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Re: Random thought: consteval member data

From: Ville Voutilainen <ville.voutilainen_at_[hidden]>
Date: Sun, 8 Aug 2021 22:38:52 +0300
On Sun, 8 Aug 2021 at 22:08, Scott Michaud via Std-Discussion
<std-discussion_at_[hidden]> wrote:
> I had a random thought while drinking my Friday coffee. This is not a proposal. I discussed it on a C++ Discord, but it was met with criticism. That said, sometimes even the biggest failures can lead to interesting successes, so I'm curious what the std-discussion list thinks about it.
> The concept is compile-time data, which I'm calling "consteval members". Its goal is to simplify some of the situations where templates and macros are abused by letting people express their intent in a more literal, OOP fashion using data that the compiler cannot allow in the actual assembly. This can be used as better control flow based on the specific value of literals, and it can be used for better compile-time error handling.
> The key is that the transforms must happen at compile-time. The consteval data cannot be in the application at runtime.
> Some concerns:
> I expect that this would be horrifyingly painful to implement in an actual C++ compiler.
> This might be difficult for debuggers to implement... although I'd expect it would be able to infer the consteval state from the source code.
> On the plus side, I also believe that it allows very readable and maintainable code, especially for library authors that hide its usage with access specifiers, so it might lead into some interesting ideas.
> This email will show a use case where a factory-style object collects properties and produces an object instance ("the builder pattern"). This mechanism will allow the various functions to guide the user (with compiler errors) if they forgot required data, or if they set incompatible properties. In this case, the user must supply an IP address and a port, but they are allowed to choose any mechanism that does that, and (in this example) they can do so in any order (although another use case could easily check for order).

This sure looks like something that falls under the general umbrella
of compile-time side effects and mutations.
We'll perhaps get there once we get the more urgent bits of
https://wg21.link/p2237 off the ground and into the standard.

Received on 2021-08-08 14:39:06