Nested Class Declarations

Inside the scope of a class, we can declare more than just data members and member functions; we can declare a class inside another class. These classes are called nested classes.

Since a nested class declaration happens inside the outer class, it has access to all the declared names as if it were part of the outer class: it can access even private declarations.

On the other hand, a nested class is not associated with any instance, so it can only access static members.

To access a nested class, we can use the double colon (::), similar to accessing static members of the outer class. Let's examine the following example:

// Declaration
class Coordinate {
...
  struct CoordinateDistance {
    float x = 0;
    float y = 0;
    static float walkingDistance(CoordinateDistance distance);
  }
};
// Create an instance of the nested class CoordinateDistance
Coordinate::CoordinateDistance distance;

/* Invoke the static method walkingDistance declared inside the nested class CoordinateDistance...