Sorry, typing on my phone and of course don't see the typos until after hitting send. Fixed below...

On Sun, 4 Jan 2026, 20:45 Jonathan Wakely, <cxx_at_[hidden]> wrote:

On Sun, 4 Jan 2026, 15:28 Jan Schultke via Std-Proposals, <std-proposals_at_[hidden]> wrote:

Leading by calling it "evil" is an extremely poor start to an argument. You have already convinced all skeptics that your argument lacks any substance. 

What's the substance of your argument?

It seemed pretty clear to me that Barry's point was that parameter names were not part of the intended API for any existing library code, and changing that now by making those names significant (against the original authors' wishes of at least without their knowledge or

or at least without...

agreement) might be bad, or could be at least surprising.

You just assert that parameter names will "probably" require an explicit opt-in without discussing the cost to the user of this choice. Then you nitpick that I've used the word "evil", totally ignore any technical argument I've made, and declare victory.

Your argument seemed to be "some other things are observable, so it doesn't make sense to say that parameter names must not be observable", which isn't really a relevant response to Barry's point. 

The things you identify as having been observable for 30 years have indeed been observable 30 years, and that's the point. That some things are already observable is not an argument for or against *changing* whether parameter names are observable. Nobody is claiming that it doesn't make sense for those names to be significant in an ideal world, only that we have existing code and constraints they

constraints that...

mean we don't have an ideal world.

On Sun, 4 Jan 2026 at 16:22, Barry Revzin <barry.revzin_at_[hidden]> wrote:

On Sun, Jan 4, 2026 at 9:15 AM Jan Schultke via Std-Proposals <std-proposals_at_[hidden]> wrote:

I think the evil of needing the explicit opt-in significantly outweighs the perceived freedom it gives library authors.

Leading by calling it "evil" is an extremely poor start to an argument. You have already convinced all skeptics that your argument lacks any substance. 

 

Note that as I've already mentioned, there are numerous properties of functions that the user can observe but which are likely implementation details, such as:

• recursive leakage through headers
• expression validity tests / SFINAE stability
• ability to form function pointers and function references
• ability to pass template arguments explicitly, especially cv-qualified types
• any property only obtainable through reflection (such as parameter names)
• lack or existence of default function arguments
• lack or existence of default template arguments
• distinct declaration vs using-declaration of an existing global entity
• various type choices, like using iterators as pointers
• existence of a noexcept specification or lack thereof

Sometimes these are stable, but very often it's not intended for users to rely on them. This has been the status quo of the last 30 years, and people talk about adding function parameter names to this list as if it was obviously unacceptable, but it really isn't.

On the other hand, the ability to rely on function parameter names is not really a problem in practice in languages with support for named arguments, like Kotlin. It's only conjectured to be a major problem in C++. However, an explicit opt-in would require every library author to do significant work to permit passing named function arguments, and this work isn't exactly clean and easy. Except for fresh C++29-only codebases, this would devolve into code looking like:

f(int x, int y);

f(

    int MY_LIB_NAMED_SINCE_CXX29(x),

    int MY_LIB_NAMED_SINCE_CXX29(x)

);

I don't want to see every code base looking like this for the next 10-20 years, or authors understandably not bothering with named parameters because this looks disgusting. There could still be a per-function opt-in rather than a per-parameter opt-in, but I'm not sure if that's worth the trouble.

On Sun, 4 Jan 2026 at 15:42, Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]> wrote:

Parameter names are optional in function declarations. (And can be named differently from their counterpart in definitions.)

So far they were either

 - kept the same between declaration and definition -> leak of internal (implementation) data

 - used as reminder for the programmer or for popups and autocompletes in IDEs; possibly also for compile error messages

 

So the programmer already has a lot of flexibility. Nevertheless you are right, that the parameter names of included functions never leaked into the caller code up till now.

 

-----Ursprüngliche Nachricht-----
Von: Barry Revzin via Std-Proposals <std-proposals_at_[hidden]>
Gesendet: So 04.01.2026 15:13
Betreff: Re: [std-proposals] Labelled parameters
An: std-proposals_at_[hidden];
CC: Barry Revzin <barry.revzin_at_[hidden]>;

 

Even when the parameter names are normal-ish, that's probably not something that library authors want users to rely on. For instance, in range-v3, the predicates and projections are pretty consistently named pred and proj, but is that the correct user-facing name? Eric and Casey have never had to think about this before. max(r, .proj=key) is definitely nicer to read than max(r, {}, key), but not as nice as max(r, .key=key) (or ".by" or something else). Lack of explicit opt-in cements a decision that up until now was never even a known decision. 

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