An empty class is pretty useless. Classes are designed to bring together related information and functionality — time to add stuff inside!

Components of a class are called class members. Just like you can get a string’s length using .length(), you can access class members using the dot operator (object.class_member).

There are two types of class members:

• Attributes, also known as member data, consist of information about an instance of the class.
• Methods, also known as member functions, are functions that you can use with an instance of the class. We use a . before method names to distinguish them from regular functions.

We encapsulate — or enclose for simpler user access — attributes and methods in a class like this:

class City {

  // attribute
  int population;

// we'll explain 'public' later
public:
  // method
  void add_resident() {
    population++;
  }

};

Unless we have a mostly empty class, it’s common to split function declarations from definitions. We declare methods inside the class (in a header), then define the methods outside the class (in a .cpp file of the same name).

How can we define methods outside a class? We can do this using ClassName:: before the method name to indicate its class like this:

int City::get_population() {
  return population;
}

Unlike with regular functions, we need to include the header file in the .cpp file where we define the methods.

Note: We must declare a method inside the class if we want to define it outside.