Balancing Ease of Use and Purity in Inheritance and Polymorphism

In the world of Object-Oriented Programming (OOP), the concepts of inheritance and polymorphism play a critical role in how we design our applications. While they offer programming ease, they also pose challenges, particularly in defining the relationships between objects. This blog post uncovers the dilemma often faced by developers: the balance between ease of use and purity in code design. Specifically, we will explore how to utilize inheritance and polymorphism effectively without compromising the integrity of the relationships among objects.

The Dilemma: Inheritance vs Polymorphism

Many developers find themselves in a scenario where they need different objects to perform similar actions. For instance, in a project aimed at processing data sets, various objects may need to maintain a damage counter. It’s easy to think of using polymorphism to allow these different objects to “act the same.” However, polymorphism inherently follows an “is a” relationship, whereas in many cases, we find it more appropriate to describe them as “has a” relationship.

Key Differences:

• Inheritance: Implies an “is a” relationship (e.g., a Person is a Damage counter).
• Composition: Refers to a “has a” relationship (e.g., a Person has a damage counter).

This distinction raises the question: Should we sacrifice the ideal of clarity in relationships for the sake of ease of programming?

Possible Solution: Embracing Multiple Inheritance

For languages like C++, a robust solution to this problem is to employ multiple inheritance alongside the use of pure virtual classes to create interfaces. This approach allows for flexibility without compromising the logical models often needed in application development.

Step-by-Step Approach:

1. Define Interfaces: Start by creating pure virtual classes that define the desired interfaces. For example, you might define a Damage interface.

   class Damage {
       virtual void addDamage(int d) = 0;
       virtual int getDamage() = 0;
   };
   

2. Implement the Interface: Next, implement this interface in the classes where the behavior is needed. Both the Person and Car classes could implement the Damage interface:

   class Person : public virtual Damage {
       void addDamage(int d) {
           // Implementation for Person
           damage += d * 2;
       }
       int getDamage() {
           return damage;
       }
   };
   
   class Car : public virtual Damage {
       void addDamage(int d) {
           // Implementation for Car
           damage += d;
       }
       int getDamage() {
           return damage;
       }
   };
   

3. Maintain Relationships: By doing this, both Person and Car now implement the Damage interface, which satisfies the logic of “is a” at the same time as respecting their inherent qualities of “has a”.

Advantages of This Approach:

• Clarity: It maintains a clear model of relationships between objects.
• Flexibility: Future changes in implementation do not affect the system adversely. This adheres to the Open-Closed Principle, which states that software entities should be open for extension but closed for modification.

Conclusion

The balance between ease of use and purity in code design is a common challenge in OOP. By strategically employing multiple inheritance and utilizing pure virtual classes, developers can achieve the desired polymorphic behavior while keeping their code’s logical structure intact. This approach allows for clear relationships between objects, ultimately leading to an even more maintainable codebase.

In the ever-evolving landscape of programming, it’s crucial to find solutions that promote both functionality and clarity. Embracing these practices can lead to more robust and understandable applications that stand the test of time.