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Subject: Re: [std-proposals] Operator functions to fix ADL
From: Barry Revzin (barry.revzin_at_[hidden])
Date: 2020-10-11 14:48:59


On Sun, Oct 11, 2020 at 9:16 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.
>

We can't deprecate ADL, we're never going to remove it - there's too much
code that depends on its existence and there is little benefit to changing
it

>
> 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
>

Marking customization points is certainly worthwhile - to have a more
refined lookup mechanism than whatever is in a namespace. I really like
Matt Calabrese's idea here in P1292:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1292r0.html. You
declare a function as being customizeable, and you declare your functions
that customize it. That way, you wouldn't have to annotate the call-site or
provide a separate library mechanism to do dispatch.

Barry



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