In real-world programs an object is made up of several parts. For example a car is made up of several parts (objects) like engine, wheel, door etc. This kind of whole part relationship is known as composition which is also known as has-a relationship. Composition allows us to create separate class for each task. Following are the advantages or benefits of composition:

Each class can be simple and straightforward.
A class can focus on performing one specific task.
Classes will be easier to write, debug, understand, and usable by other people.
Lowers the overall complexity of the whole object.
The complex class can delegate the control to smaller classes when needed.

Following program demonstrates composition:


#include<iostream>

using namespace std;
class Engine {
  public:
    int power;
};
class Car {
  public:
    Engine e;
  string company;
  string color;
  void show_details() {
    cout << "Compnay is: " << company << endl;
    cout << "Color is: " << color << endl;
    cout << "Engine horse power is: " << e.power;
  }
};
int main() {
  Car c;
  c.e.power = 500;
  c.company = "hyundai";
  c.color = "black";
  c.show_details();
  return 0;
}

#include<iostream>

using namespace std;

class Engine {

public:

int power;

};

class Car {

public:

Engine e;

string company;

string color;

void show_details() {

cout << "Compnay is: " << company << endl;

cout << "Color is: " << color << endl;

cout << "Engine horse power is: " << e.power;

}

};

int main() {

Car c;

c.e.power = 500;

c.company = "hyundai";

c.color = "black";

c.show_details();

return 0;

}

Output for the above program is as follows:


Compnay is: hyundai
Color is: black
Engine horse power is: 500

Compnay is: hyundai

Color is: black

Engine horse power is: 500

Friend Functions

A friend function is a non-member function which can access the private and protected members of a class. To declare an external function as a friend of the class, the function prototype preceded with friend keyword should be included in that class. Syntax for creating a friend function is as follows:


class ClassName
{
...
friend return–type function–name(params–list);
...
};

class ClassName

{

...

friend return–type function–name(params–list);

...

};

Following are the properties of a friend function:

Friend function is a normal external function which is given special access to the private and protected members of a class.
Friend functions cannot be called using the dot operator or -> operator.
Friend function cannot be considered as a member function.
A function can be declared as friend in any number of classes.
As friend functions are non-member functions, we cannot use this pointer with them.
The keyword friend is used only in the declaration. Not in the definition.
Friend function can access the members of a class using an object of that class.

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What is object composition in C++ programming2 min read

Friend Functions

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Friend Functions

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