std::vector default default and move constructors

[Jonathan Wakely]

On 15/01/18 22:32 +0100, François Dumont wrote:
>On 15/01/2018 13:29, Jonathan Wakely wrote:
>>On 14/01/18 22:13 +0100, François Dumont wrote:
>>>On 11/01/2018 07:57, Marc Glisse wrote:
>>>>On Thu, 11 Jan 2018, François Dumont wrote:
>>>>
>>>>-Â Â Â Â Â Â  void _M_swap_data(_Vector_impl& __x) _GLIBCXX_NOEXCEPT
>>>>+Â Â Â Â Â Â  void
>>>>+Â Â Â Â Â Â  _M_swap_data(_Vector_impl_data& __x) _GLIBCXX_NOEXCEPT
>>>>Â Â Â Â Â Â Â  {
>>>>Â Â Â Â Â Â Â Â Â  std::swap(_M_start, __x._M_start);
>>>>Â Â Â Â Â Â Â Â Â  std::swap(_M_finish, __x._M_finish);
>>>>Â Â Â Â Â Â Â Â Â  std::swap(_M_end_of_storage, __x._M_end_of_storage);
>>>>Â Â Â Â Â Â Â  }
>>>>
>>>>
>>>>I don't remember earlier discussions about this patch, but is 
>>>>this piece of code still needed? std::swap(*this, __x) looks 
>>>>like it should work.
>>>>
>>>This patch has been arround for quite a long time now, initial 
>>>submission was:
>>>
>>>https://gcc.gnu.org/ml/libstdc++/2017-07/msg00060.html
>>>
>>>even if I sent several updates since then.
>>
>>I know, but it doesn't actually fix any bugs so has been low priority
>>and I've not been able to make time to finish reviewing it.
>No problem, it just make the implementation more compiler friendly by 
>leaving as much job as possible to it.
>>
>>In fact it introduces a serious regression because of this line:
>>
>>>-Â Â Â Â Â  vector(vector&& __x) noexcept
>>>-Â Â Â Â Â  : _Base(std::move(__x)) { }
>>>+Â Â Â Â Â  vector(vector&&) = default;
>>
>>Consider what happens if we have an allocator that is not
>>nothrow-move-constructible, which can happen if the allocator has a
>>noexcept(false) move constructor, or more likely just has no move
>>constructor but has a noexcept(false) copy constructor:
>>
>>#include <vector>
>>
>>template<typename T> struct Alloc : std::allocator<T> {
>>Â Alloc() { }
>>Â Alloc(const Alloc&) { }
>>Â template<typename U> Alloc(const Alloc<U>&) { }
>>Â template<typename U> struct rebind { using other = Alloc<U>; };
>>};
>>
>>int main()
>>{
>>Â static_assert( std::is_nothrow_move_constructible<std::vector<int, 
>>Alloc<int>>>::value, "" );
>>}
>>
>>The standard says this program must compile, and with current GCC
>>trunk it does compile.
>>
>>After your change it fails, because the vector(vector&&) constructor
>>is not explicitly noexcept, instead it deduces its exception
>>specification from its base classes, which includes the allocator.
>
>Shouldn't gcc raise a compilation error ? At least if the use 
>explicitely qualified its alloc move constructor with noexcept(true).
>
>I should be more careful when the Standard explicitely qualified 
>constructor, and I might need an up to date version of the Standard 
>too.


The latest draft is always available from https://isocpp.org (on the
left sidebar).

>We can still write:
>
>+Â Â Â Â Â  vector(vector&&) noexcept = default;
>
>You already pointed to me recently on the istreambuf_iterator copy 
>constructor that I needed to keep the noexcept qualification.

That would still be implicitly noexcept(false) because of the base
class, but now you'd get an error because the explicit exception
specification doesn't match the implicit one.

>It can even be added on the _Vector_impl so that vector(vector&&) 
>benefit from it but as you said it is better to make it explicit on 
>the vector move constructor.

One of the move constructors needs to be defined with a function body,
not defaulted, so that it can also be declared noexcept.