Re: Array-like object indexing

[Simon Wright <simon@pushface.org>]

Ryan <rtarpine@hotmail.com> writes:

> I've recently been playing with making all the standard data
> structures in Ada to prepare myself for writing my first major
> project.  Currently I'm writing a resizable-array package.  I am
> trying to make it as similar to built-in arrays as possible, with
> functions for First, Length and other useful operations and
> attributes.  It is a generic package instantiated with a type for
> the elements to be stored and a type for indexing.  I've run into a
> small problem with respect to the indexing type.
> 
> I'm not sure how to represent the boundaries of an empty array
> without raising a constraint error in some cases.  If an array is
> being indexed by Natural, for example, and the first index should be
> 0 (as makes sense in many situations), then what should the First
> and Last functions (intended to work like their analogous
> attributes) return?  I cannot return 0 and -1 because -1 would raise
> a constraint error.

The non-Ada C++ STL approach is to denote the end of the range by a
value notionally one past the last valid element. It's not obvious
that Last is the right name for an operation returning such a value,
cf My_Array'Last.