functions in c call by value and call by reference

Question

The code below shows how to call a function in both methods. 

Please explain the main difference or meaning of call by value and call by reference. 

1. Make a value-based call. 

2. Make a reference call. 

The call by value method is demonstrated in the following code.

In the comments, I expressed my reservations.

#include<iostream>
int main(){
void change(int);//why function prototype is before function definition and what is 
int orig=10;//meaning of argument int, it did not defined any variable of type int
cout<<"The original value is: "<<orig<<"\n";
change(orig);//what is the meaning of this piece of code
cout<<"Value after change() is over:"<<orig<<"\n";
return 0;
};
void change(int orig){
orig=20;
cout<<"Value of orig in function change() is:"<<orig<<"\n";
return;
}

According to the book, function definition should come before function prototype.

Answer 1

Calling a function by value copies the argument and stores it in a local variable for use by the function, so the argument is unaffected if the value of the local variable changes. 

The argument is passed to the function as a reference rather than a copy, so if the function changes the value of the argument, the argument is also changed.

 

The void change(int); function prototype informs the compiler that there is a function named change that takes a single int argument and returns void (i.e. nothing). 

Because there is no & with the argument, it is called by value. 

You have the line change(orig); later in your code, which actually calls the function with.

Take a look at the output of this program:

#include<iostream>

using namespace std;

int main(){
  void change(int);
  void change2(int&);
  int x = 10;
  cout<<"The original value of x is: "<< x <<"\n";
  change(x); // call change(), which uses call by value
  cout<<"Value of x after change() is over: "<< x <<"\n";
  change2(x); // call change2(), which uses call by reference
  cout<<"Value of x after change2() is over: "<< x <<"\n";
  return 0;
};

void change(int orig){
    cout<<"Value of orig in function change() at beginning is: "<<orig<<"\n";
    orig=20;
    cout<<"Value of orig in function change() at end is: "<<orig<<"\n";
  return;
}

void change2(int &orig){
    cout<<"Value of orig in function change2() at beginning is: "<<orig<<"\n";
    orig=20;
    cout<<"Value of orig in function change2() at end is: "<<orig<<"\n";
  return;
}

I've changed int orig in main() to int x to hopefully avoid name confusion, and I've added change2() which uses call by reference.