Sams Teach Yourself C++ in 21 Days

Exploiting References 263

9

Passing an object by reference enables the function to change the object being referred

to. On Day 5, you learned that functions are passed their parameters on the stack. When

a function is passed a value by reference (using either pointers or references), the address

of the original object is put on the stack, not the entire object. In fact, on some comput-

ers, the address is actually held in a register and nothing is put on the stack. In either

case, because an address is being passed, the compiler now knows how to get to the orig-

inal object, and changes are made there and not in a copy.

Recall that Listing 5.5 on Day 5 demonstrated that a call to the swap()function did not

affect the values in the calling function. Listing 5.5 is reproduced here as Listing 9.5, for

your convenience.

LISTING9.5 Demonstrating Passing by Value

1: //Listing 9.5 - Demonstrates passing by value

2: #include <iostream>

3:

4: using namespace std;

5: void swap(int x, int y);

6:

7: int main()

8: {

9: int x = 5, y = 10;

10:

11: cout << “Main. Before swap, x: “ << x << “ y: “ << y << endl;

12: swap(x,y);

13: cout << “Main. After swap, x: “ << x << “ y: “ << y << endl;

14: return 0;

15: }

16:

17: void swap (int x, int y)

18: {

19: int temp;

20:

21: cout << “Swap. Before swap, x: “ << x << “ y: “ << y << endl;

22:

23: temp = x;

24: x = y;

25: y = temp;

26:

27: cout << “Swap. After swap, x: “ << x << “ y: “ << y << endl;

28: }

Main. Before swap, x: 5 y: 10

Swap. Before swap, x: 5 y: 10

Swap. After swap, x: 10 y: 5

Main. After swap, x: 5 y: 10

OUTPUT