Re: Question about generics.

["Peter C. Chapin" <pchapin@sover.net>]

"Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org> wrote in
news:7Q2qg.812592$084.507058@attbi_s22: 

> 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?

Why not a compile time error when the instantiation is attempted?

> 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;

Yes. However, this means at the point of instantiation I have to do 
something like

    	type Dummy_Type is mod 8;
    	package Fizzle is new Xyzzy(Index => Dummy_Type);

The Dummy_Type will never be used in the context of the instantiation; 
it only exists to pass size information into the generic package. That 
seems pretty counter-intuitive and awkward. I guess I was hoping Ada 
would have a nicer solution... generic parameters that are named numbers 
or something.

>> 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:

Don't be distracted by the fact that I used an array declaration above 
to demonstrate the "static-ness" of size. I realize that array bounds 
don't have to be static in Ada. However, my point was that C++ templates 
do allow non-type parameters to be used in contexts where compile time 
constants are required.

For my particular situation I believe I could declare a subtype inside 
the generic package instead of a modular type. Alas, that means I'll 
have to manually wrap around values of my type. It's not the end of the 
world but, as I said, I was hoping for a nicer solution.

Peter