Composite design pattern in C++ with real-life examples

The composite design patterns is a part of the structural design pattern s which is used when you are creating a system or software in a tree structure

We can also say that it is easy to implement a tree structure with a composite design pattern

A real-life example

Suppose you are making a software application called a school bag and the application is implemented using a tree structure which is represented in the below object structure diagram

The application school bag contains three sections

If we open section 1 we can draw a water bottle and section 3

If we open section 3 we can draw the Book and lunch box

If we open section 2 then we can draw colour pencils and a drawing book

The software application called school contains  4 composite objects and  5 leaf objects

Composite: composite object can have multiple objects

Leaf: leaf object should not have objects

Object structure

Structure

C++ example

In the below example I have used list data structure  you can use any data structure like arrays, hash tables, trees  

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

class School_bag
{
public:
    virtual void draw() = 0;
};
class Water_bottle : public School_bag
{
public:
    void draw(){
        cout << "Water bottle" << endl;
    }
};
class Colour_pencils : public School_bag
{
public:
    void draw(){
        cout << "Colour_pencils" << endl;
    }
};
class Lunch_box : public School_bag
{
public:
    void draw(){
        cout << "Lunch_box" << endl;
    }
};
class Drawing_book : public School_bag
{
public:
    void draw(){
        cout << "Drawing_book" << endl;
    }
};
class Book: public School_bag
{
public:
    void draw(){
        cout << "Books" << endl;
    }
};
class CompositeSchool_bag : public School_bag
{
private:
    list<School_bag *> child_Schoolbag;
public:
    void draw()
    {
        for(list<School_bag *>::iterator Schoolbag  = child_Schoolbag.begin();  Schoolbag != child_Schoolbag.end(); ++Schoolbag){
            (*Schoolbag)->draw();
        }
    }
    void add(School_bag *Schoolbag)
    {
        child_Schoolbag.push_back(Schoolbag);
    }
    void remove(School_bag *Schoolbag)
    {
		child_Schoolbag.remove(Schoolbag);
    }
};
int main()
{
    Water_bottle *Water_bottle1 = new Water_bottle();
	 Colour_pencils*Colour_pencils1 = new Colour_pencils();
	 Lunch_box*Lunch_box1=new Lunch_box();
	 Book *Book1= new Book();
	Drawing_book *Drawing_book1 = new Drawing_book();
	CompositeSchool_bag *Schoolbag = new CompositeSchool_bag();
	CompositeSchool_bag *Section1 = new CompositeSchool_bag();
	CompositeSchool_bag *Section2= new CompositeSchool_bag();
	CompositeSchool_bag *Section3= new CompositeSchool_bag();
	Schoolbag->add(Section1);
	Schoolbag->add(Section2);
	Section1->add(Water_bottle1);
	Section1->add(Section3);
	Section2->add(Colour_pencils1);
	Section2->add(Drawing_book1);
	Section3->add(Lunch_box1);
	Section3->add(Book1);
	Schoolbag->draw();
    return 0;
}

Output

Water bottle
Lunch_box
Book
Colour pencils
Drawing_book

You can use the same operation on leaf and composite but if it is leaf object it will only print the value of the leaf

For example, the leaf object Book1 will print only the value of the leaf

Book1->draw();

Suppose if it is a composite object then it will print all values of leaf object

Section1->draw();