Section 7.1 - Object-Oriented Programming: Overview

A major new capability of Ada 95 is the addition of direct support for object-oriented (OO) programming. Ada has always strongly supported a related approach to software development called the ``object-based'' approach. It's important to understand the basics of the OO approach before learning how Ada now supports OO programming, so this section provides a basic overview of the OO approach. If you already understand the OO approach well, feel free to skip to the next section.

Functional Decomposition

First, some history. Software development has always involved using approaches to manage complexity. One well-known approach is called ``functional decomposition.'' In this approach, the program's ``function'' is divided (decomposed) into smaller component functions. These smaller functions are broken into still smaller functions, and so on. For example, the function ``eat_lunch'' might be decomposed into the functions ``remove_packaging'', ``eat_food'', and ``throw_away_trash.'' Functional decomposition can be easily implemented in Ada using subprograms. Functional decomposition is still a useful technique for some problems, but it does not work well if the structure of the data is complex. Thus, other approaches have been developed for systems with nontrivial data structures.

Object-Based and Object-Oriented Approaches

Two of these other approaches are strongly related to each other, and are called the ``object-oriented'' (OO) approach and the ``object-based'' approach (the latter is also called the ``abstract data type'' approach). Both are approaches to managing software complexity that are quite different from functional decomposition. In both approaches, a system is described in terms of ``objects''; each object contains data and has a set of operations that can be performed on that data. Objects represent real or abstract things important to the problem being solved. The object's structure and its set of operations are defined by the object's type (also called a class). For example, we might create a type called a vehicle with a data element amount_of_fuel_left and operations refuel and drive_to(location). We could then create two objects of type vehicle called my_vehicle and your_vehicle. Each vehicle would have its own amount_of_fuel_left, and each would respond to the operations refuel and drive_to(location). Ada has always supported this approach with its packages and types.

The difference between the OO and object-based approaches is that the OO approach adds the concept of inheritance to mimic the way people normally think when they classify objects. Inheritance permits new types (also called classes) to be defined as extensions of other existing types, forming a hierarchy of type definitions. Inheritance represents the relation ``is a kind of'' (as opposed to the relation ``is a part of'' or some other relation). To continue our example, we could create two new types called ``motorcycle'' and ``bus'' as kinds of vehicles. Thus bus and motorcycle would inherit from vehicle. Note that a type called ``wheel'' should generally not inherit from type vehicle; a wheel is a part of a vehicle, but a wheel (by itself) is not a vehicle.

A type inherits all of the data structure and operations, so in our example a bus would also have a drive_to(location) operation. More importantly, once we create a new type, we can create additional operations that only apply to it, or redefine existing operations to perform special actions for this type. For example, we could add operations to a bus to allow it to accept_passengers, and this new operation would apply to buses, not to motorcycles or to all vehicles. We could also redefine the ``drive_to(location)'' operation of a bus so it would do something different with buses.

OO programming is an approach to implementing software using the OO approach. Grady Booch [1994] defined OO programming this way: ``OO programming is a method of implementation in which programs are organized as cooperative collections of objects, each of which represents an instance of some class, and whose classes are all members of a hierarchy of classes united via inheritance relationships.''

It is difficult to use OO and object-based techniques if the underlying programming language does not support certain capabilities well. In particular, OO techniques are difficult to use if the programming language used does not directly support inheritance (and a related concept called polymorphism or dynamic dispatching, which will be explained later). Ada was explicitly designed from the beginning to support an object-based approach, and in 1995 Ada was extended to support OO programming.

Warning: Oversimplification

It is important to understand that the discussion in this section is a vast oversimplification. In particular, defining the term ``object-oriented'' is not easy and has provoked a great deal of debate; I've chosen to use a ``classical languages'' kind of definition here. The document Object-Oriented Frequently Asked Questions (OO FAQ) includes some discussion of various definitions and approaches along with various technical arguments about OO approaches and implementations. Note that Ada emphasizes safety and efficiency, while some OO languages emphasize other virtues (such as linguistic power, simplicity, or compatibility with an older language). The OO FAQ also includes a bibliography of textbooks for more information on OO approaches.

The definition of ``functional decomposition'' given above is the usual approach when it is implemented in Fortran and Pascal, though again, definitions are not universally agreed upon. In particular, do not confuse this with ``functional programming languages'', which support an approach that is a very different extension of functional decomposition. There are other system decomposition approaches, such as logical programming, that are way beyond the scope of this tutorial. It can be argued that abstract data types and object-based approaches aren't identical; such arguments generally hinge on detailed definitions which have never had universal agreement.

Quiz:

Here are simplified descriptions of two different systems. Which one is described in a more OO manner?

System 1 is an anti-missile system with 3 major types of components: a radar, a launcher, and a missile. A radar searches the portion of the sky defined by its area_of_search information. When a radar sees a target it sends a `target sighting' message to a launcher. When a launcher receives a `target sighting' message and that launcher's combat_state is `armed', it selects a missile to launch and sends that missile a launch command. A missile accepts a `launch command', which includes the launch time and the expected target location; it will then launch at the given time. There are two kinds of missiles, a long_range_missile and a short_range_missile.

System 2 is a coin-operated soup dispenser. It accepts coins, then accepts a user's soup choice, and then dispenses soup. To accept coins it counts each coin's value until the total equals or exceeds the cost of a cup of soup. To accept a user's soup choice, the system waits for a selection button to be pressed. To dispense soup, it drops a cup into the user-accessible cup holder, dispenses the selected powdered soup into the cup, dispenses boiling water into the cup, and then stirs the soup. To stir the soup, a plastic stirrer is inserted into the cup, the stirrer is vigorously moved about inside the cup, and the stirrer is then removed.

Which system has been described in a more object-oriented manner?

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David A. Wheeler (dwheeler@ida.org)