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Subject: Re: How much is too much with C++20 concepts?
From: Yehezkel Bernat (yehezkelshb_at_[hidden])
Date: 2020-02-29 22:44:06


For the original example, I'd say this is a good example only if the source
was a template, or an overload set as suggested by Chris. If the function
was just fine with taking the widest integral type (probably `long long`
was a better choice?) and relaying on integral promotion from whatever the
caller sent, there is probably no reason to make it a template, even if by
using a concept.

If it was a template (or a good candidate to become one), then yes, putting
a constraint on it to express its requirements is a good usage of concepts.

On Sun, Mar 1, 2020, 4:35 AM Christopher Di Bella via SG20 <
sg20_at_[hidden]> wrote:

> +1 Though I'd recommend using std::ranges::sort in the body, so readers
> don't think they need to do this themselves by mistake.
>
> On Sat, Feb 29, 2020 at 6:32 PM Tony V E <tvaneerd_at_[hidden]> wrote:
>
>> I've just started on a quick explanation of concepts:
>> https://github.com/tvaneerd/cpp20_in_TTs/blob/master/concepts.md
>>
>> I went with
>>
>> void sort(std::ranges::random_access_range auto & c)
>> {
>> std::sort(c.begin(), c.end());
>> }
>>
>>
>> The definition of random_access_range might be a bit ugly to look at, but
>> the idea is simple - if you don't have random access you probably want to
>> sort a different way.
>>
>>
>>
>> On Sat, Feb 29, 2020 at 8:42 PM Christopher Di Bella via SG20 <
>> sg20_at_[hidden]> wrote:
>>
>>> Concepts exist primarily to express requirements on algorithms. I would
>>> recommend doing your best to introduce them in a light that reflects this
>>> first and foremost. If your foo can work with any integral, *but
>>> requires some integral*, then it could potentially be a candidate.
>>> Although I haven't tried what I'm about to suggest, I can see the use of
>>> foo potentially being a good way forward for introducing templates too:
>>> when showing that multiple overloads that are textually identical otherwise
>>> is cumbersome and error-prone, foo(std::integral auto) is the solution
>>> for integrals (and don't show full template syntax for a while).
>>>
>>> I certainly recommend deferring the design of concepts for a long while,
>>> and to come up with a genuine use-case before showing them off.
>>>
>>> template<typename T>
>>> concept has_size = requires(T t) { t.size(); }; // BAD
>>>
>>> In general, although simple, a concept that starts with has_ is
>>> probably not good to show off at all (unless you're showing it as an
>>> anti-pattern).
>>>
>>> On Sat, 29 Feb 2020, 17:22 Amir Kirsh via SG20, <sg20_at_[hidden]>
>>> wrote:
>>>
>>>> A short intro
>>>> ---
>>>> When teaching a feature I'm always trying to bring the minimal example
>>>> that is still a good one in terms of best practices. I'm trying to avoid
>>>> minimal examples that are convincing but rely on bad practice that might be
>>>> adopted by the students.
>>>>
>>>> now to concepts
>>>> ---
>>>> One of the minimal first examples that can be given with concepts is:
>>>>
>>>> void foo(std::integral auto number) { /* */ }
>>>>
>>>> As a substitute for:
>>>>
>>>> void foo(long number) { /* */ }
>>>>
>>>> But then I'm a bit reluctant. Is it really a proper substitute? The two
>>>> are not the same. What are the pros and cons of having a template function,
>>>> avoiding type conversion, compared to a simple function? Is it a good
>>>> example or a misuse? In which cases this is a good substitute?
>>>>
>>>> I tried to raise this question in SO but didn't get much support for it:
>>>>
>>>> https://stackoverflow.com/questions/60470216/how-much-is-too-much-with-c20-concepts (SO
>>>> question has an additional example inside).
>>>>
>>>> This is not a yes/no question - I do believe things might be a *pro*
>>>> in one case and a *con* in another. Analyzing the pros and cons for
>>>> different use cases can surely assist in understanding the proper choice
>>>> per each case.
>>>> As a teacher I believe this is important, even if not discussed in
>>>> class, to understand the pros and cons.
>>>> (If there are only pros than the example becomes a new "best practice".
>>>> And if there are only cons it becomes an "anti-pattern". I think above is
>>>> neither, it probably has good and bad use cases which worth understanding).
>>>>
>>>> Would appreciate your thoughts (on the specific matter, as well as
>>>> opinions on understanding pros and cons as a teaching requisite, i.e. is
>>>> this a relevant question in your opinion).
>>>>
>>>> Amir
>>>>
>>>> --
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>>>> SG20_at_[hidden]
>>>> https://lists.isocpp.org/mailman/listinfo.cgi/sg20
>>>>
>>> --
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>>>
>>
>>
>> --
>> Be seeing you,
>> Tony
>>
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