Class

A class is a user-defined type that groups fields (member variables) and methods (member functions) together. Declare a class with the class keyword and end the definition with a semicolon:

class Circle {
public:
  double radius;
};

Object

An object is an instance of a class. Members of an object are accessed with the dot operator (.):

Circle c;
c.radius = 2.0;
std::cout << c.radius << "\n";  // 2

Each object has its own copy of the fields:

Circle a;
Circle b;
a.radius = 1.0;
b.radius = 5.0;  // a.radius is still 1.

Method

Methods can be defined inside the class and use the fields of the object they are called on:

class Circle {
public:
  double area() { return 3.14 * radius * radius; }
  double radius;
};

Methods can be defined outside the class using the scope resolution operator (::):

class Circle {
public:
  double perimeter();
  double radius;
};

double Circle::perimeter() {
  return 2 * 3.14 * radius;
}