RFC: Allow moved-from strings to be non-empty

[Marc Glisse]

On Fri, 26 Oct 2018, Ville Voutilainen wrote:

> On Fri, 26 Oct 2018 at 12:55, Jonathan Wakely <jwakely@redhat.com> wrote:
>>>> Why not? There are already several, and it helps find bugs. Maybe you
>>>> could convince libc++ to change as well if you want to keep the behavior
>>>> the same?
>>>
>>> What bugs?
>>
>> Assuming the string is empty after a move and appending to it without
>> calling clear() first.
>
> I find it bloody unfortunate that the standard considers that a bug. It 
> seems quite convenient to me that it's possible to have a bag of 
> strings, move some of them out to be processed, and be able to tell the 
> processed ones from the unprocessed ones without having to separately 
> clear them when moving.

We all seem to want different things from move:

1) copy, possibly with a speed up because I don't care what the source 
looks like afterwards. That's what was standardized, and for a small 
string that means that moving it should just copy.

2) move and clear

3) move and destroy

(most likely many others)

The convenience does not seem that great to me, especially if it has a 
performance cost.

>>>>> It does slightly concern me that some users might
>>>>> actually semantically expect a moved-from string to be empty, even
>>>>> though that's not guaranteed, although for non-SSO cases
>>>>> it *is* guaranteed.
>>>>
>>>> Is it? In debug mode, I'd be tempted to leave the string as "moved" (size
>>>> 5, short string so there is no allocation).
>>>
>>> Sigh. Apparently it isn't, because the standard doesn't bother placing
>>> complexity
>>> requirements on string constructors.
>>
>> Writing 5 bytes into the SSO buffer would be constant complexity :-P
>
> Indeed it would, but the standard doesn't seem to have complexity 
> requirements on string constructors at all. If it did, moving a non-sso 
> string would *have to* steal from the source, and the sso case would not 
> have to do that, but could.

I am not sure what you are saying exactly, but I think "noexcept" is (kind 
of) playing the role of a complexity requirement here.

-- 
Marc Glisse