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Subject: Re: [std-proposals] Low-hanging fruit: virtual plain-old-data members
From: Brian Bi (bbi5291_at_[hidden])
Date: 2019-10-14 14:40:03


On Mon, Oct 14, 2019 at 2:23 PM denis bider via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Example of problem:
>
> struct GenericService {
> virtual bool DoOtherThing() const = 0;
>
> void DoComplexThing() {
> ...
> bool doOtherThing = DoOtherThing();
> if (doOtherThing) { ... }
> ...
> }
> };
>
> struct SpecificService : GenericService {
> bool DoOtherThingToo() const override final { return true; }
> };
>
> Issues:
>
> - It's a violation of the zero-overhead rule that I have to call a virtual
> function just to get a polymorphic constant.
>
> - No formal way to specify if DoOtherThing() should always return the same
> value.
>
> Proposed solution:
>
> struct GenericService {
> virtual bool const vc_doOtherThing = 0;
>
> void DoComplexThing() {
> ...
> if (vc_doOtherThing) { ... }
> ...
> }
> }
>
> struct SpecificService : GenericService {
> bool const vc_doOtherThing override final { true };
> };
>
> Instead of storing a function pointer in the virtual table, and then
> calling the function pointer to get a value; it should be possible to just
> store the value in the virtual table.
>
> This should work for plain-old-data types with trivial construction and
> destruction. There's no desperate need to support virtual std::string
> members or std::vectors, which would involve significant complications.
> Storing plain-old-data types is not complex, though.
>
> denis
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>

Previous thread
<https://groups.google.com/a/isocpp.org/d/msg/std-proposals/VmTtQs8Iwlw/WxAkwl0188cJ>
Even older thread
<https://groups.google.com/a/isocpp.org/d/msg/std-proposals/Z1QG_3Mrn0Y/lzrwNxDeKUQJ>

The tone I got from these older discussions was that nobody was strongly
opposed to adding "virtual static constexpr" data members---but when
proposing any new feature, you have the burden of demonstrating that there
is a compelling need for it.

-- 
*Brian Bi*


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