guideline

Derive from controlled types in preference to using limited private types.

Use limited private types in preference to private types.

Use private types in preference to nonprivate types.

Explicitly export needed operations rather than easing restrictions.

------------------------------------------------------------------------
with Ada.Finalization;
package Packet_Telemetry is
   type Frame_Header is new Ada.Finalization.Controlled with private;
   type Frame_Data   is private;
   type Frame_Codes  is (Main_Bus_Voltage, Transmitter_1_Power);
   ...
private
   type Frame_Header is new Ada.Finalization.Controlled with
      record
         ...
      end record;
   -- override adjustment and finalization to get correct assignment semantics
   procedure Adjust (Object : in out Frame_Header);
   procedure Finalize (Object : in out Frame_Header);
   type Frame_Data is
      record
         ...
      end record;
   ...
end Packet_Telemetry;
------------------------------------------------------------------------

rationale

Limited private types and private types support abstraction and information hiding better than nonprivate types. The more restricted the type, the better information hiding is served. This, in turn, allows the implementation to change without affecting the rest of the program. While there are many valid reasons to export types, it is better to try the preferred route first, loosening the restrictions only as necessary. If it is necessary for a user of the package to use a few of the restricted operations, it is better to export the operations explicitly and individually via exported subprograms than to drop a level of restriction. This practice retains the restrictions on other operations.