Re: Move, std::vector and gcc: which is this correct?

From:
=?UTF-8?B?RGFuaWVsIEtyw7xnbGVy?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 18 Oct 2012 06:16:45 -0700 (PDT)
Message-ID:
<k5o8io$2n4$1@dont-email.me>
On 2012-10-18 01:25, Edward Rosten wrote:

I have a simple program below which defines or defaults various
combinations of move and copy constructors and puts the classes into a
std::vector.


[snip]

This is what I expected. When the vector resizes itself, the move
constructor is always called. In the final case, this is implied by the
copy constructor not being called.


[snip]

In other words, when both copy and move are defined explicitly, the copy
constructor gets called but if the move constructor is defaulted, then it
gets called in preference to the copy constructor.

I would have expected the GCC 4.6 behaviour to be correct. Am I mistaken?


You are mistaken. Let me explain a bit the reasoning of my assertion:

Originally move construction was considered as a "must-be-nothrow"
operation in the library, but during the standardization process several
good reasons were brought on the table that argued in favour for
allowing potentially throwing move-operations. Potentially throwing
move-operations were the main reason why noexcept was invented as both
deduction mechanism and exception-specifier. A fundamental difference of
move operations versus copy operations is that the former can
irreversibly change the state of the *source* of the move. This would
mean that a throwing move prevents that an operation can hold the strong
exception guarantee, that is, either the operation has succeeded or
there are no effects.

If you look at the specification of vector's push_back (and some other
insertion operations) you find the following specification:

"If an exception is thrown other than by the copy constructor, move
constructor, assignment operator, or move assignment operator
of T or by any InputIterator operation there are no effects. If an
exception is thrown by the move constructor of a non-CopyInsertable T,
the effects are unspecified."

So for every type that has a copy constructor (using the
std::allocator), the standard requires that vector's push_back either
succeeds or has no effects. If your type has a potentially throwing move
constructor (This is so in your example), it has to fall-back to a copy
operation when it can. If there is no copy possible, there is no more
the guarantee that this operation has the strong exception guarantee.
This explains why gcc 4.7 correctly falls back to copy operations for
type Both (The first move can be done safely, because it does not affect
the existing container elements), because it has a potentially throwing
move constructor and an accessible copy constructor. For a move-only
type an implementation is no longer bound to the "no-effects-on-failure"
constraint (It would be impossible to realize that), therefore uses the
move constructor irrespective whether it could throw an exception or not.

There is a simple way to realize that only your move-operations are
called instead of copy operations for type Both: Ensure that the move
constructor does not throw operations and mark it with the equivalent of
noexcept(true) or throw().

HTH & Greetings from Bremen,

Daniel Kr??gler

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