Re: numbers as 'generics' parameters

On Sun, 18 May 2008 13:37:47 -0700 (PDT), Maciej Sobczak wrote:

On 18 Maj, 16:52, "Dmitry A. Kazakov" <mail...@xxxxxxxxxxxxxxxxx>
wrote:

Note also that the constraint to have equal strings is only an example
and probably does not show the full problem.

(The problem; I don't understand, BTW)

I might have written my code like this instead:

template <size_t N>
void foo(const fixed_size_string<N> & s1,
const fixed_size_string<2 * N> & s2);

Easily:

generic
N : Natural;
package Silly_String is
type Pascal_String is new String (1..N);
end Silly_String;

with Silly_String;
generic
with package One_Silly is new Silly_String (<>);
with package Twice_As_Silly is
new Silly_String (One_Silly.Pascal_String'Length * 2);
procedure Foo
( S1 : One_Silly.Pascal_String;
S2 : Twice_As_Silly.Pascal_String
);

If you replaced the type declaration in Strings_Nobody_Ever_Needed to

subtype Pascal_String is String (1..N);

you would start to see what's wrong with all this... (:-))

No, I don't follow. Could you please explain?

This Pascal_String would clash when multiple instantiated. The constraint
in your examples is a part of the type. This is wrong. The constraint of
the subtype is a proper constraint. To violate it is *legal*, so in order
to check it statically you need other (finer) means.

Talking about strings, they are allowed to have statically unknown length

Yes - the string was only a vehicle to show the problem. It might be a
matrix of size N or just about anything else.

Still, for a vast majority of them, it is wrong to have the constraint a
part of the type. This is a fundamental issue, why generics are not a
solution.

--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
.

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