## Concept of single and multi-instance design

Below code shows how C implements multi-instance design and how it is done by C++.
Also, the efficiency of C++ is equivalent to C, which can be shown with the compiler
generated code.

In code, it is also been shown that templates instead of creating any overhead,
makes the code even faster by compiling only the relevant part.
```C++
// design_pattern4_day3.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"


using namespace std;


// Implement Stack data structure in C language

// Version - 1 - Single Instance design in C
/*int items[10];
int top = 0;

void push(int item)
{
	items[top] = item;
	top++;
}

int pop()
{
	top--;
	return items[top];
}*/

// Version - 2 - Multi Instance design in C

/*struct Stack
{
	int items[10];
	int top = 0;
};

void push(Stack *s, int item)
{
s->items[s->top] = item;
s->top++;
}

int pop(Stack *s)
{
s->top--;
return s->items[s->top];
}*/

// Version - 3 - Multi Instance design in C++

struct Stack
{
	int items[10];
	int top = 0;
	// gauranteed initialization
	Stack()
	{

	}
	// gauranteed de-initialization
	~Stack()
	{

	}

	//operator overaloding, which is not in C

	void push(int item) // void push(Stack *this, int item)
	{
		items[top] = item; // this->items[this->top] = item;
		top++; // this->top++;
	}

	int pop()
	{
		top--;
		return items[top];
	}
};


// Templated class are never classes, instead they are used to create new classes out of them
// And once class object is created, only those functions are compiled which are called.
template<class T>
class LinkedList
{
public:
	void add()
	{
		abdhdfff;
	}
};

// ---------------------------Client--------------------------
int main()
{
	// Multi-instance design will require address
	/*Stack s1, s2, s3;

	push(&s1, 100);
	push(&s1, 200);
	push(&s1, 300);*/
	Stack s1, s2, s3;

	s1.push(100); //push(&s1, 100);
	s1.push(200); //push(&s1, 100);
	s1.push(300); //push(&s1, 100);

	LinkedList<int> l1;
	//l1.add(5); // If this is uncommented, then only compilation error will happen.
	return 0;
}
```