Can a record discriminant determine array lengths indirectly in Ada?

user_4616944 0
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I'm trying to account for a special case in which several array lengths should be shorter than normal, but where there's no direct relationship between the discriminant and the array lengths. A simplified record might look like this in the normal case:

type Int_Array is array(Integer range <>);

type My_Record is
record
    A : Integer;
    B : Integer;
    C : Int_Array(1..10);
    D : Int_Array(1..6);
end record;

But if some discriminant is 320, it should look like this:

type My_Record is
record
    A : Integer;
    B : Integer;
    C : Int_Array(1..4);
    D : Int_Array(1..2);
end record;

I've kinda been playing around with it some but can't make anything compile. Here are some things I've tried:

type My_Record(Disc : Positive) is
record
    A : Integer;
    B : Integer;
    C : Int_Array(1..(if Disc = 320 then 4 else 10));
    D : Int_Array(1..(if Disc = 320 then 2 else 4));
end record;

But this results in the error "discriminant in constraint must appear alone".

If I try:

type My_Record(Disc : Positive) is
record
    A : Integer;
    B : Integer;
    case Disc is
        when 320 =>
            C : Int_Array(1..4);
            D : Int_Array(1..2);
        when others =>
            C : Int_Array(1..10);
            D : Int_Array(1..4);
    end case;
end record;

The definitions of C and D conflict with each other. Is there some other technique I can use for this?

ada record types
2 Answers

If you want to access the variables with .C and .D regardless of the variant, you could use accessor functions:

   type My_Record (IDisc : Positive) is record
      IA, IB : Integer;
      case Disc is
         when 320 =>
            C1 : aliased Int_Array (1 .. 4);
            D1 : aliased Int_Array (1 .. 2);
         when others =>
            C2 : aliased Int_Array (1 .. 10);
            D2 : aliased Int_Array (1 .. 4);
      end case;
   end record;

   type Int_Array_Accessor (Data : not null access all Int_Array) is
     limited null record with Implicit_Dereference => Data;
   function C (Object : in out My_Record) return Int_Array_Accessor is
     (Int_Array_Accessor'(if Object.Disc = 320 then
                          Object.C1'Access else Object.C2'Access));
   function D (Object : in out My_Record) return Int_Array_Accessor is
     (Int_Array_Accessor'(if Object.Disc = 320 then
                          Object.D1'Access else Object.D2'Access));

By using an accessor with Implicit_Dereference, you can assign values as if it was a record field.

user_3479640

In the case statement use different variable names in each case. Your original examples contain a few syntax errors.

package foo is
   type Int_Array is array(Integer range <>) of Integer;
   type My_Record(Disc : Positive) is
      record
         A : Integer;
         B : Integer;
         case Disc is
         when 320 =>
            C : Int_Array(1..4);
            D : Int_Array(1..2);
         when others =>
            E : Int_Array(1..10);
            F : Int_Array(1..4);
         end case;
      end record;
end foo;
user_68544070
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