undefined reference in C++ when trying to use header file

Question

I am very new to C++ and i have been given an assignment to display stack implementation using array in C++. I developed the Program using two .cpp files i.e main and calculation part. But after i submitted by assignment the teacher told me tu use the header file to develop the program . I am trying to get the same output using the header file but it gives undefined reference error. Any help would be much appreciated.

My Calc.h file

#include <iostream>
using namespace std;
#define SIZE 100
class Calc {
int *arr,n,top=-1;

public:
Calc(int size = SIZE);         // constructor
~Calc();

public:

void push(int val);

void pop();

void displayStackElems();

};

Calc::Calc(int size)
{
arr = new int[size];
n = size;
top = -1;
}

// Destructor to free memory allocated to the stack
Calc::~Calc() {
delete[] arr;
}

Calc.cpp

#include <iostream>
#include "Calc.h"
using namespace std;
void Calc::push(int val) {
        if (top >= n-1)
            cout<<"Stack Overflow"<<endl;
        else{
            top++;
            arr[top]=val;
        }
}

void Calc::pop() {
        if (top <=-1)
            cout<<"Stack Underflow"<<endl;
        else{
            cout << "The popped element is " << arr[top] << endl;
            top--;
        }
}

void Calc::displayStackElems() {
    if(top >=0){
            cout<<"Stack elements are : ";
            for (int i = top; i >=0 ; i--)
                cout << arr[i] << " ";
            cout<<endl;
        } else
            cout<<"Stack is empty";
}

Main.cpp

#include <iostream>
using namespace std;
#include "Calc.h"

int main() {
    int LengthOfStack;
    cout<<"Enter the length of the stack you want : \n"<<endl;
    cin >> LengthOfStack;

    Calc c(LengthOfStack);

    int choice,value;
    cout<<"1. Push in arr"<<endl;
    cout<<"2. Pop from arr"<<endl;
    cout<<"3. Display arr"<<endl;
    cout<<"4. Exit"<<endl;

    do {
        cout<<"Enter choice : "<<endl;
        cin >> choice;
        switch(choice) {
            case 1: {
                cout<<"Enter value to be pushed:"<<endl;
                cin >> value;
                c.push(value);
                break;
            }
            case 2: {
                c.pop();
                break;
            }
            case 3: {
                c.displayStackElems();
                break;
            }
            case 4: {
                cout<<"Exit"<<endl;
                break;
            }
            default: {
                cout<<"Invalid Choice"<<endl;
            }
        }
    } while (choice != 4);
    return 0;

The Error Log :

6_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: CMakeFiles/untitled.dir/main.cpp.o:/cygdrive/z/Academics/Tezu/DS/untitled/main.cpp:25: undefined reference to `Calc::push(int)'
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: CMakeFiles/untitled.dir/main.cpp.o: in function `main':
/cygdrive/z/Academics/Tezu/DS/untitled/main.cpp:29: undefined reference to `Calc::pop()'
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: /cygdrive/z/Academics/Tezu/DS/untitled/main.cpp:33: undefined reference to `Calc::displayStackElems()'
collect2: error: ld returned 1 exit status