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Subject: Re: [std-proposals] Operator functions to fix ADL
From: Dusan Jovanovic (DBJ) (dbj_at_[hidden])
Date: 2020-10-15 02:12:56


Jorg Brown has written: But all you've done at that point is demonstrate
why people like ADL

Faced with the representative of the people, I better prepare my defence
seriously?

"... ADL is widely known to be an essential feature. On the other hand, it is
also unarguably a source of many subtle problems..."

Who said that
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2103.pdf> ... ;)

ps: sometimes they are not but this wikipedia adl article
<https://en.wikipedia.org/wiki/Argument-dependent_name_lookup> might be a
simple gem.

On Wed, 14 Oct 2020 at 16:49, Jorg Brown <jorg.brown_at_[hidden]> wrote:

> > *Let's see where that takes us.*
>
> OK...
>
> I'm curious how you would re-write Arthur's example without ADL. It's
> pretty simple, after all, to write it in such a way that ADL is prevented:
>
> void fn() {
> std::string s;
> auto t = s.operator+(s); // ADL on operator+
> std::cout.operator<<(s); // ADL on operator<<
> (swap)(s, s); // ADL on swap
> }
>
> See https://godbolt.org/z/br6K1q
>
> So now what? How do you accomplish this without ADL?
>
> I mean, you could do it brute-force:
>
> void fn() {
> std::string s;
> auto t = std::operator+(s, s);
> std::operator<<(std::cout, s);
> std::swap(s, s);
> }
>
> But all you've done at that point is demonstrate why people like ADL.
>
> -- Jorg
>
>
> On Tue, Oct 13, 2020 at 4:17 PM Dusan Jovanovic (DBJ) via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> >* ADL is used whenever you write*
>> * std::string s;*
>> * auto t = s + s; // ADL on operator+*
>> * std::cout << s; // ADL on operator<<*
>> * swap(s, s); // ADL on swap*
>>
>> *In particular, the "std::swap two-step" is used inside all swap-based
>> STL algorithms, such as std::rotate and std::sort.*
>> *So you can't actually turn it off, because then you'd break the world
>> (because approximately every C++ program depends on ADL somewhere). But you
>> could issue a warning about it outside of system headers, just to get a
>> sense of the magnitude of the issue.*
>>
>> *> -Arthur*
>>
>> *Perhaps this is why I have written "not-a-minority not using std lib" :)
>> ... you know those pesky C++ projects using -fno-exceptions or even using
>> things like EASTL (gasp! again). I am not a "just-say-no-to-adl" zealot, I
>> am just developing your very same idea further. Let's see where that takes
>> us. Perhaps that might lead to some solution about removing the heavy
>> technical debt of ADL.*
>>
>>
>> *Enough said, peace *
>>
>> *DBJ*
>>
>>
>>
>> On Tue, 13 Oct 2020 at 21:30, Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
>> wrote:
>>
>>> On Tue, Oct 13, 2020 at 10:57 AM Dusan Jovanovic (DBJ) <dbj_at_[hidden]>
>>> wrote:
>>>
>>>> A.O.D. has written: -- I'd be interested to see a compiler patch to
>>>> warn whenever "ADL lookup occurred, on a non-operator, and ADL found a
>>>> best-matching candidate which would *not* have been found by regular
>>>> unqualified lookup." It couldn't be used in production because
>>>> approximately every C++ program depends on ADL *somewhere*. But it
>>>> could give us a sense of the magnitude of the issue. --
>>>>
>>>> Why not just going further: *-fno-adl*
>>>>
>>>> That would certainly "rock the boat". In my not-a-compiler-writer
>>>> naivete, I might think, contrary to A.O.D. (gasp!) a lot of code will
>>>> work unbroken with that switch.? Especially that silent not-minority not
>>>> using std lib.
>>>>
>>>
>>> ADL is used whenever you write
>>> std::string s;
>>> auto t = s + s; // ADL on operator+
>>> std::cout << s; // ADL on operator<<
>>> swap(s, s); // ADL on swap
>>>
>>> In particular, the "std::swap two-step" is used inside all swap-based
>>> STL algorithms, such as std::rotate and std::sort.
>>> So you can't actually turn it *off*, because then you'd break the world
>>> (because approximately every C++ program depends on ADL *somewhere*).
>>> But you could issue a warning about it outside of system headers, just to
>>> get a sense of the magnitude of the issue.
>>>
>>> -Arthur
>>>
>> --
>> Std-Proposals mailing list
>> Std-Proposals_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
>



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