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Subject: Re: [std-proposals] count-based for loop
From: Andrew Tomazos (andrewtomazos_at_[hidden])
Date: 2019-08-29 12:21:19


I plan to target C++23 with it.

I proposed a similar core language feature in 2016 in P0286:

    http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0268r0.pdf

but the proposal was rejected.

On Fri, Aug 30, 2019 at 2:46 AM Sophia Poirier <spoirier_at_[hidden]> wrote:

> Thanks Andrew, that's a great collection of utilities. Is that a proposal
> you are working towards submitting? (I note it has no paper number.)
>
> I do prefer to see it language level as I think it is intuitive as I have
> suggested, plus it is such a basic and fundamental logical operation (and
> allows serving those who cannot use the STL), however I do recognize there
> are fewer hurdles to landing library features.
>
> - Sophia
>
>
> On Aug 28, 2019, at 5:23 AM, Andrew Tomazos <andrewtomazos_at_[hidden]>
> wrote:
>
> See:
>
>
> https://docs.google.com/document/d/1gBdBualdIU1bpgW_El4GT-p0he9Yr7D52xmLmHes5Qo/edit?usp=sharing
>
> On Wed, Aug 28, 2019 at 3:42 AM Sophia Poirier via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> On Aug 27, 2019, at 10:26 AM, Barry Revzin <barry.revzin_at_[hidden]>
>> wrote:
>>
>> On Tue, Aug 27, 2019, 12:02 PM Sophia Poirier via Std-Proposals <
>> std-proposals_at_[hidden]> wrote:
>>
>>> Thanks, yes I have that as an alternate example in my longer notes. My
>>> understanding is that it still suffers from the type-matching problem as
>>> traditional for loops. Your example of:
>>>
>>> for (const auto i : std::views::iota(0, count))
>>>
>>> only works when count is an int. Otherwise, if for example count is
>>> uint32_t, it would need to be:
>>>
>>> for (const auto i : std::views::iota(uint32_t{0}, count)
>>> or:
>>> for (const auto i : std::views::iota(0u, count))
>>> or:
>>> for (const auto i : std::views::iota<uint32_t>(0, count))
>>> or:
>>> for (const auto i : std::views::iota<decltype(count)>(0, count))
>>>
>>> or something along those lines, or you will get template instantiation
>>> failure compiler error. I think that if std::views::iota had a constructor
>>> overload that was simply the second argument (count) with implicit zero
>>> start, then it would be a good option. However I believe there is interest
>>> to reserve such an overload perhaps for infinite ranges?
>>>
>>> thanks,
>>> Sophia
>>>
>>
>> This is true. But we can write a helper function to get the correct type
>> of 0 so we don't need the ugliness at point of use:
>>
>> template <std::integral T>
>> auto upto(T n) {
>> return views::iota(T{0}, n);
>> }
>>
>> We end up with:
>>
>> for (const auto i : upto(count))
>>
>> Barry
>>
>>>
>> This could be the basis of an alternate library proposal, true.
>>
>> - Sophia
>>
>
>



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