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New
Term: A
pointer is a variable that holds a memory
address.
To
understand pointers, you must know a little about
computer memory. Computer memory is divided into
sequentially numbered memory locations. Each variable
is located at a unique location in memory, known
as its address.
Different
computers number this memory using different, complex
schemes. Usually programmers don’t need to know
the particular address of any given variable, because
the compiler handles the details. If you want this
information, though, you can use the address of
operator (&), which is illustrated in Listing
8.1.
Listing
8.1. Demonstrating address of variables.
1:
// Listing 8.1 Demonstrates address of operator
2:
// and addresses of local variables
3:
4:
#include <iostream.h>
5:
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int main()
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{
8:
unsigned short shortVar=5;
9:
unsigned long longVar=65535;
10:
long sVar = -65535;
11:
12:
cout << "shortVar:\t" << shortVar;
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cout << " Address of shortVar:\t";
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cout << &shortVar _<< "\n";
15:
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cout << "longVar:\t" << longVar;
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cout << " Address of longVar:\t" ;
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cout << &longVar _<< "\n";
19:
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cout << "sVar:\t" << sVar;
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cout << " Address of sVar:\t" ;
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cout << &sVar _<< "\n";
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return 0;
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}
Output:
shortVar: 5 Address of shortVar: 0x8fc9:fff4
longVar: 65535 Address of longVar: 0x8fc9:fff2
sVar: -65535 Address of sVar: 0x8fc9:ffee
(Your printout may look different.)
ANALYSIS: Three variables are declared
and initialized: a short in line 8, an unsigned
long in line 9, and a long in line 10. Their values
and addresses are printed in lines 12-16, by using
the address of operator (&). The value of shortVar
is 5, as expected, and its address is 0x8fc9:fff4
when run on my 80386-based computer. This complicated
address is computer-specific and may change slightly
each time the program is run. Your results will
be different. What doesn’t change, however, is that
the difference in the first two addresses is two
bytes if your computer uses two-byte short integers.
The difference between the second and third is four
bytes if your computer uses four-byte long integers.
Figure 8.2 illustrates how the variables in this
program would be stored in memory.
Figure 8.2. Illustration of variable storage.
There
is no reason why you need to know the actual numeric
value of the address of each variable. What you
care about is that each one has an address and that
the right amount of memory is set aside. You tell
the compiler how much memory to allow for your variables
by declaring the variable type; the compiler automatically
assigns an address for it. For example, a long integer
is typically four bytes, meaning that the variable
has an address to four bytes of memory.
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