Modifying Arguments passed by reference in the function call in a recursive function

Question

Here is a simple code that takes counter as a reference passed argument and then prints it:

  #include <iostream>
using namespace std;
void Fun(int &counter, int n)
{
    if(n==0)
      return;
    Fun(counter+1,n--);
}


int main() {
    int counter = 0;
    int n = 5;
    Fun(counter,n);
    cout<<counter<<endl;
    return 0;
}

I'm getting this error.

  prog.cpp: In function ‘void Fun(int&, int)’:
prog.cpp:7:16: error: invalid initialization of non-const reference of type ‘int&’ from an rvalue of type ‘int’
     Fun(counter+1,n);
                ^
prog.cpp:3:6: note:   initializing argument 1 of ‘void Fun(int&, int)’
 void Fun(int &counter, int n)
  ^

Can somebody please help, why this error?

Answer 1

In Fun(counter+1,n--); you are not passing counter to the function. You create a temporary from counter+1 that you pass to the function. To extend the life of a temporary taken by reference, it needs to be const , so void Fun(const int &counter, int n) would be compileable.

Counter will however be 0 when the function ends since you never change counter and the function will never return since you do not decrease the n you pass to the function. You call the function with n and then decrease n .

An alternative:

void Fun(int &counter, int n)
{
    if(n==0)
      return;
    Fun(counter += 1, n - 1); // or Fun(++counter, --n);
}

Both counter += 1 and ++counter return a reference to counter which is why this works.

counter++ and n-- will however not work, because the post-increment operators return temporaries too, like in:

int old = n;
n = n - 1;
return old;