Actual argument and Formal parameter:

• An actual argument is used in a function call.
• A formal parameter is used in a function header.
• Formal parameters receive the values of the respective actual arguments when a function is called.

• A value parameter receives a copy of the actual argument.
• A reference parameter receives the address of the actual argument.
• Operations on value parameters within the function do not affect the respective actual argument values.
• Operations on reference parameters within the function do affect the respective actual argument values.

#include <iostream.h> void getData(int &x, int &y); void main () {  int num1=100, num2=200;     getData(num1, num2);  cout<<"num1= "<<num1<<", num2= "<<num2<<endl; }  void getData(int &x, int &y) {  cout<<"x= "<<x<<", y= "<<y<<endl;  cout<<"Enter two integers."<<endl;  cin>>x>>y;  cout<<"x= "<<x<<", y= "<<y<<endl; }	#include <iostream.h> void getData(int x, int y); void main () {  int num1=100, num2=200;     getData(num1, num2);  cout<<"num1= "<<num1<<", num2= "<<num2<<endl; }  void getData(int x, int y) {  cout<<"x= "<<x<<", y= "<<y<<endl;  cout<<"Enter two integers."<<endl;  cin>>x>>y;  cout<<"x= "<<x<<", y= "<<y<<endl; }
Pass by reference: num1 and x use the same box,  num2 and y use the same box.	Pass by value, num1 and x are different boxes. when main()calls getaDat(), the value of num1 is copied to x and the value of num2 is copied to y. But, when you finish executing getData(), and go back to main(), the value of x is not copied back to num1, similar to y and num2.
num1 and num2 are local variables for main() In the getData(), you can not reference num1 and num2 .  i.e. You can not use them in the getData(). That is, the scope of num1 and num2 is main() only. Similarly, the scope of x and y is getData() only.

See an additional example