How-to define array of generic elements - arrays

Question

With help from many sources, I have a working generic thing for a ring-buffer, with push and read of single elements:
q.ads:
generic
Q_SIZE : POSITIVE;
type T is private;
package Q is
subtype SIZE_TYPE is NATURAL range 0 .. Q_SIZE;
subtype Q_INDEX_TYPE is SIZE_TYPE range 1 .. SIZE_TYPE'last;
type Q_ARRAY_TYPE is array (Q_INDEX_TYPE) of T;
procedure INITIALIZE;
procedure PUSH(element : T);
function READ return T;
end Q;
q.adb:
package body Q is
Q_ARRAY : Q_ARRAY_TYPE;
TAIL : Q_INDEX_TYPE;
HEAD : Q_INDEX_TYPE;
...
end Q;
My test program instantiates the above for bytes and exercises the ring. It is basically as follows:
package body main is
package RING is new Q (15, UNSIGNED.BYTE);
procedure TEST is
byteval : UNSIGNED.BYTE;
begin
byteval := 16;
RING.PUSH(byteval);
...
I would now like to add the ability to pass an array of T in. I've add this to the ADS and ADB files:
procedure PUSH_ARRAY(DATA_ARRAY : Q_ARRAY_TYPE; COUNT : SIZE_TYPE);
My problem is in the test program. I've changed it to this:
BYTE_ARRAY : array (1 .. 10) of UNSIGNED.BYTE;
procedure TEST is begin
-- initialize the first 5 elements of BYTE_ARRAY, then
RING.PUSH_ARRAY(BYTE_ARRAY, 5);
this last line gives me an error message: expected type Q_ARRAY_TYPE defined at Q.ADS:xx. How do I pass a BYTE ARRAY to my method which expects an instance of the generic array?

Answer 1

Question: What is the purpose of SIZE_TYPE?
In Ada, the 'Length attribute will return the size of the array in Natural (the nonnegative Integer subtype). With that in mind, it doesn't seem to make sense to declare an extra subtype with an extra value for the index.
Ring_Buffer.ads
Generic
Type T is private;
Default : T;
Size : Positive;
Package Ring_Buffer is
SubType Index is Positive range 1..Size;
Type Ring is private;
Function Length( Obj : Ring ) return Natural;
Function Pop( Obj : in out Ring ) return T
with Pre => Length(Obj) in Positive,
Post => Length(Obj'Old)-1 = Length(Obj); --' --Highlight fix
Procedure Push( Obj : in out Ring; Item : in T )
with Pre => Length(Obj) < Size,
Post => Length(Obj'Old)+1 = Length(Obj); --'
Private
Type Internal_Data is Array(Index) of T;
Type Ring is record
Start : Positive:= Internal_Data'First; --'
Size : Natural:= 0;
Data : Internal_Data:= (Others => Default);
end record
with Type_Invariant => Ring.Size <= Size;
Function Length( Obj : Ring ) return Natural is
( Obj.Size );
End Ring_Buffer;
Ring_Buffer.adb
Package Body Ring_Buffer is
Function Pop( Obj : in out Ring ) return T is
Begin
return Result : constant T := Obj.Data(Obj.Start) do
Obj.Size:= Natural'Pred( Obj.Size ); --'
Obj.Start:= (if Obj.Start = Size then 1 else Obj.Start + 1);
end return;
End Pop;
Procedure Push( Obj : in out Ring; Item : in T ) is
Begin
Obj.Data( Natural'Succ((Obj.Start-1) + Obj.Size mod Size) ):= Item; --'
Obj.Size:= Obj.Size + 1;
End Push;
End Ring_Buffer;