"Given an optimizing compiler, the first function (count(a)) is likely to just immediately return the size of the backing vector. The function is nearly free."

As far as I can see, neither gcc nor clang actually do this (unless the array has a fixed size; and even then in the gcc case only if the array is zero or one element long).

Probably still more efficient than a virtual call, but also a lot more complex on both the programmer and the compiler, and a lot harder to debug.