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Subject: Re: [std-proposals] Operator functions to fix ADL
From: Jody Hagins (coachhagins_at_[hidden])
Date: 2020-10-11 13:07:36


Hi Gregory.

I knew Arthur would be the first to respond :-)

I understand the general issues, andI am in favor of addressing some of the issues surrounding naming and ADL. However, I would like to know why you say "Most of the time invoking ADL is not what a user wanted" - for two reasons.

First, while I know there are issues, I certainly do not have this experience - yes there are issues, but most of the time invoking ADL is exactly what I wanted - it is those few startling times that I don't like. So, I would like to better understand the issues that cause, in your experience, for most usage to result in something unexpected since I'm obviously missing something.

Second, I've noticed a number of good proposals being taken down a slippery slope merely because the reasoning could be considered inflammatory, or in this case possibly exaggerated. Unless, of course, you can give good reasoning to back the claim that "most of the time invoking ADL is not what a user wanted," in which case that reasoning should be presented rather than just a blanket statement that may otherwise derail discussion.

Hmmm... Maybe I'm the one taking it into a rabbit hole this time - I hope not - but this group is partly for that purpose - to bring things up informally so that once things are brought up more officially those things will have already been addressed.

In any case, I would love to know why you make that claim because I obviously need to learn something in my naïveté.

Thanks!

> On Oct 11, 2020, at 10:15 AM, Григорий Шуренков via Std-Proposals <std-proposals_at_[hidden]> wrote:
>
> Hi!
>
> C++ has rather complex name lookup rules for unqualified function calls that involve argument-dependent lookup and may lead to surprising results. And from time to time there appear proposals to fix those rules. This is one of such proposals.
>
> Most of the time invoking ADL is not what a user wanted. However, ADL also has legitimate use cases: operators and "customization points" (swap, begin, end are examples). The idea of the proposal is that we can mark customization points with operator keyword and deprecate ADL for all functions that are not marked with such keyword.
>
> That is if ADL was triggered for an unqualified function call and a function was found via ADL that was not an operator, then a compiler may issue a warning. Currently it's not feasible to have such a warning, because we cannot not discriminate between "good" ADL and "bad" ADL.
>
> Declaration of a custom swap function then will look like this:
> void operator swap (A& fst, A& snd);
>
> There are other things that might be included in the proposal:
> 1) requesting ADL via operator keyword at the call-site: operator swap(a, b);
> 2) searching "operator functions" among class members, thus allowing uniform function call syntax at least for them.
> But before exploring those directions further I would like to get feedback on the main idea.
>
> Regards,
> Gregory
>
>
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