9. Object-Oriented Features
This chapter recommends ways of using Ada's object-oriented features. Ada supports inheritance and polymorphism, providing the programmer some effective techniques and building blocks. Disciplined use of these features will promote programs that are easier to read and modify. These features also give the programmer flexibility in building reusable components.
The following definitions are provided in order to make this chapter more understandable. The essential characteristics of object-oriented programming are encapsulation, inheritance, and polymorphism. These are defined as follows in the Rationale (1995, §§4.1 and III.1.2):
A means for incrementally building new abstractions from an existing one by "inheriting" their properties without disturbing the implementation of the original abstraction or the existing clients.
- Multiple Inheritance
The means of inheriting components and operations from two or more parent abstractions.
- Mixin Inheritance
Multiple inheritance in which one or more of the parent abstractions cannot have instances of their own and exist only to provide a set of properties for abstractions inheriting from them.
A means of factoring out the differences among a collection of abstractions, such that programs may be written in terms of the common properties.
- Static polymorphism is provided through the generic parameter mechanism whereby a generic unit may be instantiated at compile time with any type from a class of types.
- Dynamic polymorphism is provided through the use of so-called class-wide types and the distinction is then made at runtime on the basis of the value of the tag ("effectively a hidden discriminant identifying the type" [Rationale 1995, §II.1]).
As stated in the Ada Reference Manual (1995, Annex N):
A type has an associated set of values and a set of primitive operations that implement the fundamental aspects of its semantics.
A class is a set of types that is closed under derivation, which means that if a given type is in the class, then all types derived from that type are also in the class. The set of types of a class share common properties, such as their primitive operations. The semantics of a class include expected behavior and exceptions.
An object is either a constant or variable defined from a type (class). An object contains a value. A subcomponent of an object is itself an object.
Guidelines in this chapter are frequently worded "consider ..." because hard and fast rules cannot apply in all situations. The specific choice you make in a given situation involves design tradeoffs. The rationale for these guidelines is intended to give you insight into some of these tradeoffs.