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

From: Nicolai Josuttis <nico_at_[hidden]>
Date: Fri, 13 Nov 2020 13:07:33 +0100
Hi guys,

attached is a new draft version with several updated due to your
feedback. For example:
- add a tony table
- remove "Heal C++:" in the title
- add Arthur is author
- discuss not to extend lifetime of the optional initializers
- fix the std::ranges example
- add a std::tuple example
- add std::as_const() example
- propose as first option that the new definition does
  not use references and "as-if" code at all
- fix he/she
- ...

Thanks you so much for all of your feedback.
We welcome further feedback, of course.


Am 13.11.2020 um 08:48 schrieb Nicolai Josuttis:
> Am 12.11.2020 um 02:17 schrieb Giuseppe D'Angelo:
>> Hi,
>> On 10/11/2020 16:58, Nicolai Josuttis wrote:
>>> Feel free, to propose this attribute as an orthogonal paper.
>> Thanks for the encouragement, this sounds like a major uptake and an
>> uphill battle though, and I'm kind of new at the game. :)
> your're still welcome
>>>> The other is minor and purely educational, and has to do with the (if
>>>> you want) "straightforward" definition of a range-based for loop in
>>>> terms of a "traditional" for loop. An argument that I sometimes use
>>>> towards using the new shiny toys from C++11 and beyond (range-based,
>>>> lambdas) is that they are truly zero cost abstractions; they are
>>>> *defined* to be exactly like the good ol' C++98 for loops using
>>>> iterators, or function objects, just with convenient syntax.
>>>> If we give special semantics to the range-based for loop (and only to
>>>> that) then this is no longer true, and one risks objections "this thing
>>>> has embedded hidden magic => the costs are not clear w.r.t. a non-ranged
>>>> loop => I am not going to use it".
>>> In the area of compiler optimizations, a traditional for loop
>>> also no longer expands to what we think.
>>> It might for example suddenly introduce vectorization behind the scenes.
>> I wasn't talking about what wonders compiler optimizations can do, I was
>> talking about the "mental model" of what a for loop is for someone
>> coming from C or from C++98. One of the arguments to buy in the "fancy
>> new toys" (ranged-for, lambdas) is that they're almost literally
>> expressed in terms of pre-existing, similar, well established
>> constructs. This generally resonates quite well.
>> (I know and you know that for an optimizing compiler how things are
>> "specified" doesn't really matter that much. For people, it does matter.)
>> On the other hand I have absolutely no idea (yet) how to "sell" the
>> mental model of "OK, this is a for, but it's actually specified as a
>> *function call* with a for inside!".
>> Maybe it's just easier to discuss it in terms of an "algorithmic
>> function" like for_each, kind of a black box as you said before,
>> although the fact that it's a _language_ feature could make it hard to
>> approach it like this.
> I agree,
> it is more like for_each(coll).
> The only question is how to deal with the other "argument" "auto elem" then.
> We pass the collection to iterate over and the declaration for each
> element inside the loop.
> Yes, that's OK to teach.
> Although, people might wonder why the element is a copy, then.
> Nevertheless, this is ONLY a performance issue, not undefined behavior.
>> Do you have some experience to share?
>> --
>> One more technical note. Do you think that with your change proposed,
>> one could also think of dropping the deleted rvalue reference overload
>> of std::as_const? (And possibly similar other functions.) As far as I
>> understand, with your proposed changes, something like
>>> for (auto & elem : std::as_const(getVector())) { ~~~ }
>> would now work. Is it worth including it?
> THAT is an interesting example, thanks.
> In principle, you are right, but exactly to avoid problems like the one
> above as_const() is deleted for temporary objects:
>> template <class T> void as_const(const T&&) = delete;
> So, while this could work now, it still would not compile.
> Thanks a lot.
> Best
> Nico

Nicolai M. Josuttis
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Received on 2020-11-13 06:07:40