Comment by repsilat

Yeah I guess a c++ programmer might say `std::array::operator[]` is a function (or method, whatever), just like `std::array::size` is. And that identifying the array with just one of those functions (or even the collection of all methods offered) is to miss the point -- not just contiguous indices, but contiguous storage. A red-black tree with added constraints is not an array.

TFA does allude to this. An "array indexing function" that just met the API could be implemented as an if-else chain, and would not be satisfactory.