Hi, everybody.
 
I’m sorry. It was my mistake. I read the phrase too quickly without taking into account its details.
 
But I have some more question.
 
The first one is where is there in the C++ Standard the definition of the term «synthesized three-way comparison»? What is the difference between the «synthesized three-way comparison» and the «three-way comparison»?
 
Another question is relative to this phrase
 
«2 A defaulted <=> or == operator function for class C is defined as deleted if any non-static data member of C is of reference type or C has variant members (11.5.2).»
 
Why are not relational operator functions mentioned in this phrase as deleted in this case? 
 
With best regards,
(Vlad from Moscow) 
 
You can meet me at http://cpp.forum24.ru/ or www.stackoverflow.com or http://ru.stackoverflow.com
 
 
Среда, 21 декабря 2022, 0:19 +03:00 от Lénárd Szolnoki via Std-Discussion <std-discussion@lists.isocpp.org>:
 
On Mon, 19 Dec 2022 21:22:37 +0100
Edward Catmur <ecatmur@googlemail.com> wrote:
 
> On Mon, 19 Dec 2022 at 21:09, Lénárd Szolnoki <cpp@lenardszolnoki.com>
> wrote:
>
> >
> >
> > On 19 December 2022 19:05:33 GMT, Edward Catmur
> > <ecatmur@googlemail.com> wrote:
> > >On Mon, 19 Dec 2022 at 19:05, Lénárd Szolnoki
> > ><cpp@lenardszolnoki.com> wrote:
> > >
> > >> Hi,
> > >>
> > >> On 19 December 2022 17:10:18 GMT, Edward Catmur
> > >> <ecatmur@googlemail.com
> > >
> > >> wrote:
> > >> >On Mon, 19 Dec 2022 at 17:56, Lénárd Szolnoki via
> > >> >Std-Discussion < std-discussion@lists.isocpp.org> wrote:
> > >> >
> > >> >> My reading in pseudocode it would look something like:
> > >> >>
> > >> >> /* ...comparing corresponding elements xi and yi in the
> > >> >> expanded
> > lists
> > >> of
> > >> >> subobjects for x and y (in increasing index order) until the
> > >> >> first
> > >> index i
> > >> >> where xi == yi yields a result value which, when contextually
> > >> >>
> > converted
> > >> to
> > >> >> bool, yields false. */
> > >> >> bool mismatch_found = false;
> > >> >> for (size_t idx=0; idx < member_count; ++idx) {
> > >> >> if (not (lhs_members[idx] == rhs_members[idx]) {
> > >> >> mismatch_found = true;
> > >> >> break;
> > >> >> }
> > >> >> }
> > >> >>
> > >> >> // If no such index exists, ...
> > >> >> if (not mismatch_found) {
> > >> >> return true; // ... V is true.
> > >> >> } else { // Otherwise, ...
> > >> >> return false; // ... V is false.
> > >> >> }
> > >> >>
> > >> >> The first part describes the side effects of the defaulted
> > definition,
> > >> and
> > >> >> the short-circuiting in particular. The second part describes
> > >> >> the
> > >> returned
> > >> >> value.
> > >> >>
> > >> >> Obviously this can be written more clearly and concisely in
> > pseudocode.
> > >> >> Would be nice if it could similarly be simplified in English.
> > >> >>
> > >> >
> > >> >Close, but `not` doesn't necessarily do what it should. I'd say
> > >> >this
> > would
> > >> >be more correct and idiomatic:
> > >>
> > >> Yes, you are right. At first I wanted to write !=, but then I
> > "corrected"
> > >> myself... It's pseudocode :)
> > >>
> > >> >/* ...comparing corresponding elements xi and yi in the
> > >> >expanded lists
> > of
> > >> >subobjects for x and y (in increasing index order) until the
> > >> >first
> > index i
> > >> >where xi == yi yields a result value which, when contextually
> > converted to
> > >> >bool, yields false. */
> > >> >size_t idx=0;
> > >> >for (; idx != member_count; ++idx) {
> > >> > if (lhs_members[idx] == rhs_members[idx]) {
> > >> > ;
> > >> > } else {
> > >> > break;
> > >> > }
> > >> >}
> > >> >
> > >> >// If no such index exists, ...
> > >> >if (idx == member_count) {
> > >> > return true; // ... V is true.
> > >> >} else { // Otherwise, ...
> > >> > return false; // ... V is false.
> > >> >}
> > >>
> > >> I wonder if the English wording would be clearer if the if-else
> > >> would be switched around.
> > >>
> > >> The current wording reads like:
> > >>
> > >> if (no such index exists) {
> > >> V is true
> > >> } else {
> > >> V is false
> > >> }
> > >>
> > >> I try to avoid if(!cond)...else in code, as now the else branch
> > >> has a double negative.
> > >>
> > >> Replacing the last part of the wording with this might be
> > >> clearer:
> > >>
> > >> "If such index exists, V is false. Otherwise V is true."
> > >>
> > >
> > >I think part of the reason for the current wording is that it
> > >parallels that for spaceship, where the double negative does make
> > >sense: https://eel.is/c++draft/class.compare#class.spaceship-3
> >
> > This would be a good point, if the wording here actually mirrored
> > [class.eq], but it does not. Here the value for the "no such
> > index" case is described later to the short-circuit case. An
> > equivalent [class.eq] wording would be the following:
> >
> > The return value V of a defaulted == operator function with
> > parameters x and y is determined by comparing corresponding
> > elements xi and yi in the expanded lists of subobjects for x and y
> > (in increasing index order) until the first index i where xi == yi
> > yields a result value which, when contextually converted to bool,
> > yields false; V is false. If no such index exists, V is true.
> >
>
> You're completely correct; I was looking at the overall form (and the
> last sentence). And that "equivalent" wording is, if a bit clumsy,
> still more unambiguously readable that what we have today.
>
> >>Of course in real code I would just return from the loop on
> >>mismatch, but
> > I
> > >> don't know how to convey that in English.
> > >>
> > >
> > >It's tricky; English (and in particular Standardese) isn't really
> > >lent to conveying that kind of algorithm. Quite honestly I don't
> > >feel that there's much need to improve the wording; most people
> > >who read the specification itself will know how to write a
> > >short-circuiting equality operator anyway, so need only check that
> > >the wording can be read with that in mind.
> >
> > It's of course subjective. I agree that the current wording is
> > unambiguous, but I also needed to read it a couple of times to
> > parse the last "otherwise". And apparently several other people
> > also had trouble with it.
> >
> > It's probably a very minor thing to nitpick about, but it might
> > worth an editorial change.
> >
>
> Yes, editorial should do it. Do you want to enter a PR?

I raised https://github.com/cplusplus/draft/issues/6036 .

I didn't want to commit to one particular wording, so no PR.

Cheers,
Lénárd

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