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Re: [std-proposals] Proposing Mixed Template Parameter Packs [initial discussion]

From: Marcin Jaczewski <marcinjaczewski86_at_[hidden]>
Date: Fri, 18 Feb 2022 23:30:26 +0100
pt., 18 lut 2022 o 22:54 Barry Revzin via Std-Proposals
<std-proposals_at_[hidden]> napisaƂ(a):
> See P1985, which uses "template auto T" as a template paramater of arbitrary kind. I wish there was a good non-auto-related syntax for this, but I have no idea what that would be and this one is certainly sensible.
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1985r1.pdf

Why not `template<using A>`? as `using` as it is currently used to
introduce names to contexts, and this in some way introduces a new
name to the template context.

> Barry
> On Fri, Feb 18, 2022, 2:56 PM mail--- via Std-Proposals <std-proposals_at_[hidden]> wrote:
>> Dear all,
>> I have been running into limitations with respect to template parameters,
>> which prevents me from writing some general-purpose helpers (e.g., for use
>> in ensuring type safety and in template meta-programming). To address these
>> limitations, I have worked out a small proposal for the addition of ``mixed
>> template parameter packs'' to the language.
>> I have attached an initial draft of this feature and I welcome any input
>> (e.g., whether there is demand for the feature and any comments) before I
>> pursue further formalization of this feature and proposing it for inclusion
>> in the standard.
>> Below is the abstract of my proposal:
>> We propose the addition of mixed template parameter packs as an option for
>> parameter lists in template declarations. These mixed template parameter
>> packs represent zero-or-more template parameters of unknown kinds and can
>> represent any combination of type, non-type, and template template
>> parameters.
>> The motivating use case for mixed template parameter packs is the definition
>> of general-purpose concepts that hold only for a given class template X,
>> this independent of the kind of template parameters X requires. Furthermore,
>> we show that mixed template parameter packs provide a general-purpose
>> strengthening of the already-excellent template meta-programming facilities
>> provided by C++.
>> With kind regards,
>> Jelle Hellings
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Received on 2022-02-18 22:30:38