Re: Concatenation and Characters

[Jeffrey Carter <jrcarter@acm.org>]

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.

> 
> In any case, although I put my foot in my mount about *that* example, I
> 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.

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).

-- 
Jeff Carter
"English bed-wetting types."
Monty Python & the Holy Grail