Introduction, Classes and Data Abstraction

• Basic characteristics of O-O languages
  • Everything is an object.
  • Object-orientation is a natural way of thinking about the world and of writing computer programs.
  • Objects are all around us-people, animals, plants, cars, planes, buildings, computers, etc.
  • Abstractions allow us to view screen images as objects such as people, planes, trees, etc. rather than as individual dots of color.
  • Abstractions allow us to think in terms of beaches rather than grains of sand, houses rather than bricks.
  • All objects have attributes such as size, shape, color, weight, etc.
  • All objects exhibit various behaviors. A baby cries, sleeps, crawls, walks; a car accelerates, brakes, turns, etc.
  • Humans learn about objects by studying their attributes and observing their behaviors.
  • Different objects can have many of the same attributes and exhibit similar behaviors.
    • Comparisons can be made between babies and adults, and between humans and chimpanzees.
    • Cars, trucks, little red wagons, and roller blades have much in common.
  • Object-oriented programming (OOP) models real-world objects with software counterparts.
    • It takes advantage of class relationships where objects of a certain class, such as a class of vehicles, have the same characteristics.
    • It takes advantage of inheritance relationships, and even multiple inheritance relationships, where newly created classes are derived by inheriting characteristics of existing classes, yet contain unique characteristics of their own.
  • A program is a bunch of objects telling each other what to do, by sending messages.
  • Each object has its own memory, and is made up of other objects.
  • Every object has a type (class).
  • All objects of the same type can receive the same messages.
• Objects
  • An object has an interface, determined by the class it's an instance of.
  • A class is an abstract data type (or user-defined data type).
  • Defining a class requires defining its interface.
  • What about built-in types?
    • Think of an int
    • What's its interface?
    • How do you "send it messages"?
    • How do you make (construct) one?
• The interface is the critical part, but the details (implementation) are important too
• Users use the interface (the "public part"), the implementation is hidden by "access control".
• C libraries have always been like this, sort of:
  • The library designer invents a useful struct.
  • Then she provides some useful functions for the struct.
  • The user creates an instance of the struct, then applies library functions to it.
• C++ uses "access specifiers": public, protected, and private to determine who can use the attribute or function.
• Two Ways of Reusing Classes
  • Composition: One class has another as a "part".
  • Inheritance: One class is a specialized version of another
• Polymorphism: Different subclasses respond to the same message, possibly with different actions.
• Creating and Destroying Objects
  • We usually get this for free with built-in types like int or char, we just say
    • int i;
    • char c;
  • With user-defined types (the ones we make), we need to be explicit about what we want:
    • constructor function
    • destructor function
    • C++ has new and delete (similar to malloc and free in C)
    • This is a very important issue! What is a memory leak?
• A compiler typically does
  • preprocessing
  • first pass to make parse tree
  • second pass to generate code
• The result is an object module (.obj file).
• A linker produces an .exe file by
  • Resolving references between compilation units (i.e., separate source files)
  • Adding code from libraries
  • Adding special startup code
  • Building the final executable file
• In C++, variables and functions must be both declared and defined. The rules:
  • A declaration tells the compiler that you intend to use a variable/function with a certain name.
  • A variable declaration specifies the type (int, float, etc.) so the compiler can check your usage.
  • A variable declaration doesn't allocate space for the variable.
  • A function declaration specifies the function name, argument types, and return type, so the compiler can check your usage.
  • A function declaration doesn't allocate space for the function code.
  • A variable definition causes memory to be allocated to hold its value. This can only be done (must be done) exactly once in the entire program. Why?
  • And so for functions.
• Libraries are collections of compiled function definitions.
  • Library header files (.h files, or files with no extension) are collections of (uncompiled e.g., ASCII) function declarations.
  • $\char93$includeing a header file is a fast and painless way of providing the declarations the compiler insists on.
  • The compiler is happy, since it has declarations from the .h file(s)
  • The linker is happy, because there is exactly one definition of a library function.
  • The linker resolves references to variables/functions that are spread across files.

  Figure 1: Survey of Programming Techniques; unstructured, procedural, modular, and object-oriented programming.
• Survey of Programming Techniques (see Fig. 1)
  • Unstructured programming.
    • Simple sequence of command statements.
    • Operates directly on global data.
    • Not good for large programs.
    • Repetitive statement segments are copied over.
    • The repetitive sequences extracted and named so that they can be called and values returned leads to the idea of procedures.
  • Procedural programming.
    • Combines returning sequences of statements into one function.
    • Procedure calls are used to invoke procedures.
    • Programs are now more structured.
    • Errors are easier to detect.
    • Combining procedures into modules is the next logical extension.
  • Modular programming.
    • Procedures with common functionality are grouped into modules.
    • Main program coordinates calls to procedures within modules.
    • Each module has its own data and isolated for other modules.
  • Object-oriented programming.
    • Data and the functions that operate on that data are combined into an object.
    • Programming is not function based but object based.
    • Objects are base on three basic ideas: Encapsulation, Inheritance and Polymorphism.