Inheritance

    Inheritance uses the idea of reusability or genericity.  A programmer creates an object that is general in nature and adds to it after it has been added to another object.  This new object is more specific and can be inherited by other objects.  Figure 1 shows how inheritance works.  The object at the top is the generic type.  The objects become more specific as they move down with the most specific objects on the bottom.

    Ada 95 is a language which has single inheritance.  Table 1 shows the comparison of Java, C++, and Ada 95.  Even though Ada 95 is a single inheritance language, programs can be written to support multiple inheritance.

    Most object-oriented languages have single inheritance.  C++ and Eiffel are exceptions and support multiple inheritance.

    At the time this was written, information in Table 1 could be found at http://www.adahome.com/Resources/Languages/.
 

Java C++ Ada 95
Inheritance Single
(but with multiple sub typing)		 Multiple Single
(but supports MI)
Separate
Interface/Implementation		 No
(interface generated from code)		 Yes
(header files)		 Yes
(specifications)
Garbage Collection Yes No No**
Operator Overloading No Yes Yes
Generics No
(but extensive polymorphism)		 Yes
("templates")		 Yes
Exceptions Yes Yes Yes
Native Multi-threading Yes No Yes
Table 1  Language comparison
       ** Garbage collection is not required but is allowed.  Only commercial Ada compilers which emit Java Byte Code currently implement garbage collection.
 
 

Figure 1  Inheritance diagram






    In Ada 95 inheritance is accomplished with the help of expandable types.  Expandable types are also known as tagged types because they contain a hidden tag.  If an object is not tagged it can not be inherited.  Figure 2 shows how an object is tagged in Ada 95.
 


type Animal is tagged
    record
         Name  :  string (1..30);
    end record;
Figure 2  Tagging an object

    Figure 3 is an example of what Ada 95 code looks like for a series of objects inheriting from each other.
 


type Animal is tagged
    record
         Name  :  String (1..30);
    end record;
 

type Mammal is new Animal with
    record
        Bear_Young_Live : Boolean;
    end record;
 

type Dog is new Mammal with
    record
        ...  -- attributes
    end record;

type German_Shepherd is new Dog with
    record
        ...  -- attributes
    end record;

 
Figure 3  Inheriting an object

    In figure 3 Animal is the parent type to Mammal and Mammal is the child type to Animal.  We can also say that Mammal is derived from Animal.  This can be carried on down the chain as Dog is derived from Mammal and German_Shepherd is derived from Dog.
 

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