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Subject: Re: P1887R0 attribute predicades
From: Corentin Jabot (corentinjabot_at_[hidden])
Date: 2020-11-10 11:35:27


On Tue, 10 Nov 2020 at 15:22, Cleiton Santoia via SG7 <sg7_at_[hidden]>
wrote:

> namespace std::meta {
> template consteval <typename Attr> // Need to be a template ?
> bool has_attribute(info x);
>
> template consteval <typename Attr> // template parameter only in result
> ?
> Attr attribute(info x);
> }
>
> Given:
>
> public X {};
> public Y { Y(int) {} };
>
> [[X]]
> struct V {};
> [[Y(10)]]
> struct W {};
>
> constexpr auto x = std::meta::attribute<X>(reflexpr(V)); // ok x = X();
> constexpr auto y = std::meta::attribute<Y>(reflexpr(W)); // ok y = Y(10);
>
> but what if you ask for
>
> constexpr auto y2 = std::meta::attribute<Y>(reflexpr(V)); ???
>
> since Y cannot be default initialized ?
>

Program is il-formed

>
>
>
> I believe std::meta::attribute should a plural filtered funcion
>
> template consteval
> std::vector<info> attributes_of( info class_type, Args ...filters );
>

How do you get a value out of the attribute ?

>
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