Oh ffs, have you actually calculated this "cost" on a CPU made after 1985?
- binary size increase (e.g look at that : https://gcc.godbolt.org/z/xre7cp)
- more symbols (I guess you never hit the per-object file symbol limit on windows, even with /BIGOBJ - when you do the only thing left is despair).
- most callbacks will have to copy a kind of `this` pointer so the size of your objects increase and they stop fitting into cache lines
- when you have "normal" inheritance, the vtable of your parent object is generally already in your cache, while here there will be one vtable per-object (and no, the vtables aren't optimized to be part of the object itself so you still get that indirection even with SBO). so goodbye memory coherency. I've been working on my own callback implementation with an inlined vtable which allowed me to grasp sometimes more than 30% perf improvement vs std::function, in callback-heavy code : https://github.com/jcelerier/smallfunction/blob/master/small...