02_Composition

Composition

Defination

It is case where one object(part) is a** part** of another object(whole).It is also known as part-whole relationship.It is** strong relationship** because ownership is involved.

To qualify for composition 😎 ?

1. The part(member) is part of the object(class).

explanation

As in the picture, class B is a part of class A, it means that class B behaves as a data member for class A. This is an example of aggregation, where class B is being used by class A but class B is independent of class A and can exist on its own.

2. The part(member) can only belong to one object(class) at a time.

explanation

As in the above picture, it is clear that class B is a private data member of class A, so it can not be a data member of another class at the same time. This makes sense, since class B is a part of class A and cannot be shared with another class. This relationship between class A and class B is an example of aggregation, where class B is used by class A but is independent and can exist on its own.

3. The part(member) has its existence managed by the object(class).

explanation

When object of** A** is created object of ** B** is created inside** A** too. When object of** A** is destroyed part object of** B** is also destroyed.

4. The part(member) does not know about the existence of the object(class).

explanation

The creation and destruction of class B (a data member of class A) does not affect or control the creation and destruction of an object of class A. This is an important aspect of aggregation, where the lifetime of the contained class (class B) is independent of the containing class (class A). The containing class (class A) may create and destroy instances of the contained class (class B) as needed, but the contained class (class B) can also exist independently and potentially be used by other classes.

why there is a need of composition ? 👀

Note

In the above picture : Single class person controls every thing.

Disadvantages

• Not scalable
• Error prone
• Not reusable in other class
• Redefine all attributes and functions separately for other classes
• For example student, doctor teacher, and patient

Note

In the above picture : -Design separate classes(name, date, address, person).

• Add objects(name, date, address) as variables in class(** person**).

Advantages

• Scalable
• Less Error prone
• Reusable in other classes such as student, doctorand teacher, patient
• No need to redefine all attributes and functions separately for other classes

calling sequence 👀

-Default constructor: in same order as defined objects in class 1)name 2)date 3)address 4)person

• Destructor: in reverse order as defined objects in class 1)person 2)address 3)date 4)name
• Parametrized constructor: called in order of member initializer syntax : dateofBirth(d, m, y), pname(fn, ln), paddress(city, country, street, house) 1)date 2)name 3)address 4)person

#include<iostream>
using namespace std;
#include<cstring>

class Address
{
    int houseNo;
    char blockNo;
    char town[30];
    char city[30];

public:
    Address(int h, int b, char* t, char* c) // Parameterized constructor
    {
        houseNo = h;
        blockNo = b;
        strcpy(town, t); // strcpy( ) is a built - in function used
        //to copy one string(character array) to another, However, we can also define our own function.strcpy(city, c);
    }
    friend ostream& operator<<(ostream& out, const Address& a)
    {
        out << "House No:" << a.houseNo << '\n';
        out << "Block No:" << a.blockNo << '\n';
        out << "Town:" << a.town << '\n';
        out << "City:" << a.city << '\n';
        return out;
    }
};

class Employee
{
    static int employeeCount; // To Assign employeeID
    int employeeID;
    char name[50];
    Address address;
    int salary;
public:
    // Parameterized Constructor to initialize the Address of the employee

    Employee(int s, char* n, int h, char b, char* t, char* c)
        : address(h, b, t, c)
    {
        employeeID = employeeCount;
        strcpy(n, name);
        salary = s;
        employeeCount++;
    }
    friend ostream& operator << (ostream& out, const Employee& a)
    {
        out << "Employee No:" << a.employeeID << '\n';
        out << "Name :" << a.name << '\n';
        out << "Salary:" << a.salary << '\n';
        out << "Address\n" << a.address; // Calls the stream operator from Address class
        return out;
    }
};
int Employee::employeeCount = 1;

int main()
{
    char name[] = "Waleed";
    char town[] = "Johar Town";
    char city[] = "Lahore";

    Employee Emp_1(100000, name, 46, 'H', town, city);
    cout << Emp_1;
    return 0;

}