Implementing Stack Using Class Templates in C++

The task is to implement some important functions of stack like pop(), push(), display(), topElement(), isEmpty(), isFull() using class template in C++. Stack is a linear data structure that follows a particular order in which the operations are performed. The order may be LIFO(Last In First Out) or FILO(First In Last Out).

The simple idea is to pass data type as a parameter so that we don’t need to write the same code for different data types. For example, a software company may need sort() for different data types. Rather than writing and maintaining the multiple codes, we can write one sort() and pass data type as a parameter.

C++ adds two new keywords to support templates: ‘template’ and ‘typename’. The second keyword can always be replaced by the keyword ‘class’.

Illustration:

Consider an example of plates stacked over one another in the canteen. The plate which is at the top is the first one to be removed, i.e. the plate which has been placed at the bottommost position remains in the stack for the longest period of time. So, it can be simply seen to follow LIFO(Last In First Out)/FILO(First In Last Out) order.

Example:

C++14

// C++ Program to Implement stack using Class Templates

// Including input output libraries
#include <iostream>
// Header File including all string functions
#include <string>

using namespace std;

// Taking size of stack as 10
#define SIZE 5

// Class
// To represent stack using template by class
// taking class in template
template <class T> class Stack {
    // Public access modifier
public:
    // Empty constructor
    Stack();

    // Method 1
    // To add element to stack which can be any type
    // using stack push() operation
    void push(T k);

    // Method 2
    // To remove top element from stack
    // using pop() operation
    T pop();

    // Method 3
    // To print top element in stack
    // using peek() method
    T topElement();

    // Method 4
    // To check whether stack is full
    // using isFull() method
    bool isFull();

    // Method 5
    // To check whether stack is empty
    // using isEmpty() method
    bool isEmpty();

private:
    // Taking data member top
    int top;

    // Initialising stack(array) of given size
    T st[SIZE];
};

// Method 6
// To initialise top to
// -1(default constructor)
template <class T> Stack<T>::Stack() { top = -1; }

// Method 7
// To add element element k to stack
template <class T> void Stack<T>::push(T k)
{

    // Checking whether stack is completely filled
    if (isFull()) {
        // Display message when no elements can be pushed
        // into it
        cout << "Stack is full\n";
    }

    // Inserted element
    cout << "Inserted element " << k << endl;

    // Incrementing the top by unity as element
    // is to be inserted
    top = top + 1;

    // Now, adding element to stack
    st[top] = k;
}

// Method 8
// To check if the stack is empty
template <class T> bool Stack<T>::isEmpty()
{
    if (top == -1)
        return 1;
    else
        return 0;
}

// Utility methods

// Method 9
// To check if the stack is full or not
template <class T> bool Stack<T>::isFull()
{
    // Till top in inside the stack
    if (top == (SIZE - 1))
        return 1;
    else

        // As top can not exceeds th size
        return 0;
}

// Method 10
template <class T> T Stack<T>::pop()
{
    // Initialising a variable to store popped element
    T popped_element = st[top];

    // Decreasing the top as
    // element is getting out from the stack
    top--;

    // Returning the element/s that is/are popped
    return popped_element;
}

// Method 11
template <class T> T Stack<T>::topElement()
{
    // Initialising a variable to store top element
    T top_element = st[top];

    // Returning the top element
    return top_element;
}

// Method 12
// Main driver method
int main()
{

    // Creating object of Stack class in main() method
    // Declaring objects of type Integer and String
    Stack<int> integer_stack;
    Stack<string> string_stack;

    // Adding elements to integer stack object
    // Custom integer entries
    integer_stack.push(2);
    integer_stack.push(54);
    integer_stack.push(255);

    // Adding elements to string stack
    // Custom string entries
    string_stack.push("Welcome");
    string_stack.push("to");
    string_stack.push("GeeksforGeeks");

    // Now, removing element from integer stack
    cout << integer_stack.pop() << " is removed from stack"
         << endl;

    // Removing top element from string stack
    cout << string_stack.pop() << " is removed from stack "
         << endl;

    // Print and display the top element in integer stack
    cout << "Top element is " << integer_stack.topElement()
         << endl;

    // Print and display the top element in string stack
    cout << "Top element is " << string_stack.topElement()
         << endl;

    return 0;
}

Output

Inserted element 2
Inserted element 54
Inserted element 255
Inserted element Welcome
Inserted element to
Inserted element GeeksforGeeks
255 is removed from stack
GeeksforGeeks is removed from stack 
Top element is 54
Top element is to

Output explanation:

Here two data types (integer and string) are implemented using a single stack class. First, two objects are taken one is for integer class and the second is for the string class, Elements are inserted in both the classes using push() and isFull() method of stack class. Elements are removed using pop and isEmpty() functions of stack class. Finally, the top element is printed for each class using the top element() function.