[Development] C++20 ctor-level [[nodiscard]] (was: Re: C++20 @ Qt)

[Marc Mutz]

On 19.06.23 09:55, Ivan Solovev via Development wrote:
>  > (*)  That macro that does nothing until C++20 anyway, so I’d also add 
> the support for the optional string-literal that C++20 also introduces..
> 
> Our tests actually show that this macro provides an expected warning on 
> most of the compilers that we use in the CI, even in C++17 mode.
> IIRC, in my tests I only had issues with INTEGRITY, GCC 9.3.1 on 
> OpenSuse 15.4, and QCC 8.3.0 on QNX.
> For these platforms the macro currently does nothing.
Right. P1771 encourages to retroactively apply [[nodiscard]] on ctors to 
C++17. It seems only GCC misinterpreted the C++17 wording as not 
applying to ctors. So it's really not a C++20 feature, but more like a 
[[nodiscard]] bugfix. Thankfully, the feature test macro value was 
updated, so we have an easy check for the presence of the bugfix.

 > > I’d also add the support for the optional string-literal that C++20
 > > also introduces..

That _is_ a new feature. I have written in the corresponding Jira epic¹ 
that I don't see a strong use-case for [[nodicard("reason")]]. Expanding 
on that: there's just one action if you get the warning: _use_ the 
value. It's not like [[deprecated("alternative")]] where the string 
provides essential information (what to do instead). So I've suggested 
to no supply a Q_NODISCARD_X/Q_REQUIRED_RESULT_X, as we've just replaced 
Q_REQUIRED_RESULT with [[nodiscard]] across the code-base. It's easily 
supplied if someone comes up with a use-case, though.

¹ https://bugreports.qt.io/browse/QTBUG-104134

Thanks,
Marc

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

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