Classes are the most fundamental feature in C++. Classes let us define new types for our applications, making our programs shorter and easier to modify.

Data abstraction—the ability to define both data and function members—and encapsulation—the ability to protect class members from general access—are fundamental to classes. We encapsulate a class by defining its implementation members as private. Classes may grant access to their nonpublic member by designating another class or function as a friend.

Classes may define constructors, which are special member functions that control how objects are initialized. Constructors may be overloaded. Constructors should use a constructor initializer list to initialize all the data members.

Classes may also define mutable or static members. A mutable member is a data member that is never const; its value may be changed inside a const member function. A static member can be either function or data; static members exist independently of the objects of the class type.