Re: Concatenation and Characters

[Jeffrey Carter <jrcarter@acm.org>]

Matthew Heaney wrote:
> This isn't quite right.  If an object of a discrete type is used to index an
> array, the compiler is required to ensure that the object --even if
> uninitialized-- is only used to index an actual component of the array
> object.
> 
> For example:
> 
> procedure Op (S : String) is
>   I : Positive;
> begin
>   S (I) := 'x';
> end;

I presume you mean "S : in out String"?

> 
> The Ada95 language guarantees that index I will only touch the memory owned
> by array object S.
> 
> This is one area where Ada95 differs from Ada83, which made no such
> guarantee.

This has nothing to do with detecting a reference to an uninitialized 
variable. It is about bound checking of array indexing, which did exist 
in Ada 83. Unless run-time checks are suppressed, this behaves as if it 
were written

procedure Op (S : in out String) is
    I : Positive;
begin
    if I not in S'range then
       raise Constraint_Error;
    end if;

    S (I) := 'x';
end Op;

This was true in Ada 83. This is true even if I is initialized:

-- In Op:
I : Positive := 7;

-- Elsewhere:
X : String := "abcdefg";
...
Op (S => X (3 .. 5) ); -- Raises Constraint_Error

With run-time checks suppressed, what happens is anyone's guess, in Ada 
83 and Ada.

-- 
Jeff Carter
"This school was here before you came,
and it'll be here before you go."
Horse Feathers