On Tue, 31 Jan 2023, 20:27 Arthur O'Dwyer, <arthur.j.odwyer_at_[hidden]>
wrote:

> On Tue, Jan 31, 2023 at 3:23 PM Billy Martin via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> You can't do this because there isn't any code that knows both B and D at
>> compile time.
>>
>> The case I'm thinking of involves some templated container that has an
>> object of type D. (Maybe it's a type erasure system, a smart pointer,
>> whatever). That container knows the type D because it was passed as a
>> template argument when it was instantiated with a brand new D object.
>> Somewhere else, some other code has a pointer B* and they want it to point
>> to this object, but they don't know what type D is (it might not even be
>> knowable until runtime).
>>
>
> Can you please show some example code?
>

Yeah I'm not clear what you want to do. How will you search for B in the
bases of D if you don't know both B and D?

Your proposed trait doesn't help.

Just write the exact code you think you want to write, but pretend that
> `std::bases_of<T>::type` already exists. Show how you'll solve your problem
> using `std::bases_of<T>::type`.
>

No need to pretend, it already exists in GCC.

#include <tr2/type_traits>

https://gcc.gnu.org/git/?p=gcc.git;a=blob_plain;f=libstdc%2B%2B-v3/include/tr2/type_traits;h=8723ab521d71de9d979c45f8822cbcd21c7651ac;hb=HEAD



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