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From: Martin Dowie <martin.dowie@btopenworld.com>
Newsgroups: comp.lang.ada
Subject: Re: generics in Ada 83
Date: Tue, 13 Sep 2005 16:41:53 +0000 (UTC)
Organization: BT Openworld
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Date: 2005-09-13T16:41:53+00:00
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REH wrote:
> Martin Dowie wrote:
> 
>>REH wrote:
>>
>>>Is there a way to define a default for the with'd procedure in the
>>>following example in Ada 83:
>>>
>>>generic
>>>    type X is private;
>>>    with procedure Y(Z : in X);
>>>package Foo;
>>>
>>>I want to define a default for Y, but how can I without knowing X?
>>>
>>>I have a generic package that has several with'd procedures, but
>>>depending on what services the package provides are use, only a few of
>>>the procedures may need to be defined.  I'd like to just default the
>>>unused ones to stubs.
>>
>>Same as Ada95:
>>
>>generic
>>   type X is private;
>>   with procedure Y (Z : X) is <>;
>>package Foo is
>>end Foo;
>>
>>Or do you mean you want the compiler to magically insert a 'null' procedure
>>if there isn't a matching procedure Y for whatever type X happens to be? If
>>that's the case you can't do that. There has to be some procedure (even one
>>that's only 'begin null end;') for each type.
>>
>>Cheers
>>
>>-- Martin
> 
> 
> defining it with a box won't work.  There would have to a procedure
> already defined.  What I want is something like:
> 
> generic
>     type X is private;
>     with procedure Y(Z : in X) is P;
> package Foo
> 
> 
> but of course P must already take into account X.  With '95 I think I
> could just with a generic package containing the stubs, but I can
> figure out a way in '83.

Yes, how can you have 'P' know which X?

The best I can see is:

package A is
    type A_T is private;
    procedure Y (Z : A_T);
private
    type A_T is new Integer;
end A;

generic
    type X is private;
    with procedure Y (Z : X) is <>;
package B is
end B;

with A; use A;
with B;

package C is
    new B (A_T);  -- Picks up 'Y' by default

Cheers

-- Martin