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Re: [SG20] A draft paper to fix the range-based for loop to make it teachable

From: Nicolai Josuttis <nico_at_[hidden]>
Date: Tue, 10 Nov 2020 16:35:48 +0100
Am 10.11.2020 um 15:38 schrieb Amir Kirsh:
> The problem is there. No argument. And the paper presents it clearly and
> brilliantly.
> And it may bite our students.
> But I would argue that the problem is with using temporaries, not
> specifically with range-based-for.
> Yes, there is a need to teach our students: beware of temporaries and
> lifetime extension, it may bite you.
> For example: don't return a const reference to a parameter that you got
> as const ref. It might be a temporary (great example I send my students
> to is here <http://www.youtube.com/watch?v=lkgszkPnV8g&t=13m0s>).
> But fixing just the range-based-for may lead to even more confusion.
> That would require additional thought of how to explain things, like:
> class Person {
> std::string name;
> public:
> Person(const std::string& name): name(name) {}
> const std::string& getName() const {
> return name;
> }
> ~Person() {
> name = "dead!";
> }
> };
> int main() {
> // [a]
> auto&& name = Person("Mo").getName(); // UB
> std::cout << name << std::endl; // UB, may print "dead!"
> // [b]
> for(char c: Person("Mo").getName()) { // UB
> std::cout << c;
> } // UB, may print "dead!", should it become a defined behavior
> and print "Mo"? while leaving the above UB?
> }
> Having the desired fix for range-based-for only, when students would see
> [b] and use it in cases like [a], we would have to teach them: "no, it
> is allowed only in range-based-for, which has its own special rules".
It seems we all are incredible blind now because we see auto&& when
using the range-based for loop.
The goal of my whole proposal is NOT to see it anymore but take see it
as a function call like
  foreach(char c, Person("Mo").getName())

> If both [a] and [b] could have a defined behavior, that could be an
> interesting choice, which would be of course less bug prone and easier
> to teach.
> But since we are not dealing here with the life-time extension rules in
> general, I think there is no way to avoid teaching the fact the
> temporaries have their "strange behavior" and then I'd say it's better
> to have the same "strange behavior" for both [a] and [b] above, to
> eliminate any additional confusion.
[b] hould be no longer an example equivalent to a.
I can teach it then as a black box that iterates over the elements
(until I reveal details but only for performance details).

> For the purpose of having less bug prone code - fixing just the
> range-based-for, [b] above, and leaving the other cases, [a] above, stay
> UB - is an option.
> But if the motivation is teaching, I'd argue it would not make it any
> easier to teach. Just yet another additional difference of behavior
> between two quite similar expressions (no worries, we already go with
> the students via initialization rules, they are used to strange
> things... ;-).
> Of course that's only my subjective view.
> By the way,
> I do teach my students range-based-for and encourage them to use it.
> And I do warn them about the tricky life-time extension rules for
> temporaries, saying that if you are not sure, don't use a temporary, or
> make yourself absolutely positively sure that you use it correctly.
> Amir
> On Tue, Nov 10, 2020 at 3:21 PM Detlef Vollmann via SG20
> <sg20_at_[hidden] <mailto:sg20_at_[hidden]>> wrote:
> On 11/10/20 10:24 AM, Nicolai Josuttis via SG20 wrote:
> > May be SG20 can support the paper as a whole,
> > because IMO we really have to heal C++ to make it teachable again.
> You could ask SG12 as well.
> > Note: So far I was not a member of this email list;
> > so forgive me if this was discussed before.
> Maybe it was discussed in SG12.
> Detlef
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Nicolai M. Josuttis
+49 (0)531 / 129 88 86
+49 (0)700 / JOSUTTIS
 C++: http://cppstdlib.com, http://cppstd17.com,
      http://tmplbook.com, http://cppmove.com
 SOA: http://soa-in-practice.com

Received on 2020-11-10 09:35:53