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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: prohibit certain generic instantiations in Ada 2005
Newsgroups: comp.lang.ada
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Date: Sat, 11 Feb 2006 11:45:42 +0100
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Date: 2006-02-11T11:45:31+01:00
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On 11 Feb 2006 00:04:44 -0800, matteo.bordin@gmail.com wrote:

> I will provide a proper example in C++. With this code I can prohibit
> instantiation of template My_Class with T=int.
> 
> class Checker{
> public:
>   static void check(float){};
> private:
>   static void check(int){};
> };
> 
> template <class T>
> class My_Class{
> public:
> My_Class(){
>   T t;
>   Checker::check(t);
> 
> }
> };
> 
> int main(){
> 
>   My_Class<int> mc = My_Class<int>(); //COMPILE TIME ERROR!!!!
>   //The error comes from the fact that Checker::check(int) is private
>   My_Class<float> mc2 = My_Class<float>(); //OK!!!
> };

Your example in Ada would be:

generic
   type T is digits <>; -- Only floating-point is allowed

T is specified by its contract: a floating-point type, rather than using
ad-hoc matching.

It isn't clear what you are going to achieve using the technique like in
your example. What if T were long?

The trick you mention should be impossible in legal Ada, because generics
are compilable separately [thank to the contract model.] So if any error
may occur, then only in the formal part. [GNAT Ada has a lot of problems
with this, but these are just bugs.] In means that any trick should rely on
the formal parameter part. For example:

generic
   type T is private;
   with function "+" (Left : T) return Float is <>;

This will reject Integer and accept Float. But, again, it is an awful
style.

Now to your example. It has a direct Ada equivalent (checks are moved to
the formal part):

package Checker is
   function Check (T : Float) return Float renames "+";
end Checker;

with Checker;  use Checker;
generic
   type T is private;
   wth function Check (X : T) return T is <>;

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