ch02.htm

24小时学会vc++

<UL>
	<LI>When including library files, the file to be included is surrounded by angled
	brackets, as shown in the <TT>Hello2.cpp</TT> file shown earlier. The preprocessor
	searches a predefined path for the file.<BR>
	<BR>
	
	<LI>When including header files that are specific to a specific application, the
	filename is surrounded by quotes, such as <TT>#include &quot;stdafx.h&quot;</TT>.
	The preprocessor will search for the file in the current source file directory. If
	the file is not found, the search will continue along the predefined include path.
</UL>

<P>The second line of <TT>Hello2.cpp</TT> is also an <TT>#include</TT> directive:</P>
<PRE><FONT COLOR="#0066FF"><TT>#include &lt;string&gt;</TT>
</FONT></PRE>
<P>The string header file is part of the standard C++ library. Including the string
header file enables a C++ source file to use the standard string class, which simplifies
using text strings in a C++ application.
<H4><FONT COLOR="#000077">The std Namespace</FONT></H4>
<P><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>A collection of names and
other identifiers in C++ is known as a <I>namespace</I>. By default, any name that
is introduced in a C++ program is in the <I>global namespace</I>. All names found
in the standard C++ library are located in the <TT>std</TT> namespace.</P>
<P>Namespaces make it easier to manage names in large C++ projects, especially when
using libraries or code developed by different groups of people. Before namespaces
were introduced to C++, it wasn't unusual to have two or more libraries that were
incompatible with each other simply because they used conflicting names.</P>
<P>Namespaces allow libraries to place their names into a compartment that itself
has a name. As shown in Figure 2.3, two namespaces can each use a common name, in
this case <TT>string</TT>; because each namespace provides a compartment for the
name <TT>string</TT>, the two names do not conflict with each other.</P>
<P><A NAME="03"></A><A HREF="03.htm"><B>Figure 2.3.</B></A> <BR>
<I>Namespaces provide separate compartments for names used in a C++ program.</I></P>
<P>When using a name from a namespace, the namespace must be prefixed, like <TT>std::string</TT>
or <TT>codev::string</TT>. Alternatively, a <TT>using namespace</TT> directive can
be used to tell the compiler that an identifier can be found in the global namespace,
as in the next line of the program, which tells the compiler that the names found
in the program can be found in the <TT>std</TT> namespace:</P>
<PRE><FONT COLOR="#0066FF"><TT>using namespace std;</TT>
</FONT></PRE>
<H4><FONT COLOR="#000077">Using Comments to Document Your Code</FONT></H4>
<P><FONT COLOR="#000077"><B>New Term:</B></FONT><B> </B>A <I>comment</I> is a note
provided to the person reading the source code. It has no meaning to the compiler
or computer.</P>
<P>The next line begins with <TT>//</TT>, which is used to mark the beginning of
a single-line comment in a C++ program. By default, comments are colored green by
the Developer Studio editor. In contrast, <TT>int</TT> and <TT>return</TT> are colored
blue to indicate that they are C++ keywords.


<BLOCKQUOTE>
	<P>
<HR>
<B> </B><FONT COLOR="#000077"><B>Time Saver:</B></FONT><B> </B>It's a good idea to
	use comments to document your code. After time has passed, you can use your comments
	to help explain how your code was intended to work. 
<HR>


</BLOCKQUOTE>

<H4><FONT COLOR="#000077">The main Function</FONT></H4>
<P>The next line of <TT>Hello2.cpp</TT> is the beginning of the <TT>main</TT> function.</P>
<PRE><FONT COLOR="#0066FF"><TT>int main()</TT>
</FONT></PRE>
<P>The first line inside the <TT>main</TT> function is a variable declaration.</P>
<PRE><FONT COLOR="#0066FF"><TT>string userName;</TT>
</FONT></PRE>
<P>Don't worry too much about what this means--for now, it's enough to know that
<TT>userName</TT> is a <TT>string</TT> variable. A <TT>string</TT> is not one of
the fundamental data types; instead, it's part of the standard library. The <TT>string</TT>
type enables you to use strings of text as though they are built-in fundamental types.</P>
<P>Following the declaration of <TT>userName</TT> is a statement that displays a
message to the user as a prompt for the user's name:</P>
<PRE><FONT COLOR="#0066FF"><TT>cout &lt;&lt; &quot;What is your name? :&quot;;</TT>
</FONT></PRE>
<P>This particular statement in <TT>Hello2.cpp</TT> displays a line of characters
to the console window by using the <TT>iostream</TT> object <TT>cout</TT>. The <TT>iostream</TT>
library is included with every C++ compiler, although it is not technically part
of the C++ language definition; instead, it's part of the standard C++ library. Performing
standard input and output for your console mode program is easy using the <TT>iostream</TT>
library.</P>
<P>The <TT>iostream</TT> library uses the <TT>&lt;&lt;</TT> symbol for output and
the <TT>&gt;&gt;</TT> for input to and from IO streams. Think of a stream as a sequence
of bytes, like a disk file, or the output to a printer or a character-mode screen.


<BLOCKQUOTE>
	<P>
<HR>
<B> </B><FONT COLOR="#000077"><B>Just a Minute:</B></FONT><B> </B>One simple rule
	of thumb is that when you see the <TT>&lt;&lt;</TT> symbol, the value to the right
	of the symbol will be output to the IO object on the left. When you see the <TT>&gt;&gt;</TT>
	symbol, data from the IO object on the left is stored in a variable to the right.
	
<HR>


</BLOCKQUOTE>

<P>The next line of <TT>Hello2.cpp</TT> accepts input from the user and stores it
in <TT>userName</TT>:</P>
<PRE><FONT COLOR="#0066FF"><TT>cin &gt;&gt; userName;</TT>
</FONT></PRE>
<P>The variable <TT>userName</TT> now contains whatever value was entered by the
user.</P>
<P>The next line displays the Hello greeting and adds the contents of the <TT>userName</TT>
variable. When using <TT>cout</TT>, several different components can be output one
after another by separating them with the <TT>&lt;&lt;</TT> symbol:</P>
<PRE><FONT COLOR="#0066FF"><TT>cout &lt;&lt; &quot;Hello &quot; &lt;&lt; userName &lt;&lt; &quot;!&quot; &lt;&lt; endl;</TT>
</FONT></PRE>
<P>The last line of the <TT>main</TT> function is a <TT>return</TT> statement. When
a <TT>return</TT> statement is executed, the function <I>returns</I> or stops executing,
and the caller of the function is passed the value provided after the <TT>return</TT>
keyword. Because this return statement is inside <TT>main</TT>, the value <TT>0</TT>
is passed back to the operating system. The <TT>return</TT> keyword can appear almost
anywhere in a function. However, as a matter of style, most people prefer to have
a single <TT>return</TT> statement in a function if possible.
<H2><FONT COLOR="#000077"><B>Summary</B></FONT></H2>
<P>In this hour, you have learned more details about C++ programs. You wrote a simple
console-mode program and analyzed its parts. You also learned about the C++ preprocessor,
type-safety, and variables.
<H2><FONT COLOR="#000077"><B>Q&amp;A</B></FONT></H2>

<DL>
	<DD><B>Q When I compile the Hello2 project and enter my first and last name, only
	the first name is displayed. How can I display my first and last names?</B><BR>
	<BR>
	<B>A</B> When using <TT>cin</TT> to gather input as shown in the Hello2 project,
	white space such as the space between your first and last name will cause your names
	be parsed into two separate variables. You can use <TT>cin</TT> with multiple variables
	much like you use <TT>cout</TT> with multiple variables; just separate the variables
	with the <TT>&gt;&gt;</TT> operator. A new version of Hello2 that displays first
	and last names looks like this:
</DL>



<BLOCKQUOTE>
	<PRE><FONT COLOR="#0066FF"><TT>#include &lt;iostream&gt;</TT>
<TT>#include &lt;string&gt;</TT>
<TT>using namespace std;</TT>
<TT>int main()</TT>
<TT>{</TT>
<TT>    string strFirstName;</TT>
<TT>    string strLastName;</TT>

<TT>    cout &lt;&lt; &quot;Please enter your first and last name:&quot;;</TT>

<TT>    cin &gt;&gt; strFirstName &gt;&gt; strLastName;</TT>

<TT>    cout &lt;&lt; &quot;Hello &quot; &lt;&lt; strFirstName &lt;&lt; strLastName &lt;&lt; endl;</TT>
<TT>    return 0;</TT>
<TT>}</TT></FONT></PRE>

</BLOCKQUOTE>

<PRE><FONT COLOR="#0066FF"><TT></TT></FONT></PRE>

<DL>
	<DD><B>Q When I declare a variable, sometimes I get strange error messages from the
	compiler in the Build window. This is the line that causes the error:</B>
</DL>



<BLOCKQUOTE>
	<PRE><FONT COLOR="#0066FF"><TT>int my age;</TT></FONT></PRE>

</BLOCKQUOTE>

<PRE><FONT COLOR="#0066FF"><TT></TT></FONT></PRE>

<DL>
	<DD><B>A</B> In C++, all variables must be a single identifier. The compiler complains
	because after using the identifier as a variable name, it can't figure out what to
	do with the identifier name. One coding style is to separate the words that make
	up a variable name with an underscore, like this:
</DL>



<BLOCKQUOTE>
	<PRE><FONT COLOR="#0066FF"><TT>int my_age;</TT></FONT></PRE>

</BLOCKQUOTE>

<PRE><FONT COLOR="#0066FF"><TT></TT></FONT></PRE>
<H2><FONT COLOR="#000077"><B>Workshop</B></FONT></H2>
<P>The Workshop is designed to help you anticipate possible questions, review what
you've learned, and begin thinking ahead to putting your knowledge into practice.
The answers to the quiz are in Appendix B, &quot;Quiz Answers.&quot;
<H3><FONT COLOR="#000077"><B>Quiz</B></FONT></H3>

<DL>
	<DD>1. What is the difference between the <TT>cout</TT> and <TT>cin</TT> <TT>iostream</TT>
	objects?<BR>
	<BR>
	2. What are the two forms of the <TT>#include</TT> preprocessor directive?<BR>
	<BR>
	3. What type of variable is used to store character values?<BR>
	<BR>
	4. What is the purpose of a C++ namespace?<BR>
	<BR>
	5. How can you declare more than one variable on a single line?<BR>
	<BR>
	6. What is type-safety?<BR>
	<BR>
	7. What types of variable are used to store floating-point values?<BR>
	<BR>
	8. How do you assign a value to a variable?<BR>
	<BR>
	9. What type of variable is normally used to store integer values?<BR>
	<BR>
	10. Why would you declare a variable as unsigned?
</DL>

<H3><FONT COLOR="#000077"><B>Exercise</B></FONT></H3>

<DL>
	<DD>1. Modify the Hello2 program to ask for your age in addition to your name; display
	the name and age in the Hello message.
</DL>

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