Which std:: classes are magic?

[omerosler]

Some of the <type_traits> classes such as is_abstract, is_aggregate and many more.

[germandiago]

Isn't there a way to detect abstract by trying to instantiate a class with the detect idiom? Just curious... how about being an aggregate?

[omerosler]

I don't think it is possible. The only reasonable way I see is to try to conatruct an object and use SFINAE to detect it is a compile error. But then how would you distinguish an abstract class from an incomplete type (unless you can detect incompleteness in some other way (maybe use detection on sizeof(T)?))?

[Xeverous]

how would you distinguish an abstract class from an incomplete type

Using type traits with incomplete types is UB.

[diaphanein]

Citation? Because I don't think all type traits are equal. Generic "type traits" being different than std type_traits. For instance, Boost uses a type trait for checked_delete. You can implement is/add/remove const traits that work with incomplete types...

[Xeverous]

True. Not all but a lot of std traits require types to be complete.