Re: How can I create an empty array of a generic type?

[Keith Thompson <kst-u@mib.org>]

b.mcguinness747@gmail.com writes:
> I have a generic package
>
> generic
>   type Element_Type is private;
>   type Key_Component_Type is private;
>   with function "<" (Left, Right : Key_Component_Type) return Boolean is <>;
>   with function "=" (Left, Right : Element_Type) return Boolean is <>;
> package Ternary_Trees is
> ...
> private
> ...
>   Empty_Key  : constant Key_Type(1..0) := (others => Key_Component_Type'First);
> ...
> end Ternary_Trees;
>
> When I try to compile the package, I get an error message:
>
> ternary_trees.ads:77:54: prefix for "First" attribute may not be private type

You haven't shown us the declaration for Key_Type.  Presumably it's an
array of Key_Component_Type.

This compiles:

generic
  type Element_Type is private;
  type Key_Component_Type is private;
  with function "<" (Left, Right : Key_Component_Type) return Boolean is <>;
  with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ternary_Trees is
  type Key_Type is array(Positive range <>) of Key_Component_Type;
private
  Dummy: Key_Component_Type;
  Empty_Key  : constant Key_Type(1..0) := (others => Dummy);
end Ternary_Trees;

The Dummy value is a bit ugly.  An alternative is to require the
instantiator to provide a value:

generic
  type Element_Type is private;
  type Key_Component_Type is private;
  Default_Key_Component: Key_Component_Type;
  with function "<" (Left, Right : Key_Component_Type) return Boolean is <>;
  with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ternary_Trees is
  type Key_Type is array(Positive range <>) of Key_Component_Type;
private
  Dummy: Key_Component_Type;
  Empty_Key  : constant Key_Type(1..0) := (others => Default_Key_Component);
end Ternary_Trees;

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */