220 28310 article Path: news.gmane.org!.POSTED!not-for-mail From: Matthew Woehlke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Something is better then nothing: Please, relax Initializer List use with operators! 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spf=neutral (google.com: 195.159.176.226 is neither permitted nor denied by best guess record for domain of gclcip-std-proposals@m.gmane.org) smtp.mailfrom=gclcip-std-proposals@m.gmane.org; dmarc=fail (p=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:28310 Archived-At: On 2016-09-18 09:08, Nicol Bolas wrote: > On Sunday, September 18, 2016 at 8:41:39 AM UTC-4, mihailn...@gmail.com= =20 > wrote: >> Back to the example - it is hard to explain why this is working: >> Point p; >> p =3D {-1, -1};=20 >> >> but this is not: >> >> if(p =3D=3D {-1, -1}) return; > > Because one is assignment while the other is equality testing. They're tw= o=20 > different things. >=20 > Why is that hard to explain? ....because it *doesn't make sense*. Consider: foo.operator@({...}) // works for any operator foo @ {...} // only works for '=3D' ....where `@` is replaced with a valid operator, e.g. `=3D=3D`, `*`, `+`, etc. Why *should* it work with `=3D` but not other operators? Consider also: A a; B b; // B is convertible to A a.operator@(b); // works a @ b; // works Why do these mean the same *except* when `b` is replaced with an initializer list *and* the operator is not `=3D`? That just feels like an arbitrary rule that isn't otherwise consistent with the language. >> *But this is actually relatively easy to explain why it's not working*: >> if({-1, -1} =3D=3D p) return; > > Um, no. If we allow `X =3D=3D Y` to work, then it makes sense that we all= ow `Y=20 > =3D=3D X` to work. As Johannes already noted, your logic is faulty (non sequitor): // Let 'a' be a complex type with 'bool operator=3D=3D(bool) const' a =3D=3D true; // okay true =3D=3D a; // error Operators =E2=80=94 even the comparison operator =E2=80=94 are not necessar= ily commutative. (Similarly, I could readily create some types for which `(a =3D=3D b) !=3D (b =3D=3D a)`.) I am *quite* happy to argue that `{...} =3D=3D expr` should not be allowed. More accurately, it should be allowed iff there exists a conversion from `expr` to std::initializer_list *and* there exists an operator=3D=3D for std::initializer_list, just like if the LHS was any other type. (IOW, since those conditions are false, it follows that the operation is not permitted, *according to the same rules that apply to any other LHS type*. This is therefore logical and consistent with the language as a whole.) > *Bonus:* >> This must work, it really must work: >> const auto e =3D car ? car->engine() : {}; >> We know it does not but it should, no reason why not - first arg is=20 >> given, second arg is given, the the third must the same type as the seco= nd,=20 >> so we have everything to deduce it! >=20 > Again, that goes back to consistency. `!car ? {} : car->engine()` is=20 > conceptually the same, yet it wouldn't work under your scheme. Why couldn't it? The compiler ought to be able to deduce what to do with an initializer list as one half of a ternary, regardless of which half it appears in. The rule here is simple; if one "half" of a ternary is an initializer list and the other is not, and there exists a conversion to the type of the other "half" from an initializer list, then that conversion is implicitly applied to the initializer list. --=20 Matthew --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/nrru5o%24sa6%241%40blaine.gmane.org. .