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From: Kazutoshi Satoda <k_satoda@f2.dion.ne.jp>
Newsgroups: comp.std.c++
Subject: Contradicting definition of empty shared_ptr on shared_ptr(nullptr, d)
Date: Sat, 23 Jun 2012 13:13:02 -0700 (PDT)
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Xref: csiph.com comp.std.c++:529

20.7.2.2/1 (N3376) says:
>
> A shared_ptr object is empty if it does not own a pointer.

Please note that it says "own a pointer". This definition was added as
the resolution for LWG defect #813.
813. "empty" undefined for shared_ptr
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#813

20.7.2.2.1/9 says about the effect of shared_ptr(nullptr_t p, D d):
>
> Effects: Constructs a shared_ptr object that owns the object p and the
> deleter d.

Please note that it says "owns the object". This was intentionally
changed from "the pointer" as a part of resolution for LWG defect #758,
to cover nullptr_t case.
758. shared_ptr and nullptr
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#758

Since shared_ptr(nullptr, d) owns an object of type nullptr_t, but does
not own a pointer, it is said as "empty" by a strict reading of the
above definition.

This causes a contradiction:
20.7.2.2.1/10 sets a postcondition use_count() == 1 on
shared_ptr(nullptr, d). But 20.7.2.2.5/7 says the return value of
use_count() is "0 when *this is empty".


Proposed resolution:
Replace the last 2 words in 20.7.2.2/1
 ... empty if it does not own a pointer.
to
 ... empty if it does not own an object.


Besides that, I want to know if it is (or was) possible or not to define
shared_ptr(nullptr) (including some variants with deleter and allocator)
is empty. It seems to be less surprising. FYI, please see the following
question on stackoverflow.
http://stackoverflow.com/questions/11164354/does-the-standard-behavior-for-deleters-differ-between-shared-ptr-and-unique-ptr

--
k_satoda


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