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Subject: Re: How much is too much with C++20 concepts?
From: Tony V E (tvaneerd_at_[hidden])
Date: 2020-02-29 20:32:27


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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>>
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-- 
Be seeing you,
Tony


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