c++ - Call by reference function and syntax -

the code sorts 3 numbers in ascending order. but, swap function has been invoked reference. why must sort function invoked reference ? question swap function has been invoked reference. so, why need sort function reference ? i'm confused. secondly, cout << endl, doesn't give error so, i've pressed wrongly comma. how come ?

#include <iostream>   using namespace std;   void swap ( int& a, int& b ); void sort ( int a, int b, int c );   int main() {      int num1, num2, num3;      cout << "enter first number => ";     cin  >> num1;     cout << "enter second number => ";     cin  >> num2;     cout << "enter third number => ";     cin  >> num3;      cout << endl,     cout << "before sorting numbers\n" << num1          << " " << num2 << " " << num3 << endl;      sort( num1, num2, num3 );      cout << "after sorting numbers\n" << num1     << " " << num2 << " " << num3 << endl;      return 0; }   void swap ( int& a, int& b ) {      int temp = a;     = b;     b = temp; } void sort ( int a, int b, int c ) { //void sort ( int& a, int& b, int& c )      if (a > b)         swap(a, b);      if (a > c)         swap(a, c);      if (b > c)         swap(b, c); } 

when call sort function, 3 new variables created in scope of function. these variables copies of 3 variables pass in main function. 3 variables going passed swap function, takes 2 integer addresses. variable address you're passing scoped within sort function. use pointers keep more organized.

int main() {      int *num1, *num2, *num3;      cout << "enter first number => ";     cin  >> *num1;     cout << "enter second number => ";     cin  >> *num2;     cout << "enter third number => ";     cin  >> *num3;      cout << endl,     cout << "before sorting numbers\n" << *num1          << " " << *num2 << " " << *num3 << endl;      sort( num1, num2, num3 );      cout << "after sorting numbers\n" << *num1     << " " << *num2 << " " << *num3 << endl;      return 0; }   void swap ( int *a, int *b ) {      int *temp = a;     = b;     b = temp; } void sort ( int *a, int *b, int *c ) {      if (*a > *b)         swap(a, b);      if (*a > *c)         swap(a, c);      if (*b > *c)         swap(b, c); } 

i didn't have chance compile this, later today see if works.