And still, the main problem is that you need to be able to tag classes
you can't change.

On Tue, May 28, 2024 at 11:43 AM Frederick Virchanza Gotham via
Std-Proposals <std-proposals_at_[hidden]> wrote:
>
> On Tue, May 28, 2024 at 12:48 AM Arthur O'Dwyer wrote:
> >
> > You have to re-opt-in to `is_library3_thing` at each level
> > of inheritance (that is, at each leaf)
>
>
> I think there are two kinds of tag you would want to put on a class:
>
> (1) Hereditary
> (2) Terminal
>
> With hereditary tags, a derived class must opt out.
> With terminal tags, a derived class must opt in.
>
> And so a hereditary tag system would work like:
>
> class MyClass {
> public:
> typedef std::true_type tag_elide;
> };
>
> And a terminal tag system would work like:
>
> class MyClass {
> public:
> typedef MyClass tag_elide;
> };
>
> But instead of using typedef's for this purpose, should we instead be
> talking about adding "Class Tagging" to C++26? It would work as
> follows:
>
> class MyClass1 {
> public:
> tag [elide] hereditary;
> };
>
> class MyClass2 {
> public:
> tag [elide, some_other_tag] terminal;
> };
>
> And then instead of creating a separate concept to use as a
> constraint, you would just do:
>
> template<typename T, typename R> requires tagged[elide, some_other_tag]<T,R>
> void Func(void)
> {
> // Both T and R must have the tags "elide" and "some_other_tag"
> }
>
> If we take it a step further, the core language could then even
> interact with tags that the programmer has put on their own custom
> classes. So for example, let's say we name a tag "no_local" and we put
> it on our class "MyClass" as follows:
>
> class MyClass {
> public:
> tag [no_local] hereditary;
> };
>
> And then we try to make a local variable:
>
> int main(void)
> {
> MyClass obj; // compiler error -- cannot make on the stack
>
> MyClass *p = new MyClass; // This is okay
> }
>
> This would be a neat way of making sure massive classes don't get
> allocated on the stack.
>
> I'm sure there's a hundred other uses for this.
> --
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