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

From:

=?windows-1252?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 12 Aug 2013 17:15:19 -0700 (PDT)

Message-ID:

<kubag1$9g2$1@dont-email.me>

Am 12.08.2013 19:48, schrieb Greg Marr:

On Monday, August 12, 2013 10:23:38 AM UTC-4, SG wrote:

Actually, this looks good to me. It does not support self-reset but it
does not have to according to the standard. What do you think is
non-conforming about it?

It does support self-reset, exactly as does the MSVC implementation
that Daniel Krugler said is non-conforming, because it doesn't
call get_deleter()(old_value) if the old value and the new value
are the same.

I'm confused by your statement here: *If* the argument of reset() is the
owned pointer itself, the effects will be a double-delete: The first one
within reset and the second one in the first of another reset or the
destructor of std::unique_ptr.

This is different than the reset() later in the same file, which is

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

I'm confused a second time: Sebastian Gesemann was referring to exactly
this version of reset as I understand his response.

The difference is
if (__p != nullptr)
vs
if (__p != pointer())

This is not a semantics difference, because as of the NullablePointer
requirements imposed on type 'pointer' we can build the following logic
chain:

a) __p is a value of (possibly const) 'pointer' and nullptr is a
(possibly const) value of type std::nullptr_t, according to Table 25 ?
NullablePointer requirements [nullablepointer]

__p != nullptr

has the semantics of

!(__p == nullptr)

and the sub-expression

__p == nullptr

has the semantics of

__p == pointer()

b) Using the same starting point we note that

__p != pointer()

has the semantics of

!(__p == pointer())

and we end up with the same essential effects as in (a).

HTH & Greetings from Bremen,

Daniel Kr?gler

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