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From: "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>
Organization: jrcarter at acm dot org
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Newsgroups: comp.lang.ada
Subject: Re: Question about generics.
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Date: 2006-07-03T06:30:27+00:00
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Peter C. Chapin wrote:
> 
> generic
>   Size : in Integer;
> package Xyzzy is
>   type Index is mod Size;
>   -- etc.
> end Xyzzy;

What does a Size of -7 mean?

As you've discovered, generic formal parameters are not static in Ada, 
and so cannot be used in a type declaration.

You can do

generic
    type Index is mod <>;
package P is
    Size : constant Positive := Index'Modulus;
end P;

> I realize that I could make the Index type a generic parameter but that 
> would require me to define the type at the point of instantiation and 
> that seems unnatural. Conceptually I'm trying to parameterize the 
> package on a (static) size. It doesn't seem like I should have to define 
> a type to express that concept.

Except there's no way to do that.

> template< int size >
> class Xyzzy {
>   char array[size];  // size is a compile time constant.
> };

This is a different kettle of fish. You can't declare new integer types 
in C++, so you can't compare what you're trying to do with C++. Array 
subtype constraints don't have to be static, or even constant:

generic
    Size : Positive; -- or Natural, if zero is acceptable
package P is
    subtype S is String (1 .. Size); -- Size is a non-static constant
end P;

declare
    N : Positive := Get;
begin
    if N rem 2 /= 0 then
       N := N + 1;
    end if;

    declare
       subtype S is String (1 .. N);
    begin
       null;
    end;
end;

Note that Integer is perfectly OK for using type String, for exponents, 
and for sizes and counts that will fit in a 16-bit signed integer, but 
for just about everything else you should be using user-defined types 
based on the requirements of the application. Whichever you use, if 
negative values are not meaningful, you should explicitly exclude them 
by using an appropriate subtype.

-- 
Jeff Carter
"We'll make Rock Ridge think it's a chicken
that got caught in a tractor's nuts!"
Blazing Saddles
87