ch02.htm

24小时学会vc++

		<TD ALIGN="LEFT" VALIGN="TOP"><B>Type</B></TD>
		<TD ALIGN="LEFT" VALIGN="TOP"><B>Size (in bytes)</B></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>bool</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">1</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>char</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">1</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>short</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">2</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>int</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">4</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>long</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">4</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>float</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">4</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>double</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">8</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><TT>long double</TT></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">8</TD>
	</TR>
</TABLE>

<H3><FONT COLOR="#000077"><B>Variable Naming</B></FONT></H3>
<P>One important part of programming is the selection of names for your variables
and other parts of your programs. The program listings you've seen so far have been
very simple. As you become a more experienced user of Visual C++, you will need to
establish some sort of naming convention for your identifiers.</P>
<P>When naming your variables, use names that are as long as necessary to indicate
how the variable is used. A variable name in C++ is an example of an identifier.
Identifiers in C++ are used to name variables and functions, among other things.
In Visual C++, your identifiers can be literally hundreds of characters long and
can include any combination of letters, numbers, and underscores, as long as the
first character is a letter or underscore. Listing 2.3 is an example of several different
variable declarations.
<H4><FONT COLOR="#000077">TYPE: Listing 2.3. Some examples of good and bad variable
names.</FONT></H4>
<PRE><FONT COLOR="#0066FF"><TT>#include &lt;iostream&gt;</TT>
<TT>using namespace std;</TT>
<TT>int main()</TT>
<TT>{</TT>
<TT>    // Good declarations</TT>
<TT>    int     nEmployees;      // Number of employees</TT>
<TT>    char    chMiddleInitial; // A middle initial</TT>

<TT>    // Declarations that could be improved</TT>
<TT>    int     i, n, k;         // What are these vars used for ?</TT>
<TT>    float   temp;            // May not be enough information</TT>
<TT>    char    ch;              // Should have more information</TT>

<TT>    return 0;</TT>
<TT>}</TT>
</FONT></PRE>
<P>No matter which technique you use to name your variables, it's important to be
consistent. For example, most of the sample programs and online help examples provided
as part of Visual C++ use a naming convention known as <I>Hungarian Notation</I>.</P>
<P>When Hungarian is used properly, it's easy to tell the logical type of variable
at a glance without searching for its declaration. For example, most scalar variables
such as <TT>int</TT>, <TT>long</TT>, or <TT>short</TT> are prefixed with an <TT>n</TT>.
Variables that are used to store characters are prefixed with <TT>ch</TT>, as in
<TT>chEntry</TT> and <TT>chInitial</TT>. Most of the sample code available from Microsoft
uses Hungarian Notation, which will be used for the remainder of the code listings
in this book. A listing of common Hungarian prefixes is provided in Appendix D, &quot;Hungarian
Notation.&quot;


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>DO/DON'T:</B></FONT><B> <BR>
	DO</B> use meaningful names for your variables.<BR>
	<B>DO</B> be consistent in your naming conventions.<BR>
	<B>DO</B> use variable types that match your data.<BR>
	<B>DON'T</B> depend on capitalization to differentiate between variables. 
<HR>


</BLOCKQUOTE>

<H3><FONT COLOR="#000077"><B>Assigning Values to Variables</B></FONT></H3>
<P>In assigning values to variables, the assignment operator is just an equals sign
used as follows:</P>
<PRE><FONT COLOR="#0066FF"><TT>nFoo = 42;</TT>
</FONT></PRE>
<P>This line assigns the integer value <TT>42</TT> to <TT>nFoo</TT>.</P>
<P>If a floating-point decimal value is assigned, it's assumed by the compiler to
be a <TT>double</TT>, as follows:</P>
<PRE><FONT COLOR="#0066FF"><TT>dFoo = 42.4242;</TT>
</FONT></PRE>
<P>You can assign to a variable of type <TT>char</TT> in two ways. If you are actually
storing a character value, you can assign the letter using single quotes as shown
here:</P>
<PRE><FONT COLOR="#0066FF"><TT>chInitial = `Z';</TT>
</FONT></PRE>
<P>The compiler converts the letter value into an ASCII value and stores it in the
<TT>char</TT> variable. Small integer values can also be stored in a <TT>char</TT>,
and the assignment is done just like an <TT>int</TT> variable.</P>
<PRE><FONT COLOR="#0066FF"><TT>chReallyAnInt = 47; </TT>
</FONT></PRE>


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>Time Saver:</B></FONT><B> </B>The <TT>char</TT> variable
	type is sometimes used to store small integer values. This is useful if you are storing
	a large number of values, because an <TT>int</TT> takes up four times the storage
	of a <TT>char</TT>. 
<HR>


</BLOCKQUOTE>

<H2><FONT COLOR="#000077"><B>A Simple C++ Program</B></FONT></H2>
<P>In Hour 1, you created a C++ project named Hello that displayed a simple &quot;Hello
World!&quot; message. This hour you will make a simple modification to the Hello
project--the Hello2 project will ask you for a name and then use the name in the
greeting. Building this project will help demonstrate some common elements found
in C++ programs.
<H3><FONT COLOR="#000077"><B>Creating the Hello2 Project</B></FONT></H3>
<P>The first step in writing any Visual C++ program is to create a project, as you
did in the first hour. To review, these are the steps required to create a console-mode
project:

<DL>
	<DD>1. Begin by selecting File | New from the Visual C++ main menu. This will display
	the New dialog box.<BR>
	<BR>
	2. Select the Projects tab in the New dialog box. A list box containing different
	types of projects will be displayed.<BR>
	<BR>
	3. Select the icon labeled Win32 Console Application, as shown in Figure 2.2. You
	must also provide a name for the project--a default location will be provided for
	you automatically.
</DL>

<P><A NAME="02"></A><A HREF="02.htm"><B>Figure 2.2.</B> </A><I><BR>
The New Projects dialog box.</I></P>
<P>After you have selected the project type and the subdirectory, click OK to create
the project.
<H3><FONT COLOR="#000077"><B>Creating the Source File for Your Program</B></FONT></H3>
<P>The source file for the Hello2 project is shown in Listing 2.4. Unlike your first
Hello program, this version collects input from the user and then outputs a greeting.
<H4><FONT COLOR="#000077">TYPE: Listing 2.4. A console mode program that accepts
input.</FONT></H4>
<PRE><FONT COLOR="#0066FF"><TT>#include &lt;iostream&gt;</TT>
<TT>#include &lt;string&gt;</TT>
<TT>using namespace std;</TT>

<TT>// Prompt the user to enter a name, collect the name,</TT>
<TT>// and display a message to the user that contains</TT>
<TT>// the name.</TT>
<TT>int main()</TT>
<TT>{</TT>
<TT>    string userName;</TT>

<TT>    cout &lt;&lt; &quot;What is your name? :&quot;;</TT>
<TT>    cin &gt;&gt; userName;</TT>
<TT>    cout &lt;&lt; &quot;Hello &quot; &lt;&lt; userName &lt;&lt; &quot;!&quot; &lt;&lt; endl;</TT>

<TT>    return 0;</TT>
<TT>}</TT>
</FONT></PRE>
<P>Open a new C++ source file and type the code shown in Listing 2.4. Remember that
C++ is case-sensitive. Save the file as <TT>Hello2.cpp</TT> in the project's directory.
To review, these are the steps required to open a new C++ source file and add it
to the project:

<DL>
	<DD>1. Select File | New from the main menu, and select the Files tab in the New
	dialog box.<BR>
	<BR>
	2. Select the icon labeled C++ Source File.<BR>
	<BR>
	3. Check the Add to Project check box, and enter <TT>Hello2.cpp</TT> as the filename.<BR>
	<BR>
	4. Click OK to close the dialog box and open the file for editing.
</DL>

<P>Compile the Hello2 project by selecting Build | Build Hello2.exe from the main
menu (or press F7). If the source code was entered correctly, the project will be
built with no errors, and the last line in the status window will read</P>
<PRE><FONT COLOR="#0066FF"><TT>Hello2.exe - 0 error(s), 0 warning(s)</TT>
</FONT></PRE>
<P>If there are errors or warnings, check the source code for typographical errors
and build again.
<H3><FONT COLOR="#000077"><B>Running the Hello2 Program</B></FONT></H3>
<P>Open a DOS window and change to the <TT>DEBUG</TT> subdirectory under the Hello2
project directory. Run the Hello2 program by typing <TT>Hello2</TT> at the DOS prompt.
The program produces the following output:</P>
<PRE><FONT COLOR="#0066FF"><TT>What is your name? :Alex</TT>
<TT>Hello Alex!</TT>
</FONT></PRE>
<P>The Hello2 program accepts any name as input and uses that name for its Hello
World message.
<H3><FONT COLOR="#000077"><B>Analyzing the Hello2 Program</B></FONT></H3>
<P>Let's take a look at the Hello2 program because it has a lot in common with much
larger C++ programs. Even though it is fairly short, it has many of the elements
that you will see in more complicated Windows programs later in this book.
<H4><FONT COLOR="#000077">Include Statements</FONT></H4>
<P>The first line of <TT>Hello2.cpp</TT> is a message to the compiler to include
another file when compiling <TT>Hello2.cpp</TT>:</P>
<PRE><FONT COLOR="#0066FF"><TT>#include &lt;iostream&gt;</TT>
</FONT></PRE>
<P>This <TT>#include</TT> statement tells the compiler to look for the file named
<TT>iostream</TT> and insert it into your source file. Actually, the <TT>#include</TT>
statement is read by the preprocessor, a part of the compiler that scans the source
file before the file is compiled.</P>
<P><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>Statements read by the preprocessor
are known as <I>preprocessor directives</I> because they aren't actually used by
the compiler. Preprocessor directives always begin with a <TT>#</TT>. You will learn
more about preprocessor statements throughout the rest of the book.</P>
<P><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>The file <TT>iostream</TT>
is an example of a <I>header</I> file. A header file contains declarations or other
code used to compile your program. In order to perform common input and output operations,
you must <TT>#include</TT> the <TT>iostream</TT> file.


<BLOCKQUOTE>
	<P>
<HR>
<B> </B><FONT COLOR="#000077"><B>Just a Minute:</B></FONT><B> </B>Traditionally,
	C++ header files have an <TT>.h</TT> or <TT>.hpp</TT> file extension; the standard
	C++ library includes files such as <TT>iostream</TT> that have no extension. For
	backward compatibility, the Visual C++ compiler includes older versions of the include
	files that have the <TT>.h</TT> extension. 
<HR>


</BLOCKQUOTE>

<P>The <TT>#include</TT> preprocessor directive is seen in two basic forms: