Why don’t list or array have a capacity member

A `list` does not support random access, and element of `list` is stored seperately in memory. There is no need to reallocate its elements, and thus no need to preallocate memory. So the "`capacity`" of a `list` is the same with its `size`.

An `array` has a fixed size, there is also no need to reallocate its elements, and thus no need to preallocate memory. So the "`capacity`" of an `array` is the same with its `size` too.