What is the difference between using composition and aggregation in class and object design?

Composition and aggregation are two types of relationships between objects in object-oriented design.

• Composition is a "part-whole" relationship where the part cannot exist without the whole. If the whole object is destroyed, its parts are also destroyed. Example: class Car and class Engine. The engine exists only inside the car.

• Aggregation is a weaker relationship where the part can exist independently of the whole. For example, class Class and class Student. A student can exist without being associated with a specific class.

Example in Python:

class Engine:
    pass

class Car:
    def __init__(self):
        self.engine = Engine()  # composition

class Student:
    pass

class SchoolClass:
    def __init__(self, students):
        self.students = students  # aggregation

s1 = Student()
s2 = Student()
class1 = SchoolClass([s1, s2])

Here, Engine is created inside Car and does not exist separately, whereas Student is created independently and passed into SchoolClass.