What is the difference between association, aggregation and composition?

Question

What is the difference between association, aggregation, and composition? Please explain in terms of implementation.

Answer 1

Composition (If you remove "whole", “part” is also removed automatically– “Ownership”)

• Create objects of your existing class inside the new class. This is called composition because the new class is composed of objects of existing classes.

• Typically use normal member variables.

• Can use pointer values if the composition class automatically handles allocation/deallocation responsible for creation/destruction of subclasses.

Composition in C++

#include 
using namespace std;
/********************** Engine Class ******************/
class Engine
{
    int nEngineNumber;
    public:
    Engine(int nEngineNo);
    ~Engine(void);
};
Engine::Engine(int nEngineNo)
{
    cout<<" Engine :: Constructor " <

Output

--------------- Start Of Program --------------------
------------- Inside Car Block ------------------
Engine :: Constructor
Car :: Constructor
Car :: Destructor
Engine :: Destructor
------------- Out of Car Block ------------------
------------- Inside Bus Block ------------------
Engine :: Constructor
Bus :: Constructor
Bus :: Destructor
Engine :: Destructor
------------- Out of Bus Block ------------------
--------------- End Of Program --------------------

Aggregation (If you remove "whole", “Part” can exist – “ No Ownership”)

• An aggregation is a specific type of composition where no ownership between the complex object and the subobjects is implied. When an aggregate is destroyed, the subobjects are not destroyed.

• Typically use pointer variables/reference variable that point to an object that lives outside the scope of the aggregate class

• Can use reference values that point to an object that lives outside the scope of the aggregate class

• Not responsible for creating/destroying subclasses

Aggregation Code in C++

#include 
#include 
using namespace std;
/********************** Teacher Class ******************/
class Teacher
{
    private:
    string m_strName;
    public:
    Teacher(string strName);
    ~Teacher(void);
    string GetName();
};
Teacher::Teacher(string strName) : m_strName(strName)
{
    cout<<" Teacher :: Constructor --- Teacher Name :: "<

Output

--------------- Start Of Program --------------------
Teacher :: Constructor --- Teacher Name :: Reference Teacher
Teacher :: Constructor --- Teacher Name :: Pointer Teacher
------------- Inside Block ------------------
Department :: Constructor
Department :: Destructor
------------- Out of Block ------------------
Teacher :: Destructor --- Teacher Name :: Pointer Teacher
Teacher :: Destructor --- Teacher Name :: Reference Teacher
--------------- End Of Program --------------------