Classes are central to most C++ programs: Classes let us define our own types that are customized for the problems we need to solve, resulting in applications that are easier to write and understand. Well-designed class types can be as easy to use as the built-in types.
A class defines data and function members: The data members store the state associated with objects of the class type, and the functions perform operations that give meaning to the data. Classes let us separate implementation and interface. The interface specifies the operations that the class supports. Only the implementor of the class need know or care about the details of the implementation. This separation reduces the bookkeeping aspects that make programming tedious and error-prone.
Class types often are referred to as abstract data types. An abstract data type treats the data (state) and operations on that state as a single unit. We can think abstractly about what the class does, rather than always having to be aware of how the class operates. Abstract data types are fundamental to both object-oriented and generic programming.
Chapter 12 begins our detailed coverage of how classes are defined. This chapter covers topics fundamental to any use of classes: class scope, data hiding, and constructors. It also introduces some new class features: friends, uses of the implicit this pointer, and the role of static and mutable members.
Classes in C++ control what happens when objects are initialized, copied, assigned, and destroyed. In this respect, C++ differs from many other languages, many of which do not give class designers the ability to control these operations. Chapter 13 covers these topics.
Chapter 14 looks at operator overloading, which allows operands of class types to be used with the built-in operators. Operator over-loading is one of the ways whereby C++ lets us create new types that are as intuitive to use as are the built-in types. This chapter also presents another special kind of class member functionconversion functionswhich define implicit conversions from objects of class type. The compiler applies these conversions in the same contextsand for the same reasonsas it does with conversions among the built-in types.
