Pass by reference

Passing by by reference refers to a method of passing the address of an argument in the calling function to a corresponding parameter in the called function. In C, the corresponding parameter in the called function must be declared as a pointer type. In C++, the corresponding parameter can be declared as any reference type, not just a pointer type.

In this way, the value of the argument in the calling function can be modified by the called function.

The following example shows how arguments are passed by reference. In C++, the reference parameters are initialized with the actual arguments when the function is called. In C, the pointer parameters are initialized with pointer values when the function is called.

Beginning of C++ only.

#include <stdio.h>

void swapnum(int &i, int &j) {
  int temp = i;
  i = j;
  j = temp;
}

int main(void) {
  int a = 10;
  int b = 20;

  swapnum(a, b);
  printf("A is %d and B is %d\n", a, b);
  return 0;
}

End of C++ only.

Beginning of C only.

#include <stdio.h>

void swapnum(int *i, int *j) {
  int temp = *i;
  *i = *j;
  *j = temp;
}

int main(void) {
  int a = 10;
  int b = 20;

  swapnum(&a, &b);
  printf("A is %d and B is %d\n", a, b);
  return 0;
}

End of C only.

A is 20 and B is 10

Beginning of C++ only.

#include <iostream>
using namespace std;

void f(const int& x) {
  int& y = const_cast<int&>(x);
  y++;
}

int main() {
  int a = 5;
  f(a);
  cout << a << endl;
}

End of C++ only.