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From: "Steve" <nospam_steved94@comcast.net>
Newsgroups: comp.lang.ada
References: <BBDEBC6C.902D%office@anobject.net>
Subject: Re: How to define an array of a variant record type?
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Date: 2003-11-18T02:48:54+00:00
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To do this you must use a "default discriminant".  In your record type
definition use:

  type Name_Value( Value_Type : Name_Value_Type := String_Type ) is
  ...

There are a couple of other things you should be aware of.
1. When assigning values to the record, you can only change the type by
doing a complete record assignment.

  That is:
     Value : Name_Value;

     Value := ( Value_Type => String_Type,        --this is ok
                     String_Value => To_Unbounded_String( "Hello" );

     Value.Value_Type := Integer_Type;    -- will not work

     Value := ( Value_Type => Integer_Type,    -- ok
                     Integer_Value => 42 );

     Value.Integer_Value := 43; -- ok


2. You can only change the discriminant through the record assignment if the
record was declared using the default.

  That is:
     Value : Name_Value(Integer_Type);

     Value := ( Value_Type => String_Type,        -- will not work.
                     String_Value => To_Unbounded_String( "Hello" );


Steve
(The Duck)


"Harald Schmidt" <office@anobject.net> wrote in message
news:BBDEBC6C.902D%office@anobject.net...
> Hi,
>
> I would like to define an array, which its element type is a variant
record
> and the Value_Type is only known at runtime.
>
>    type Name_Value_Type is (String_Type,
>                             Integer_Type,
>                             Float_Type);
>
>    type Name_Value(Value_Type : Name_Value_Type) is
>       record
>          The_Name : Unbounded_String;
>          case Value_Type is
>             when String_Type =>
>                String_Value : Unbounded_String;
>             when Integer_Type =>
>                Integer_Value : Integer;
>             when Float_Type =>
>                The_Value : Float;
>          end case;
>       end record;
>
>    type Name_Value_Sequence is array(Natural range <>) of Name_Value;
>
> The compiler says "unconstrained element type in array declaration",
> ...right!
>
> I know Ada is a strong typed language, but sometimes information is only
at
> runtime available. Does anyone know some workaround, or a regulare design
> pattern for this problem?
>
> Harald
>
>
>