Object-oriented Software in Ada 95

168 Child libraries
11.2 Visibility rules of a child package
The private part of a child package can access all components of its ancestors, even the private components.
However, the visible part of a child package has no access to the private components of its ancestors. This is to
prevent possible renaming allowing a client direct access to the private components of one of the child’s
ancestors.
A child package allows a programmer the ability to extend an existing package without the need to change or
re-compile the package.
For example, Figure 11.1 illustrates a hierarchy of package specifications rooted at package P. The ancestor of
packages P.C2 and P.C1 is package P, whilst the ancestors of package P.C2.G1 are the packages P.C2 and P.
P
Child
Parent
P.C1
Visible Spec.
Private Spec.
Body
Figure 11.1 Illustration of the hierarchy of child units.
Visible Spec.
Private Spec.
Body
P.C2
Grand child
Visible Spec.
Private Spec.
Body
Key Package specification Package body
Components
of a package
in this case
package P.
package P is
-- Visible specification.
private
-- Private specification.
end P;
package body P is
-- Body of package
end P;
P.C2.G1
Visible Spec.
Private Spec.
Body
Child
Can access in -> P P.C1 P.C2 P.C2.G1
P.C1
Visible spec.
Visible specification
•
P.C1
Private specification
P.C2.G1
Visible specification
P.C2.G1
Private specification
Visible spec.
Private spec.
Visible spec.
Visible spec.
Private spec.
•
Visible spec.
when
with’ed
Visible spec.
when
with’ed
Visible spec.
when
with’ed
Visible spec.
when
with’ed
Visible spec.
Visible spec.
Private spec.
Visible spec.
when
with’ed
Visible spec.
when
with’ed
•
•
Note:
A with clause for a child package implies a with clause for all its ancestors.
© M A Smith - May not be reproduced without permission