Inheritance

Definition#

Inheritance provides a way to create a new class from an existing class. The new class is a specialized version of the existing class such that it inherits all the public attributes (variables) and methods of the existing class. The existing class is used as a starting point or base to create the new class.

The IS-A relationship#

After reading the definition above, the next question that comes to mind is, “when do we use inheritance?” Wherever we come across an IS-A relationship between objects, we can use inheritance.

So, from the description of inheritance above, we can conclude that we can build new classes by extending existing classes.

Let’s look at the figure below to visualize some examples where an IS-A relationship doesn’t exist.

Remember, we cannot use inheritance if an IS-A relationship doesn’t exist between classes.

Modes of inheritance#

Access modifiers are tags we can associate with each member to define the parts of the program they can access directly.  By using these modifiers, we define the scope of the data members and member functions for the other classes and main.

Types of inheritance#

Based on parent classes and child classes, there are five types of inheritance in general, which are explained below.

Single inheritance#

In single inheritance, there is only a single class extending from a single parent class.

Example:

• A fuel car IS-A vehicle

Multiple inheritance#

When a class is derived from more than one base class, i.e., when a class has more than one immediate parent class, it is called multiple inheritance.

Example:

• The hybrid car IS-A fuel car.

• The hybrid car IS-A electric car as well.

Multi-level inheritance#

When a class is derived from a class that itself is derived from another class, it is called multi-level inheritance. We can extend the classes to as many levels as we want.

Example:

• A fuel car IS-A vehicle

• A gasoline car IS-A fuel car

Hierarchical inheritance#

In hierarchical inheritance, more than one class extends, as per the requirement of the design, from the same base class. The common attributes of these child classes are implemented inside the base class.

Example:

• A fuel car IS-A vehicle

• An electric car IS-A vehicle

Hybrid inheritance#

A type of inheritance which is a combination of “multiple” and “multi-level” inheritance is called hybrid inheritance.

Example:

• A fuel car IS-A vehicle.

• An electric car IS-A vehicle.

• A hybrid car IS-A fuel car and IS-A electric car.

Implementation#

Let’s take an example of a Vehicle class and implement different classes that will extend from it. We will also implement hierarchical, multi-level, and multiple inheritances from this example.

Note: Some languages, such as Java, C# and JavaScript, do not support multiple inheritance through classes.

Advantages of inheritance#

The following are four main advantages of inheritance:

Let's learn about polymorphism in the next lesson.