Re: Item 22
On 5 Mrz., 19:26, "werasm" <wer...@gmail.com> wrote:
Stefan Chrobot wrote:
Later the Author states:
"Note that replacing the three-line body of Swap() with the single line
"swap( pimpl_, other.pimpl_ );" is not guaranteed to work correctly,
because std::swap() will not necessarily work correctly for auto_ptrs."
I don't think the standard (98 at least) requires that std::swap be
specialized for auto_ptr. In fact, I think it requires std::swap not
being specialized.
[98:25.2.2]
- 1 Requires: Type T is assignable (23.1).
- 2 Effects: Exchanges values...
This is not true for auto_ptr (where breaks the post-condition in
Table 64 that t should be equivalent to u).
Your argumentation is correct for both and 14882:1998 and
14882:2003.
But most recently there are some active moves to a more
relaxed definition of Swappable, see the most recent draft
N2134 available from
http://www2.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2134.pdf
and which is expressed in 20.1.4/2:
"The Swappable requirement is met by satisfying one or more
of the following conditions:
- T is Swappable if T satisfies the CopyConstructible requirements
(20.1.3) and the Assignable requirements (23.1);
- T is Swappable if a namespace scope function named swap
exists in the same namespace as the definition of T, such that
the expression swap(t,u) is valid and has the semantics described
in Table 33."
Note that this extended interpretation also allows Swappability
for types like std::auto_ptr and it's newly proposed replacement
unique_ptr (not yet found in N2134):
template <class T, class D>
inline void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y);
template <class T, class D>
inline void swap(unique_ptr<T, D>&& x, unique_ptr<T, D>& y);
template <class T, class D>
inline void swap(unique_ptr<T, D>& x, unique_ptr<T, D>&& y);
Theoretically one could provide similar declarations of
swap overloads for auto_ptr also, but maybe they wont, if
auto_ptr will be deprecated.
Note that there are also proponents for an even laxer (and
maybe more precise) rule, see
http://www2.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#594
Greetings from Bremen,
Daniel Kr?gler
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