pass a string to a function by reference in c++ code example

Example: how to pass a string by reference in c++

#include <iostream>
using namespace std;
void func (string &s)
{
  s+="d";
}
int main()
{
  string a="abc";
 	func(a);
  cout << a << endl;
  return 0;
}
// output will be abcd