Inheritance

Inheritance is the ability to create a new class based on an existing class, starting out with the same existing properties and methods. It is generally used when it’s necessary to implement a number of different objects that are all of the same type and share several common characteristics.

Syntax

The class that inherits these properties and methods is called the derived class or sub-class. The class that the sub-class inherits from is called the base class.

class BaseClass {
  accessModifier:
    dataType property1;
    dataType property2;
};

class SubClass: public BaseClass {
  accessModifier:
    dataType subProp1;
    dataType subProp2;
};

Access Specifiers

• public members of the base class are public in the derived class.
• protected members of the base class are accessible by the derived class, but not outside the class.
• private members of the base class aren’t accessible to the derived class.

Example

A good example is a Shape class that might include:

• A center property with (x, y) coordinates.
• A weight property that defines a line width.
• A color property that would define a fill color.
• A translate method that relocates the center property.

Some possible sub-classes derived from this base class might be:

• A Rectangle class which would have height and width properties. It could also have its own perimeter method.
• A Circle class that would have a radius property. It could also have its own circumference method.

Both the Circle and Rectangle classes would share the properties and methods of their base class, Shape, in addition to properties and methods unique to their own class definition.

// Create the base Shape class
class Shape {
  public:
    int center[2];
    int weight;
    int color;

  void translate(int deltaX, int deltaY) {
    center[0] += deltaX;
    center[1] += deltaY;
  }
};

// Create sub-class Rectangle
class Rectangle: public Shape {
  public:
    int height;
    int width;

  int perimeter() {
    return (height + width) * 2;
  }
};

// Create sub-class Circle
class Circle: public Shape {
  public:
    int radius;

  float circumference() {
    return 2.0 * 3.1416 * float (radius);
  }
};