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

Marc Glisse marc.glisse@inria.fr
Thu Oct 25 22:34:00 GMT 2018


On Thu, 25 Oct 2018, Jonathan Wakely wrote:

> When an SSO string is contained in the small string buffer or has an
> unequal allocator a move operation performs a copy, leaving the original
> data in the moved-from string. Setting the length of the moved-from
> string to zero is not required, so we can avoid two writes (to the
> length member and to set the first character to nul) by leaving the
> moved-from string unchanged.
>
> This might be surprising to some users, who (wrongly) expect a string
> to always be empty after a move. Is that acceptable?
>
> It's worth noting that the current behaviour, where we do more work
> than required, also surprises some people, e.g.
> https://stackoverflow.com/q/52696413/981959
>
> Some tests need to be adjusted due to the new behaviour, but they have
> comments saying they're testing specific implementation details, so I
> don't think needing to adjust them is an argument against making the
> change:
>
> // NOTE: This makes use of the fact that we know how moveable
> // is implemented on string (via swap). If the implementation changed
> // this test may begin to fail.
>
>
> Should we make this change?

I didn't read the patch, but from the description above, I say yes.

Interesting that I got my relocate patch in before, because after your 
patch relocate probably helps a little less than before.

-- 
Marc Glisse



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