Re: Concatenation and Characters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jeffrey Carter <jrcarter@acm.org> writes:

> Robert A Duff wrote:
> > Not true.  I wasn't thinking clearly.  For the example given, you are
> > correct that a bounds check is needed, at least on the upper bound.  I
> > suppose the compiler could optimize away the check on the lower bound.
> 
> I would think even on the lower bound. The compiler knows nothing about
> the bounds. The lower bound could be any value in Positive, so a
> variable in Positive could easily be less than the lower bound just as
> it could be greater than the upper bound.

Yes, that's right.

> > In any case, although I put my foot in my mount about *that* example, I
                                              ^^^^^
I can't even spell "mouth" right.  :-(

> > think my more general point is correct: there are cases where an Ada 83
> > compiler could optimize away checks (based on the erroneousness of
> > uninit vars), where an Ada 95 compiler cannot (or at least has to prove
> > more).
> 
> If the variable has the same subtype as the index of the array object, I
> can see this:
> 
> procedure Op (S : in out String) is
>     I : Positive range S'range;
> begin -- Op
>     S (I) := 'x';
> end Op;
> 
> I is either in range or execution is erroneous.

Yes (in Ada 83).

> As I said in another post, a source of unexpected Constraint_Errors I
> encountered in Ada 83 was from beginners assuming the lower bound of a
> String parameter was one. A bounds check is required unless the index is
> known to the compiler to be in the index range of the array object.
> 
> I guess String was not a good type to use for an example. If it had been
> 
> type Index is range 1 .. 100;
> type Arr is array (Index) of Integer;
> 
> I : Index;
> A : Arr
> ...
> A (I) := 1;
> 
> it would have been more obvious what we're talking about (I think).

Yes.  ;-)

- Bob