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Subject: Re: [std-proposals] count-based for loop
From: John McFarlane (john_at_[hidden])
Date: 2019-08-29 02:02:23


On Wed, 28 Aug 2019 at 13:23, Andrew Tomazos via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> See:
>
>
> https://docs.google.com/document/d/1gBdBualdIU1bpgW_El4GT-p0he9Yr7D52xmLmHes5Qo/edit?usp=sharing
>

Thanks for writing this. I think it takes us in the right direction.

I wonder whether the name `upto` should change: `upto(10)` doesn't count
'up to' 10 in the sense most users will expect. If it remains, it will mark
the lesson in which students gain hard-earned knowledge about half-closed
ranges.

`indices` should use `ssize`, and not `size`. We should be pushing to
encourage signed integers in this situation as `i` is a quantity. This
reflects advice in ES.102
<https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es102-use-signed-types-for-arithmetic>
of the C++ Core Guidelines.

John

>
> 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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>>
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