14: Inheritance & Composition

One of the most compelling features about C++ is
code reuse. But to be revolutionary, you need to be
able to do a lot more than copy code and change it.

That’s the C approach, and it hasn’t worked very well. As with most everything in C++, the solution revolves around the class. You reuse code by creating new classes, but instead of creating them from scratch, you use existing classes that someone else has built and debugged.

The trick is to use the classes without soiling the existing code. In this chapter you’ll see two ways to accomplish this. The first is quite straightforward: You simply create objects of your existing class inside the new class. This is called composition because the new class is composed of objects of existing classes.

The second approach is subtler. You create a new class as a type of an existing class. You literally take the form of the existing class and add code to it, without modifying the existing class. This magical act is called inheritance, and most of the work is done by the compiler. Inheritance is one of the cornerstones of object-oriented programming and has additional implications that will be explored in Chapter 15.

It turns out that much of the syntax and behavior are similar for both composition and inheritance (which makes sense; they are both ways of making new types from existing types). In this chapter, you’ll learn about these code reuse mechanisms.