Re: Fix move_if_noexcept usages in _Hashtable

[Jonathan Wakely <jwakely at redhat dot com>]

On 04/12/18 07:45 +0100, François Dumont wrote:
> Hi
>
>   This patch fix a minor problem with usage of std::move_if_noexcept. 
> We use it to move node content if move construtor is noexcept but we 
> eventually use the allocator_type::construct method which might be 
> slightly different. I think it is better to check for this method 
> noexcept qualification.

This is likely to pessimize some code, since most allocators do not
have an exception-specification on their construct members.

>   Moreover I have added a special overload for nodes containing a 
> std::pair. It is supposed to allow move semantic in associative 
> containers where Key is stored as const deleting std::pair move 
> constructor. In this case we should still move the Value part.
>
>   It doesn't work for the moment because the std::pair piecewise 
> constructor has no noexcept qualification. Is there any plan to add it 
> ? I think adding it will force including <tuple> in stl_pair.h, is it 
> fine ?
>
>   If this evolution is accepted I'll adapt it for _Rb_tree that has 
> the same problem.
>
>   Working on this I also notice that content of initialization_list is 
> not moved. Is there a plan to make initialization_list iterator type 
> like move_iterator ? Should containers use 
> __make_move_iterator_if_noexcept ?

No.

Whether to allow moving from std::initializer_list is an active topic,
see
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1249r0.html

>   Tested under Linux x86_64 normal mode.
>
>   Ok to commit this first step ?

No, this is not suitable for stage 3. It seems too risky.

We can reconsider it during stage 1, but I'd like to see (at least) a
new test showing a bug with the current code. For example, a type with
a move constructor that is noexcept, but when used with a
scoped_allocator_adaptor (which calls something other than the move
constructor) we incorrectly move elements, and lose data when an
exception happens.