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Subject: Re: How much is too much with C++20 concepts?
From: Christopher Di Bella (cjdb.ns_at_[hidden])
Date: 2020-02-29 19:42:10


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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