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Re: [std-proposals] Relocation in C++

From: Edward Catmur <ecatmur_at_[hidden]>
Date: Tue, 31 May 2022 10:24:27 -0600
Sorry, looks like we crossed in flight.

On Tue, 31 May 2022 at 10:06, S├ębastien Bini <sebastien.bini_at_[hidden]>
wrote:

> > But you need std::relocate, AFAIK, in order to implement an
> "emplace_back-by-relocating" operation.
>
> What is proposed is:
> std::vector<T, Alloc>::push_back(std::relocate_t, T value)
> {
> /* relocate value using std::relocate_at at the end of the vector */
> }
> std::relocate_t being a tag type to distinguish between other push_back
> overloads. It is then called like that:
>
> std::vector<T> vec;
> T obj;
> vec.push_back(std::relocate, reloc obj); // with inline constexpr
> std::relocate_t std::relocate{};
>
> That way the object is relocated into the function parameter and then from
> the function parameter to the vector. There may be ways to alleviate that
> extra relocation.
>
> It is on one hand clearer to write, and on the other hand has the
> advantage of allowing you to relocate local variables.
>

I don't see the need for push_back to take a tag parameter at all; we can
just add push_back by value: void push_back(T value);. Possibly the
overload resolution rules will need updating, or the existing overloads
(T&& / T const&) could be removed.

> q1 has no "move assignment operator", remember? We're postulating that
> gsl::non_null<int*> is one of these types that lacks a moved-from state. So
> it certainly cannot have a move assignment operator nor a move constructor.
>
> Yes, I am telling you the rules as of today. The focus wasn't put on reloc
> assignment as you pointed out. I do believe there is less need to
> "reloc-assign" than to "reloc-construct", as they solve different problems.
>

"reloc-assign" is just assignment by value: operator=(T). See my email
just now.

Received on 2022-05-31 16:24:40