Sams Teach Yourself C++ in 21 Days

Exploiting References 265

9

23:

24: temp = *px;

25: *px = *py;

26: *py = temp;

27:

28: cout << “Swap. After swap, *px: “ << *px <<

29: “ *py: “ << *py << endl;

30:

31: }

Main. Before swap, x: 5 y: 10

Swap. Before swap, *px: 5 *py: 10

Swap. After swap, *px: 10 *py: 5

Main. After swap, x: 10 y: 5

Success! On line 5, the prototype of swap()is changed to indicate that its two

parameters will be pointers to intrather than intvariables. When swap()is

called on line 12, the addresses of xand yare passed as the arguments. You can see that

the addresses are passed because the address-of operator (&) is being used.

On line 19, a local variable,temp, is declared in the swap()function. tempneed not be a

pointer; it will just hold the value of *px(that is, the value of xin the calling function)

for the life of the function. After the function returns,tempis no longer needed.

On line 24,tempis assigned the value at px. On line 25, the value at pxis assigned to the

value at py. On line 26, the value stashed in temp(that is, the original value at px) is put

into py.

The net effect of this is that the values in the calling function, whose address was passed

to swap(), are, in fact, swapped.

Implementing swap()with References ..........................................................

The preceding program works, but the syntax of the swap()function is cumbersome in

two ways. First, the repeated need to dereference the pointers within the swap()function

makes it error-prone—for instance, if you fail to dereference the pointer, the compiler

still lets you assign an integer to the pointer, and a subsequent user experiences an

addressing error. This is also hard to read. Finally, the need to pass the address of the

variables in the calling function makes the inner workings of swap()overly apparent to

its users.

It is a goal of an object-oriented language such as C++ to prevent the user of a function

from worrying about how it works. Passing by pointers puts the burden on the calling

OUTPUT

LISTING9.6 continued

ANALYSIS