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<TD class="tt2" bgcolor="#ffffff" width="100%"><center><B><FONT style="FONT-SIZE: 16.5pt" COLOR="#FF6666" FACE="楷體_GB2312">Day 8</FONT></B></center>
<LI><A HREF="#Heading1">Day 8</A>
<UL>
<LI><A HREF="#Heading2">Pointers</A>
<UL>
<LI><A HREF="#Heading3">What Is a Pointer?</A>
<UL>
<LI><A HREF="#Heading4">Figure 8.1.</A>
</UL>
<LI><A HREF="#Heading5">Listing 8.1. Demonstrating address of variables</A>
<UL>
<LI><A HREF="#Heading7">Figure 8.2.</A>
</UL>
<LI><A HREF="#Heading8">Storing the Address in a Pointer</A>
<UL>
<LI><A HREF="#Heading9">Pointer Names</A>
<LI><A HREF="#Heading10">The Indirection Operator</A>
<LI><A HREF="#Heading11">Pointers, Addresses, and Variables</A>
<UL>
<LI><A HREF="#Heading12">Figure 8.3.</A>
</UL>
<LI><A HREF="#Heading13">Manipulating Data by Using Pointers</A>
</UL>
<LI><A HREF="#Heading14">Listing 8.2. Manipulating data by using pointers</A>
<UL>
<LI><A HREF="#Heading16">Examining the Address</A>
</UL>
<LI><A HREF="#Heading17">Listing 8.3. Finding out what is stored in pointers</A>
<LI><A HREF="#Heading19">Pointers</A>
<LI><A HREF="#Heading20">Why Would You Use Pointers?</A>
<LI><A HREF="#Heading21">The Stack and the Free Store</A>
<UL>
<LI><A HREF="#Heading22">new</A>
<LI><A HREF="#Heading23">delete</A>
</UL>
<LI><A HREF="#Heading24">Listing 8.4. Allocating, using, and deleting pointers.</A>
<LI><A HREF="#Heading25">Memory Leaks</A>
<LI><A HREF="#Heading26">Creating Objects on the Free Store</A>
<LI><A HREF="#Heading27">Deleting Objects</A>
<LI><A HREF="#Heading28">Listing 8.5. Creating and deleting objects on the free store</A>
<LI><A HREF="#Heading30">Accessing Data Members</A>
<LI><A HREF="#Heading31">Listing 8.6. Accessing member data of objects</A>
<LI><A HREF="#Heading32">on the free store.</A>
<LI><A HREF="#Heading33">Member Data on the Free Store</A>
<LI><A HREF="#Heading34">Listing 8.7. Pointers as member data</A>
<LI><A HREF="#Heading36">The this Pointer</A>
<LI><A HREF="#Heading37">Listing 8.8. Using the this pointer</A>
<LI><A HREF="#Heading39">Stray or Dangling Pointers</A>
<LI><A HREF="#Heading40">Listing 8.9. Creating a stray pointer</A>
<LI><A HREF="#Heading42">const Pointers</A>
<UL>
<LI><A HREF="#Heading43">const Pointers and const Member Functions</A>
</UL>
<LI><A HREF="#Heading44">Listing 8.10. Using pointers to const objects</A>
<UL>
<LI><A HREF="#Heading46">const this Pointers</A>
</UL>
<LI><A HREF="#Heading47">Summary</A>
<LI><A HREF="#Heading48">Q&A</A>
<LI><A HREF="#Heading49">Workshop</A>
<UL>
<LI><A HREF="#Heading50">Quiz</A>
<LI><A HREF="#Heading51">Exercises</A>
</UL>
</UL>
</UL>
</UL>
<BR>
<P>
<HR SIZE="4">
<BR>
<H2 ALIGN="CENTER"><A NAME="Heading1"></A><FONT COLOR="#000077">Day 8</FONT></H2>
<H2 ALIGN="CENTER"><A NAME="Heading2"></A><FONT COLOR="#000077">Pointers</FONT></H2>
<P>One of the most powerful tools available to a C++ programmer is the ability to
manipulate computer memory directly by using pointers. Today you will learn
<BR>
<UL>
<LI>What pointers are.
<P>
<LI>How to declare and use pointers.
<P>
<LI>What the free store is and how to manipulate memory.
</UL>
<BR>
<P>Pointers present two special challenges when learning C++: They can be somewhat
confusing, and it isn't immediately obvious why they are needed. This chapter explains
how pointers work, step by step. You will fully understand the need for pointers,
however, only as the book progresses.
<H3 ALIGN="CENTER"><A NAME="Heading3"></A><FONT COLOR="#000077">What Is a Pointer?</FONT></H3>
<BR>
<DL>
<DD>
<HR>
<FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>A <I>pointer</I> is a variable
that holds a memory address.
<HR>
<BR>
</DL>
<BR>
<P>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. (This is discussed in the "Extra
Credit" section following Day 5, "Functions.") Figure 8.1 shows a
schematic representation of the storage of an <TT>unsigned long</TT> integer variable
<TT>theAge</TT>.<BR>
<BR>
<A NAME="Heading4"></A><A HREF="figure1.jpg" tppabs="http://pbs.mcp.com/ebooks/0672310708/art/ch08/figure1.jpg"><FONT COLOR="#000077">Figure
8.1.</FONT></A><I> A schematic representation of <TT>theAge</TT></I>.<BR>
<BR>
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 <TT>address of</TT> operator (<TT>&</TT>), which is illustrated in Listing
8.1.</P>
<P><A NAME="Heading5"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 8.1. Demonstrating
address of variables.</B></FONT></P>
<PRE><FONT COLOR="#0066FF" font style="font-size:10pt">1: // Listing 8.1 Demonstrates address of operator
2: // and addresses of local variables
3:
4: #include <iostream.h>
5:
6: int main()
7: {
8: unsigned short shortVar=5;
9: unsigned long longVar=65535;
10: long sVar = -65535;
11:
12: cout << "shortVar:\t" << shortVar;
13: cout << " Address of shortVar:\t";
14: cout << &shortVar _<< "\n";
15:
16: cout << "longVar:\t" << longVar;
17: cout << " Address of longVar:\t" ;
18: cout << &longVar _<< "\n";
19:
20: cout << "sVar:\t" << sVar;
21: cout << " Address of sVar:\t" ;
22: cout << &sVar _<< "\n";
23:
24: return 0;
<TT>25: }</TT></FONT>
<FONT COLOR="#0066FF">
Output: shortVar: 5 Address of shortVar: 0x8fc9:fff4
longVar: 65535 Address of longVar: 0x8fc9:fff2
sVar: -65535 Address of sVar: 0x8fc9:ffee
</FONT></PRE>
<P>(Your printout may look different.)</P>
<P><BR>
<FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>Three variables are declared
and initialized: a <TT>short</TT> in line 8, an <TT>unsigned long</TT> in line 9,
and a <TT>long</TT> in line 10. Their values and addresses are printed in lines 12-16,
by using the <TT>address of</TT> operator (<TT>&</TT>).<BR>
The value of <TT>shortVar</TT> is <TT>5</TT>, as expected, and its address is <TT>0x8fc9:fff4</TT>
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 <TT>short</TT> integers. The difference
between the second and third is four bytes if your computer uses four-byte <TT>long</TT>
integers. Figure 8.2 illustrates how the variables in this program would be stored
in memory.<BR>
<BR>
<A NAME="Heading7"></A><A HREF="figure2.jpg" tppabs="http://pbs.mcp.com/ebooks/0672310708/art/ch08/figure2.jpg"><FONT COLOR="#000077">Figure
8.2.</FONT></A><FONT COLOR="#000077"><I> </I></FONT><I>Illustration of variable storage.</I><BR>
<BR>
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 <TT>long</TT> integer is typically four bytes,
meaning that the variable has an address to four bytes of memory.
<H4 ALIGN="CENTER"><A NAME="Heading8"></A><FONT COLOR="#000077">Storing the Address
in a Pointer</FONT></H4>
<P>Every variable has an address. Even without knowing the specific address of a
given variable, you can store that address in a pointer.</P>
<P>For example, suppose that <TT>howOld</TT> is an integer. To declare a pointer
called <TT>pAge</TT> to hold its address, you would write</P>
<PRE><FONT COLOR="#0066FF" font style="font-size:10pt">int *pAge = 0;
</FONT></PRE>
<P>This declares <TT>pAge</TT> to be a pointer to <TT>int</TT>. That is, <TT>pAge</TT>
is declared to hold the address of an <TT>int</TT>.</P>
<P>Note that <TT>pAge</TT> is a variable like any of the variables. When you declare
an integer variable (type <TT>int</TT>), it is set up to hold an integer. When you
declare a pointer variable like <TT>pAge</TT>, it is set up to hold an address. <TT>pAge</TT>
is just a different type of variable.</P>
<P>In this example, <TT>pAge</TT> is initialized to zero. A pointer whose value is
zero is called a null pointer. All pointers, when they are created, should be initialized
to something. If you don't know what you want to assign to the pointer, assign <TT>0</TT>.
A pointer that is not initialized is called a wild pointer. Wild pointers are very
dangerous.
<BR>
<BR>
<BLOCKQUOTE>
<P>
<HR>
<FONT COLOR="#000077"><B>NOTE:</B></FONT><B> </B>Practice safe computing: Initialize
your pointers!
<HR>
<BR>
<BR>
</BLOCKQUOTE>
<BR>
<P>If you do initialize the pointer to <TT>0</TT>, you must specifically assign the
address of <TT>howOld</TT> to <TT>pAge</TT>. Here's an example that shows how to
do that:</P>
<PRE><FONT COLOR="#0066FF" font style="font-size:10pt">unsigned short int howOld = 50; // make a variable
unsigned short int * pAge = 0; // make a pointer
pAge = &howOld; // put howOld's address in pAge
</FONT></PRE>
<P>The first line creates a variable--<TT>howOld</TT>, whose type is <TT>unsigned
short int</TT>--and initializes it with the value <TT>50</TT>. The second line declares
<TT>pAge</TT> to be a pointer to type <TT>unsigned short int</TT> and initializes
it to zero. You know that <TT>pAge</TT> is a pointer because of the asterisk (<TT>*</TT>)
after the variable type and before the variable name.</P>
<P>The third and final line assigns the address of <TT>howOld</TT> to the pointer
<TT>pAge</TT>. You can tell that the address of <TT>howOld</TT> is being assigned
because of the <TT>address of</TT> operator (<TT>&</TT>). If the <TT>address
of</TT> operator had not been used, the value of <TT>howOld</TT> would have been
assigned. That might, or might not, have been a valid address.</P>
<P>At this point, <TT>pAge</TT> has as its value the address of <TT>howOld</TT>.
<TT>howOld</TT>, in turn, has the value <TT>50</TT>. You could have accomplished
this with one less step, as in</P>
<PRE><FONT COLOR="#0066FF" font style="font-size:10pt">unsigned short int howOld = 50; // make a variable
unsigned short int * pAge = &howOld; // make pointer to howOld
</FONT></PRE>
<P><TT>pAge</TT> is a pointer that now contains the address of the <TT>howOld</TT>
variable. Using <TT>pAge</TT>, you can actually determine the value of <TT>howOld</TT>,
which in this case is <TT>50</TT>. Accessing <TT>howOld</TT> by using the pointer
<TT>pAge</TT> is called indirection because you are indirectly accessing <TT>howOld</TT>
by means of<TT> pAge</TT>. Later today you will see how to use indirection to access
a variable's value.</P>
<BR>
<DL>
<DD>
<HR>
<FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>Indirection means accessing
the value at the address held by a pointer. The pointer provides an indirect way
to get the value held at that address.
<HR>
<BR>
</DL>
<BR>
<H4 ALIGN="CENTER"><A NAME="Heading9"></A><FONT COLOR="#000077">Pointer Names</FONT></H4>
<P>Pointers can have any name that is legal for other variables. This book follows
the convention of naming all pointers with an initial <TT>p</TT>, as in <TT>pAge</TT>
or <TT>pNumber</TT>.
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