[Development] C++20 comparisons @ Qt (was: Re: C++20 @ Qt)

[Marc Mutz]

On 21.09.23 18:02, Thiago Macieira wrote:
> On Thursday, 21 September 2023 02:10:22 PDT Ivan Solovev via Development
> wrote:
>> But I'd say that if someone wants to implement three-way comparison for
>> their classes in C++17, then a bit better understanding of the language
>> features is a reasonable expectation.
> 
> Allowing class developers to implement the three-way comparison such that Qt
> can make use of it is fine. That's basically allowing them to use the same
> technique we will be using to make our classes three-way comparable.
> 
> Providing an API for three-way comparison in C++17 does not need to happen.

Where do you draw the line between "Allowing class developers to 
implement the three-way comparison such that Qt can make use of it is 
fine." and "Providing an API for three-way comparison in C++17 does not 
need to happen."?

A Qt class author is supposed to implement a (however named) eq(lhs, 
rhs) and cmp(lhs, rhs) function. These functions are then translated into

- in C++17:
   - by our macros:
     - ==, !=
     - <, >, <=, >=
     - ==, != (reversed)
     - <, >, <=, >= (reversed)
- in C++20:
   - by our macros:
     - ==
     - <=>
   - from these, by the compiler:
     - !=
     - ==, != (reversed)
     - <, >, <=, >=
     - <, >, <=, >= (reversed)

How is said Qt class author supposed to implement his cmp(lhs, rhs) if 
there is no "API" for doing this on his data members? As a concrete 
example, how is the author of a `struct QFoo { int; QString; }` supposed 
to implement his cmp() if all he gets is op<=>, a C++20-only API?

I don't see how we can manage without an API for doing three-way 
comparisons in C++17.

We can try without (4), but (1)-(3) must needs be "semi-public" API (not 
in _p.h's).

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at qt.io>
Principal Software Engineer

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