albasani.net>

Am 12.08.2013 09:18, schrieb Greg Marr:

On Sunday, August 11, 2013 6:10:02 PM UTC-4, Daniel Kr?gler wrote:

At least in VS2010 and 2012, unique_ptr::reset checks for setting
the same pointer and ignores it.

This is indeed a non-conforming C++11 implementation, because
those effects were intentionally changed as of

I guess the libstdc++ implementation is non-conforming too.

      void
      reset(pointer __p = pointer()) noexcept
      {
    using std::swap;
    swap(std::get<0>(_M_t), __p);
    if (__p != pointer())
      get_deleter()(__p);
      }

(though the array specialization matches the standard)

http://gcc.gnu.org/viewcvs/gcc/trunk/libstdc%2B%2B-v3/include/bits/unique_pt
r.h?view=markup

I read the defect report, but I still can't understand why someone would
explicitly change from specifying that p.reset(p.get()) was safe to it
resulting in undefined behavior (dangling pointer immediately and then a
double-delete in the unique_ptr destructor),

I don't see why unique_ptr should support

   p.reset(p.get());

reset is supposed to take a nullptr or pointer to an object that's not
already owned by someone else. So, if you invoke reset with an address
of some object that another (or the same) unique_ptr instance already
owns, you did something wrong.

Can you come with with an example that relies on a self-reset-test but
is not considered broken by others?

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