Pass-by-reference means to pass the reference of an argument in the calling function to the corresponding formal parameter of the called function.

The called function(callee) can modify the value of the argument by using its reference passed in.

The reference parameters are initialized with the actual arguments when the function is called.

#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;
}

To modify a reference that is qualified by the const qualifier, use const_cast operator to cast away its constness.

(Maybe it would be strange to change the value of const variable)

#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;
}

Compared to pass-by-value

• Pass-by-reference does not copy the arguments. (More efficient)
• Actual modification is made with pass-by-reference, while pass-by-value is not.

Compared to pass-by-pointer

• Pointers can be NULL or reassigned whereas references cannot.
• Use pass-by-pointer if NULL is a valid parameter value or if you want to reassign the pointer.
• Otherwise, use constant or non-constant references to pass arguments.

My thought

• It would be generally better to use reference than value/pointer.
• Let’s use const reference instead of value.
• Let’s use reference instead of pointer when we definitely don’t need NULL.

See also

https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/cplr233.htm