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Re: Fix move_if_noexcept usages in _Hashtable


On 12/4/18 3:38 PM, Jonathan Wakely wrote:
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.

Perhaps but the Standard mandates to call allocator construct so we don't have choice. It is surprising to consider value_type move constructor when we don't know what allocator construct does.

Most users do not change from std::allocator or, even if they do, do not implement construct so impact should be limited.

  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 ?

No feedback on this point ? Is using std::pair piecewise constructor ok ?


  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

Ok, nice, allowing emplace build of values would be even better, I'll have a closer look.

  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.


Ok, I'll try.


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