C++ Logo

STD-PROPOSALS

Advanced search

Subject: Re: [std-proposals] std::duplicate
From: Gašper Ažman (gasper.azman_at_[hidden])
Date: 2019-10-02 13:02:38


Fair enough :).

Seems like a plausible proposal. The name is bikesheddable, because it's
not quite the same as auto{value}, which I would consider to be "duplicate"
- it always creates a new object, unlike the proposed std::duplicate.

Are you intending to write the proposal?

On Wed, Oct 2, 2019 at 6:59 PM Michael Daum <mike.daum_at_[hidden]> wrote:

> Pass by value could be argued to be the current best practice, but:
> - It can be slightly sub-optimal by one move to the best practice I am
> proposing.
> - Rather than provide the caller with a clear choice on lvalue parameters,
> it will silently copy in if the lvalue parameter is not moved
> - Value parameters in the signature of the function provided no indication
> that they are intended to be sunk
> Which is why I think all around I have come around to liking && for sink
> params.
>
> On Wed, Oct 2, 2019 at 1:46 PM Gašper Ažman <gasper.azman_at_[hidden]>
> wrote:
>
>> Not to bash on your proposal - but why should sink parameters not be
>> by-value? Duplicate is default, move an optimization.
>>
>> There is also auto{v} which gives you a copy if lvalue, or a moved-to if
>> rvalue.
>>
>> G
>>
>> On Wed, Oct 2, 2019, 17:03 Michael Daum via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>
>>> I've become quite fond of a utility called `CopyTemp` which exists in
>>> the Unreal Engine code base. So much so that I think that it should be
>>> standardized. For lack of a better name, I'll call it `std::duplicate`.
>>>
>>> Like std::move, std::duplicate would take an argument of any value
>>> category and provide a return which is an rvalue. However, std::duplicate
>>> would accomplish this by copying when it had to. Sample Implementation
>>> <https://godbolt.org/z/7nIJNT>
>>>
>>> Once we have this utility, we are able to say that the best practice for
>>> providing a sink parameter is to declare it with rvalue reference type.
>>> Attempts to call the sink with lvalue arguments will cause a compiler error
>>> forcing to caller to either `std::move` or `std::duplicate` the arg when
>>> passed into the call. Compilers would be able to provide very clean,
>>> readable errors if they wanted to, and the resulting code would be optimal
>>> in terms of number of copies and moves.
>>> Example <https://godbolt.org/z/Q_E_J->
>>>
>>> The strengths of this proposal are:
>>> - It is library only
>>> - It is tiny, self contained, and easy to implement
>>> - Helps solve a problem for which we have no great best practice (sink
>>> parameters) in a teachable, optimal way which actually makes the calling
>>> code more safe and readable
>>> --
>>> Std-Proposals mailing list
>>> Std-Proposals_at_[hidden]
>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>>
>>



STD-PROPOSALS list run by std-proposals-owner@lists.isocpp.org

Standard Proposals Archives on Google Groups