[Interest] Future of constexpr in Qt?

[Schimkowitsch Robert]

Hi Volker,

do you think that the language (let's say up to 23, because constexpr could be a conditional macro anyway) would even allow making constexpr useful, bigger portions such as
- basic implicit sharing mechanism
- basic usage of QString
- basic usage of QList?

These are so common in our codebase that without them, only a relatively few functions can ever be made constexpr.

Herb Sutter highlighted constexpr as a way to increase programming language safety in his recent keynote at Meeting C++. Things like making a runtime function constexpr, so you can, at compile time, check for UB using static_asserts or such when feeding it with various ranges of values (min, max, zero, empty list,...). Now I'm thinking about how we could apply those ideas in real code - and Qt mostly stops us from doing that, right there and then.

Kind regards

Robert

-----Original Message-----
From: Volker Hilsheimer <volker.hilsheimer at qt.io>
Sent: Thursday, 21 November 2024 17:42
To: Schimkowitsch Robert <Robert.Schimkowitsch at andritz.com>
Cc: interest at qt-project.org
Subject: Re: [Interest] Future of constexpr in Qt?

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> On 21 Nov 2024, at 10:55, Schimkowitsch Robert <Robert.Schimkowitsch at andritz.com> wrote:
>
> I’d like to use constexpr more in the future in order to catch UB in compile-time tests of functions.
> Qt has a little constexpr sprinkled here and there, but not a lot. Does anyone know if there are plans to expand Qt in this direction?
>  Kind regards
>  Robert

Hi Robert,

It’s generally not an item on the roadmap to make constexpr everything that can be. But when we touch code with expressions that could be constexpr, then we might make that change as a drive-by, if the code would otherwise benefit from it.

If there’s any part of Qt where you’d see significant value from making things constexpr, then you’re welcome to push things for review.

Cheers,
Volker

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