[libcxx-dev] std::vector using copy constructor on existing elements during reserve()

[Duncan P. N. Exon Smith via libcxx-dev]

Concretely, in LLVM, I committed r266909 and r267299 (use SmallVector instead of std::vector) because it didn't make sense to litter an -fno-exceptions code base with `noexcept`.

> On Apr 17, 2019, at 09:39, Duncan P. N. Exon Smith via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> 
> I'm curious about how people feel this should/could behave with -fno-exceptions.  IIRC, Libc++ calls the copy constructor (following the standard), but -fno-exceptions isn't really standard, and from a user perspective that doesn't make sense.  `noexcept` is essentially meaningless with -fno-exceptions.
> 
> Thoughts?
> 
>> On Apr 16, 2019, at 23:19, Eric Fiselier via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> 
>> 
>> 
>> On Tue, Apr 16, 2019 at 2:25 PM Michael Narigon via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> std:vector is using the element copy constructor on existing elements during its resizing operation in reserve(). I would expect it to use the move constructor for efficiency. Is this intended behavior?
>> 
>> Yes, this behavior is mandated by the standard. The copy constructor is used when the move constructor is not noexcept. From cppreference's `std::move_if_noexcept` documentation:
>> 
>> std::vector::resize <https://en.cppreference.com/w/cpp/container/vector/resize>, which may have to allocate new storage and then move or copy elements from old storage to new storage.
>> If an exception occurs during this operation, std::vector::resize <https://en.cppreference.com/w/cpp/container/vector/resize> undoes everything it did to this point, which is only possible if
>> std::move_if_noexcept was used to decide whether to use move construction or copy construction. (unless copy constructor is 
>> not available, in which case move constructor is used either way and the strong exception guarantee may be waived)  
>>  https://en.cppreference.com/w/cpp/utility/move_if_noexcept <https://en.cppreference.com/w/cpp/utility/move_if_noexcept>
>> 
>> If you add `noexcept` to your move constructor, vector should call the move constructor.
>> 
>> 
>>  
>> 
>> This is with the libc++ delivered with Xcode 10.1.
>> 
>> Attached is a sample program displaying the behavior.
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