Question

Many programs written with inheritance could be written with composition instead, and vice versa. Rewrite class BasePlusCommissionEmployee of the CommissionEmployeeBasePlusCommissionEmployee hierarchy to use composition rather than inheritance. After you do this, assess the relative merits of the two approaches for designing classes commissionEmployee and BasePlusCommissionEmployee, as well as for object-oriented programs in general. Which approach is more natural? Why?

Short answer

Composition provides flexibility and simplicity in class design, making it a preferred approach in many cases over inheritance.

Step-by-step solution

Understand Inheritance

In inheritance, a class (child class) is derived from another class (parent class). The `BasePlusCommissionEmployee` class inherits properties from the `CommissionEmployee` class. It adds a base salary feature on top of commission calculations.

Problem with Inheritance

Using inheritance can lead to issues if `BasePlusCommissionEmployee` is considered to logically not a 'kind of' `CommissionEmployee`. Inheritance results in a tightly coupled relationship that makes code difficult to modify or test.

Understanding Composition

Composition involves including an instance of one class (part class) within another (whole class). For `BasePlusCommissionEmployee`, instead of inheriting from `CommissionEmployee`, it includes a `CommissionEmployee` object as an attribute.

Rewriting with Composition

Modify `BasePlusCommissionEmployee` to contain a private `CommissionEmployee` object. Provide methods within `BasePlusCommissionEmployee` to access `CommissionEmployee`'s functionalities and add its own logic for handling the base salary.

Create Composition Structure

```python class CommissionEmployee: # existing implementation class BasePlusCommissionEmployee: def __init__(self, commission_employee, base_salary): self.commission_employee = commission_employee self.base_salary = base_salary def earnings(self): return self.base_salary + self.commission_employee.earnings() # additional methods as required ``` In this structure, `BasePlusCommissionEmployee` uses an instance of `CommissionEmployee` and separate logic for base salary.

Assessing Relative Merits of Both Approaches

Inheritance promotes a hierarchy where `BasePlusCommissionEmployee` is viewed as a type of `CommissionEmployee`, suitable when there is a true 'is a' relationship. However, composition is more flexible and better suited for cases where `BasePlusCommissionEmployee` 'has a' `CommissionEmployee`, providing easier maintenance and scalability.

Conclusion on Preferred Approach

Composition is often a more natural approach in scenarios where components of a program need to be reusable and not strongly hierarchically related. It allows for greater flexibility and adaptability in design, reducing dependencies.

Key concepts

inheritance in C++

Imagine you have a family tree, where characteristics are passed down from one generation to the next. In the world of programming, inheritance works similarly. This concept allows a new class, called a derived or child class, to acquire properties and behavior from an existing class, known as the base or parent class. In C++, inheritance is a cornerstone of object-oriented programming (OOP), enabling classes to build on one another.

The purpose of inheritance is to promote code reusability and establish a logical hierarchy between classes. For example, in a payroll system, a `BasePlusCommissionEmployee` could inherit from a `CommissionEmployee`. This means `BasePlusCommissionEmployee` will automatically have the characteristics of a `CommissionEmployee`, such as calculating commissions. However, inheritance suggests a strong "is a" relationship, implying that a `BasePlusCommissionEmployee` is a type of `CommissionEmployee`, which may not always be logical or appropriate.

• Pros: Simplifies the code by reducing redundancy, easy to understand relationships between objects.
• Cons: Can introduce rigidity due to tight coupling, which makes future changes and testing difficult.

In essence, while inheritance offers simplicity and elegance in some situations, it should be used judiciously to avoid complications in code maintenance and extension.

composition in C++

Consider composition like putting together a puzzle to form a complete picture. In C++, composition involves creating classes that contain instances of other classes as members. This design principle focuses on the "has a" relationship rather than "is a".

For instance, instead of a `BasePlusCommissionEmployee` inheriting from `CommissionEmployee`, it can include a `CommissionEmployee` as a member object. This means that `BasePlusCommissionEmployee` uses the services of `CommissionEmployee` to perform its calculations but retains its own identity and can add new functionalities independently. With this approach, the two classes are not tightly bound, allowing changes in one without affecting the other significantly.

• Pros: Greater flexibility, easier to maintain and extend, components are more reusable.
• Cons: Can lead to more verbose code as interactions between class members need explicit handling.

By utilizing composition, programmers can build robust and scalable systems where components can be easily modified or replaced, fostering more adaptable code structures.

class design concepts

Designing classes is akin to constructing a building plan, where every element has a specific role and purpose. In object-oriented programming, classes serve as blueprints for creating objects, encapsulating data and functionalities.

An effective class design not only tackles current needs but anticipates future adjustments and extensions. Here are some guiding principles:

• Encapsulation: Enclosing data and methods within a class, restricting direct access from outside.
• Single Responsibility: Each class should have a single purpose or responsibility, ensuring clarity and simplicity.
• Open/Closed Principle: Design classes that are open for extension but closed for modification, enhancing adaptability without altering existing codebase.
• DRY (Don't Repeat Yourself): Avoid redundancy by ensuring each piece of knowledge has a single, unambiguous place within the system.

Balancing inheritance and composition in class design is crucial. While inheritance provides clarity in "is a" hierarchies, composition offers flexibility in "has a" relationships. Making informed decisions about which to use based on context and requirements leads to more maintainable and scalable code nests.