An example where we call a function called sum() from main();

sum1.cpp

// Prototypes:
int sum(int, int);

/* Main program */

int main()
{
  
  int z = sum(1, 1);    //  z will become 2 here

  return 0;
}

/* Program definition */

int sum(int x, int y)
{
  return x+y;
}