Re: Ada 83 versions of Ada 95 packages

["Norman H. Cohen" <ncohen@watson.ibm.com>]

johnherro@aol.com wrote:

> I agree that the name of my package may be confusing, because what I
> really wrote was a *substitute* for unbounded strings for use with an Ada
> 83 compiler.  I still hope that the package will prove useful to others.

But a discriminant named Length with no default value suggests that
every object of the type will be constrained to a particular length. 
Thus there is no way to implement procedures such as Append,
Replace_Slice, and Delete that modify their Unbounded_String
parameters.  That is what Robert Dewar meant when he said that this
looked like a fixed-length-string type.

The natural way to implement Unbounded_String in Ada 83 is with an
access-to-String type (adding an explicit Free operation as Robert
suggested).  

The natural way to implement a Bounded_String type in Ada 83 is with a
record consisting of a fixed-length string (whose length is the maximum
length for the type) and a current-length component; Bounded_String
should be limited so that its "=" can be overridden to ignore string
components that are present in the record but not logically part of the
value.  This means you will have to provide a Copy operation to replace
assignment, but that's desirable anyway to avoid unneeded copying.  ("="
can be overridden for limited or nonlimited types in Ada 95, but only
for limited types in Ada 83.)

-- 
Norman H. Cohen
mailto:ncohen@watson.ibm.com
http://www.research.ibm.com/people/n/ncohen