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Once
a pointer is assigned the address of a variable,
you can use that pointer to access the data in that
variable. Listing 8.2 demonstrates how the address
of a local variable is assigned to a pointer and
how the pointer manipulates the values in that variable.
Listing
8.2. Manipulating data by using pointers.
1:
// Listing 8.2 Using pointers
2:
3:
#include <iostream.h>
4:
5:
typedef unsigned short int USHORT;
6:
int main()
7:
{
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USHORT myAge; // a variable
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USHORT * pAge = 0; // a pointer
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myAge = 5;
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cout << "myAge: " << myAge <<
"\n";
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pAge = &myAge; // assign address of myAge to
pAge
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cout << "*pAge: " << *pAge <<
"\n\n";
16:
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cout << "*pAge = 7\n";
18:
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*pAge = 7; // sets myAge to 7
20:
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cout << "*pAge: " << *pAge <<
"\n";
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cout << "myAge: " << myAge <<
"\n\n";
23:
24:
25:
cout << "myAge = 9\n";
26:
27:
myAge = 9;
28:
29:
cout << "myAge: " << myAge <<
"\n";
30:
cout << "*pAge: " << *pAge <<
"\n";
31:
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return 0;
33:
}
Output:
myAge: 5
*pAge: 5
*pAge = 7
*pAge: 7
myAge: 7
myAge = 9
myAge: 9
*pAge: 9
Analysis: This program declares two
variables: an unsigned short, myAge, and a pointer
to an unsigned short, pAge. myAge is assigned the
value 5 on line 10; this is verified by the printout
in line 11.
On
line 13, pAge is assigned the address of myAge.
On line 15, pAge is dereferenced and printed, showing
that the value at the address that pAge stores is
the 5 stored in myAge. In line 17, the value 7 is
assigned to the variable at the address stored in
pAge. This sets myAge to 7, and the printouts in
lines 21-22 confirm this.
In
line 27, the value 9 is assigned to the variable
myAge. This value is obtained directly in line 29
and indirectly (by dereferencing pAge) in line 30.
Examining
the Address
Pointers
enable you to manipulate addresses without ever
knowing their real value. After this unit, you’ll
take it on faith that when you assign the address
of a variable to a pointer, it really has the address
of that variable as its value. But just this once,
why not check to make sure? Listing 8.3 illustrates
this idea.
Listing
8.3. Finding out what is stored in pointers.
1:
// Listing 8.3 What is stored in a pointer.
2:
3:
#include <iostream.h>
4:
5:
typedef unsigned short int USHORT;
6:
int main()
7:
{
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unsigned short int myAge = 5, yourAge = 10;
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unsigned short int * pAge = &myAge; // a pointer
10:
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cout << "myAge:\t" << myAge <<
"\tyourAge:\t" << yourAge << "\n";
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cout << "&myAge:\t" << &myAge
<< "\t&yourAge:\t" << &yourAge
<<"\n";
13:
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cout << "pAge:\t" << pAge << "\n";
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cout << "*pAge:\t" << *pAge <<
"\n";
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pAge = &yourAge; // reassign the pointer
18:
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cout << "myAge:\t" << myAge <<
"\tyourAge:\t" << yourAge << "\n";
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cout << "&myAge:\t" << &myAge
<< "\t&yourAge:\t" << &yourAge
<<"\n";
21:
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cout << "pAge:\t" << pAge << "\n";
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cout << "*pAge:\t" << *pAge <<
"\n";
24:
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cout << "&pAge:\t" << &pAge
<< "\n";
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return 0;
27:
}
Output:
myAge: 5 yourAge: 10
&myAge: 0x355C &yourAge: 0x355E
pAge: 0x355C
*pAge: 5
myAge: 5 yourAge: 10
&myAge: 0x355C &yourAge: 0x355E
pAge: 0x355E
*pAge: 10
&pAge: 0x355A
(Your output may look different.)
Analysis: In line 8, myAge and yourAge
are declared to be variables of type unsigned short
integer. In line 9, pAge is declared to be a pointer
to an unsigned short integer, and it is initialized
with the address of the variable myAge. Lines 11
and 12 print the values and the addresses of myAge
and yourAge. Line 14 prints the contents of pAge,
which is the address of myAge. Line 15 prints the
result of dereferencing pAge, which prints the value
at pAge—the value in myAge, or 5.
This
is the essence of pointers. Line 14 shows that pAge
stores the address of myAge, and line 15 shows how
to get the value stored in myAge by dereferencing
the pointer pAge. Make sure that you understand
this fully before you go on. Study the code and
look at the output.
In
line 17, pAge is reassigned to point to the address
of yourAge. The values and addresses are printed
again. The output shows that pAge now has the address
of the variable yourAge and that dereferencing obtains
the value in yourAge.
Line
25 prints the address of pAge itself. Like any variable,
it has an address, and that address can be stored
in a pointer. (Assigning the address of a pointer
to another pointer will be discussed shortly.)
DO
use the indirection operator (*) to access the data
stored at the address in a pointer. DO initialize
all pointers either to a valid address or to null
(0). DO remember the difference between the
address in a pointer and the value at that address.
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