On Mon, Jul 1, 2024 at 12:25 PM Daniel Krügler via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> Am Mo., 1. Juli 2024 um 17:33 Uhr schrieb Yongwei Wu via
> Std-Discussion <std-discussion_at_[hidden]>:
> >
> > On this page <URL:
> https://en.cppreference.com/w/cpp/container/vector/vector>,
> > I saw the wording "Move constructor. Constructs the container with the
> > contents of other using move semantics.... After the move, other is
> > guaranteed to be empty()."
> >
> > Does the standard really provide this empty() guarantee (for vector,
> > or any other containers)? I cannot find it. I only see the general
> > specification that a moved-from object is in a valid but unspecified
> > state.
> >
> > (If it does not provide this guarantee, should it?)
>
> The wording doesn't say that the post-condition is an empty state, but
> it has the requirement for all sequence containers (except for
> std::array) that the move operation has a constant complexity. This
> more or less implies an empty state for std::vector, since the
>

"more or less" - yes.

There is only one reasonable way to implement the move constructor for
std::vector, and it will leave the source object empty.

One can imagine a deliberately hostile implementation of the move
constructor, which works like this: first, the destination object steals
the buffer from the source object. Then, only for a trivially copyable type
such as `int`, the source object attempts to allocate a new buffer and
construct some elements into it. If the allocation fails, the exception is
swallowed and the source object is left empty. If the allocation succeeds,
we have a non-empty source object and the implementation hasn't violated
any of the rules in the standard.


> move-target will typically just "steal" the guts from the move-source.
> Since you have the valid-but-unspecified guarantee you can call every
> function without preconditions (Such as size() or capacity()) and
> inspect the actual situation, if that would be relevant for you.
>
> Thanks,
>
> - Daniel
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-- 
*Brian Bi*