Section 7.4 - Encapsulation

Ada provides a number of mechanisms to ``hide'' information from the users (``clients'') of a given type. Making information inaccessible to others who should not use it is called encapsulation. Encapsulation improves a program's maintainability and reliability.

In the lesson on types we saw how Ada permits types to be declared as ``private.'' This works with tagged types as well, so you can declare tagged types as ``private'' and then hide the implementation details from everyone who uses the type. Ada provides a number of variations on this theme to provide control over what information is visible and what is not.

The most common way to hide implementation details is to define a type publicly in a package declaration as ``tagged private'' (if you don't want the user to know about its parent) or ``new parent_name with private'' (if you want the user to know what its parent is). Follow each type declaration with declarations of subprograms that operate on the type. In the ``private'' part of the package declaration, define the type.

Here's another example. Let's create a type called a `File' with a file name, and a derived type called an `Ada_File' which also stores whether or not the file has been compiled. Both have a ``View'' subprogram. Here's how that might look:

  package File_System is
    type File is tagged private;
    procedure View(F : File);

    type Ada_File is new File with private;
    procedure View(F : Ada_File);

  private
    type File is tagged
     record
       -- We'll discuss strings later in Lovelace
       Name : String(1..20);
     end record;

    type Ada_File is new File with
     record
       Compiled : Boolean := False;
     end record;
  end File_System;

You would then create a package body to define the subprograms:

  package body File_System is
    procedure View(F : File) is
    begin
      -- ...
    end View;

    procedure View(F : Ada_File) is
    begin
      -- ...
    end View;
  end File_System;

In general, in Ada you'd define a package with a set of types inside it. The package declaration would contain a set of types declared as ``tagged private'' or ``new Parent_Type with private''. In the private part of the package declaration you'd define the type ``for real''. In the package body you'd define the subprograms.

Quiz:

Given procedure Try_Stuff:

  with File_System;
  procedure Try_Stuff is
   My_Ada_File : File_System.Ada_File;
  begin
   -- To be done.
  end Try_Stuff;

Let's say that at the line labelled ``To be done'' you'd like to set My_Ada_File's ``Compiled'' value to ``True''. How could you do this?

1. Replace the line with My_Ada_File.Compiled := True;
2. Replace the line with Compiled := True;
3. I couldn't - it can't be done using the material presented so far.

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