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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Generic default parameters
Date: Fri, 10 May 2002 14:27:39 -0500
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Date: 2002-05-10T14:27:39-05:00
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Preben Randhol wrote in message ...
>On Fri, 10 May 2002 16:22:50 +0200, Thomas Wolf wrote:
>>
>> It seems I'm not alone with these ideas... so let's summarize
>> and see if we can get a halfway decent proposal for the ARG
>> out of the discussions:
>>
>> Ada 95 lacks features for:
>>
>> 1. specifying a default type for generic formal type parameters
>>
>>    The idea would be to allow something like
>>
>>    generic
>>       type Something is range <> := Natural;
>>
>>    and if an instantiation does *not* supply an actual for
>>    'Something', 'Natural' will be taken.
>
>I actually like that Ada95 doesn't allow a default value, and I don't
>understand what the gain is in a default value. Expecially not for
>generic pacakges. If one want a default value why don't just do:
>
>package Some_Package is
>   new Some_Generic_Pacakge (Data_Type => Some_Default_Type);
>
>in a package and use this?

The point of course is to provide defaults for some (not all)
parameters, so the "package" you're talking about would have to be
generic.

The case that brought this up is something like:

    generic
        type Element_Type is private;
        type Count_Type is (<>) use Natural;
    package Lists is ...

Generally, the default Count type is sufficient. But there may be
occassions where some user-defined type would be valuable. A typical
instantiation would look like:

    package My_List is new Element (Some_Record);

To get that with a package, you'd have to have a generic package
wrapper:

    generic
        type Element_Type is private;
    package List_with_Natural_Count is
        package List is new Lists (Element_Type, Natural);
    end List_with_Natural_Count;

and then you would have complications in naming the items in the
instantiation (especially if you do not want to use a "use clause" on
the package). The net effect (either way) is to make it harder to create
a general component that is also relatively easy to use.

                       Randy.