Re: Address arithmetic alternatives

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

On 09.06.11 12:40, milouz wrote:
> 
>> More correctly, you *think* you need to play with address arithmetic. You may be
>> right, but address arithmetic is needed extremely rarely in Ada. Ada has
>> alternatives to most address arithmetic in C.
> 
> You're probably right. But what are those alternatives ?

I'd think array indexing (including ptrdiff_t stuff) is
a prominent example:

while (*s++) {
   ...
}

for K in S'Range loop
   ...
end loop;


Another is a C functions that reads two pointer, one a start
pointer and the other an offset pointer.

typedef ... t;

t* foo(const t* source, t* offset) { ...

In Ada, giving some more information about pointed to array
and the pointers via index ranges:

type T is ...;
type Indexing is range 0 .. Some_Maximum;
subtype Position is Indexing range 16#100# .. 16#1FF#;
subtype Distance is indexing
  range 0 .. Position'Last - Position'First;

type Array_of_T is array ( Position range <> ) of T;

function foo (Source : Array_of_T; Offset: Distance)
  return Distance;

(This doesn't mean that in C one couldn't write s[d], d++ or
that in Ada one couldn't use pointers with array components.
In fact, Ada.Containers features cursors. There is just much
expressive information in typed, constrained index values.
Depending on how much type information there is at compile
time, the compiler will reject a program if it can thus detect
a bounds violation (value not in type).)