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<H1></H1>
<UL>
<LI><A HREF="#Heading1">Day 17</A>
<UL>
<LI><A HREF="#Heading2">The Preprocessor</A>
<UL>
<LI><A HREF="#Heading3">The Preprocessor and the Compiler</A>
<LI><A HREF="#Heading4">Seeing the Intermediate Form</A>
<LI><A HREF="#Heading5">Using #define</A>
<UL>
<LI><A HREF="#Heading6">Using #define for Constants</A>
<LI><A HREF="#Heading7">Using #define for Tests</A>
<LI><A HREF="#Heading8">The #else Precompiler Command</A>
</UL>
<LI><A HREF="#Heading9">Listing 17.1. Using #define.</A>
<LI><A HREF="#Heading10">Inclusion and Inclusion Guards</A>
<UL>
<LI><A HREF="#Heading11">Defining on the Command Line</A>
<LI><A HREF="#Heading12">Undefining</A>
</UL>
<LI><A HREF="#Heading13">Listing 17.2. Using #undef</A><A HREF="#Heading14">.</A>
<UL>
<LI><A HREF="#Heading15">Conditional Compilation</A>
</UL>
<LI><A HREF="#Heading16">Macro Functions</A>
<UL>
<LI><A HREF="#Heading17">Why All the Parentheses?</A>
</UL>
<LI><A HREF="#Heading18">Listing 17.3. Using parentheses in macros</A><A HREF="#Heading19">.</A>
<UL>
<LI><A HREF="#Heading20">Macros Versus Functions and Templates</A>
</UL>
<LI><A HREF="#Heading21">Inline Functions</A>
<LI><A HREF="#Heading22">Listing 17.4. Using</A>
<LI><A HREF="#Heading23">inline rather than a macro.</A>
<LI><A HREF="#Heading24">String Manipulation</A>
<UL>
<LI><A HREF="#Heading25">Stringizing</A>
<LI><A HREF="#Heading26">Concatenation</A>
</UL>
<LI><A HREF="#Heading27">Predefined Macros</A>
<LI><A HREF="#Heading28">assert()</A>
<LI><A HREF="#Heading29">Listing 17.5. A simple assert() macro</A><A HREF="#Heading30">.</A>
<UL>
<LI><A HREF="#Heading31">Debugging with assert()</A>
<LI><A HREF="#Heading32">assert() Versus Exceptions</A>
<LI><A HREF="#Heading33">Side Effects</A>
<LI><A HREF="#Heading34">Class Invariants</A>
</UL>
<LI><A HREF="#Heading35">Listing 17.6. Using Invariants().</A>
<UL>
<LI><A HREF="#Heading36">Printing Interim Values</A>
</UL>
<LI><A HREF="#Heading37">Listing 17.7. Printing values in DEBUG mode</A><A HREF="#Heading38">.</A>
<UL>
<LI><A HREF="#Heading39">Debugging Levels</A>
</UL>
<LI><A HREF="#Heading40">Listing 17.8. Levels of debugging</A><A HREF="#Heading41">.</A>
<LI><A HREF="#Heading42">Summary</A>
<LI><A HREF="#Heading43">Q&A</A>
<LI><A HREF="#Heading44">Workshop</A>
<UL>
<LI><A HREF="#Heading45">Quiz</A>
<LI><A HREF="#Heading46">Exercises</A>
</UL>
</UL>
</UL>
</UL>
<P>
<HR SIZE="4">
<H2 ALIGN="CENTER"><A NAME="Heading1"></A><FONT COLOR="#000077">Day 17</FONT></H2>
<H2 ALIGN="CENTER"><A NAME="Heading2"></A><FONT COLOR="#000077">The Preprocessor</FONT></H2>
<P>Most of what you write in your source code files is C++. These are interpreted
by the compiler and turned into your program. Before the compiler runs, however,
the preprocessor runs, and this provides an opportunity for conditional compilation.
Today you will learn
<UL>
<LI>What conditional compilation is and how to manage it.
<P>
<LI>How to write macros using the preprocessor.
<P>
<LI>How to use the preprocessor in finding bugs.
</UL>
<H3 ALIGN="CENTER"><A NAME="Heading3"></A><FONT COLOR="#000077">The Preprocessor
and the Compiler</FONT></H3>
<P>Every time you run your compiler, your preprocessor runs first. The preprocessor
looks for preprocessor instructions, each of which begins with a pound symbol (<TT>#</TT>).
The effect of each of these instructions is a change to the text of the source code.
The result is a new source code file, a temporary file that you normally don't see,
but that you can instruct the compiler to save so that you can examine it if you
want to.</P>
<P>The compiler does not read your original source code file; it reads the output
of the preprocessor and compiles that file. You've seen the effect of this already
with the <TT>#include</TT> directive. This instructs the preprocessor to find the
file whose name follows the <TT>#include</TT> directive, and to write it into the
intermediate file at that location. It is as if you had typed that entire file right
into your source code, and by the time the compiler sees the source code, the included
file is there.
<H3 ALIGN="CENTER"><A NAME="Heading4"></A><FONT COLOR="#000077">Seeing the Intermediate
Form</FONT></H3>
<P>Just about every compiler has a switch that you can set either in the integrated
development environment (IDE) or at the command line, and that instructs the compiler
to save the intermediate file. Check your compiler manual for the right switches
to set for your compiler, if you'd like to examine this file.
<H3 ALIGN="CENTER"><A NAME="Heading5"></A><FONT COLOR="#000077">Using #define</FONT></H3>
<P>The <TT>#define</TT> command defines a string substitution. If you write</P>
<PRE><FONT COLOR="#0066FF">#define BIG 512
</FONT></PRE>
<P>you have instructed the precompiler to substitute the string <TT>512</TT> wherever
it sees the string <TT>BIG</TT>. This is not a string in the C++ sense. The characters
<TT>512</TT> are substituted in your source code wherever the token <TT>BIG</TT>
is seen. A token is a string of characters that can be used wherever a string or
constant or other set of letters might be used. Thus, if you write</P>
<PRE><FONT COLOR="#0066FF">#define BIG 512
int myArray[BIG];
</FONT></PRE>
<P>The intermediate file produced by the precompiler will look like this:</P>
<PRE><FONT COLOR="#0066FF">int myArray[512];
</FONT></PRE>
<P>Note that the <TT>#define</TT> statement is gone. Precompiler statements are all
removed from the intermediate file; they do not appear in the final source code at
all.
<H4 ALIGN="CENTER"><A NAME="Heading6"></A><FONT COLOR="#000077">Using #define for
Constants</FONT></H4>
<P>One way to use <TT>#define</TT> is as a substitute for constants. This is almost
never a good idea, however, as <TT>#define</TT> merely makes a string substitution
and does no type checking. As explained in the section on constants, there are tremendous
advantages to using the <TT>const</TT> keyword rather than <TT>#define</TT>.
<H4 ALIGN="CENTER"><A NAME="Heading7"></A><FONT COLOR="#000077">Using #define for
Tests</FONT></H4>
<P>A second way to use <TT>#define,</TT> however, is simply to declare that a particular
character string is defined. Therefore, you could write</P>
<PRE><FONT COLOR="#0066FF">#define BIG
</FONT></PRE>
<P>Later, you can test whether <TT>BIG</TT> has been defined and take action accordingly.
The precompiler commands to test whether a string has been defined are <TT>#ifdef</TT>
and <TT>#ifndef</TT>. Both of these must be followed by the command <TT>#endif</TT>
before the block ends (before the next closing brace).</P>
<P><TT>#ifdef</TT> evaluates to <TT>TRUE</TT> if the string it tests has been defined
already. So, you can write</P>
<PRE><FONT COLOR="#0066FF">#ifdef DEBUG
cout << "Debug defined";
#endif
</FONT></PRE>
<P>When the precompiler reads the <TT>#ifdef</TT>, it checks a table it has built
to see if you've defined <TT>DEBUG</TT>. If you have, the <TT>#ifdef</TT> evaluates
to <TT>TRUE</TT>, and everything to the next <TT>#else</TT> or <TT>#endif</TT> is
written into the intermediate file for compiling. If it evaluates to <TT>FALSE</TT>,
nothing between <TT>#ifdef DEBUG</TT> and <TT>#endif</TT> will be written into the
intermediate file; it will be as if it were never in the source code in the first
place.</P>
<P>Note that <TT>#ifndef</TT> is the logical reverse of <TT>#ifdef</TT>. <TT>#ifndef</TT>
evaluates to <TT>TRUE</TT> if the string has not been defined up to that point in
the file.
<H4 ALIGN="CENTER"><A NAME="Heading8"></A><FONT COLOR="#000077">The #else Precompiler
Command</FONT></H4>
<P>As you might imagine, the term <TT>#else</TT> can be inserted between either <TT>#ifdef</TT>
or <TT>#ifndef</TT> and the closing <TT>#endif</TT>. Listing 17.1 illustrates how
these terms are used.</P>
<P><A NAME="Heading9"></A><FONT SIZE="4" COLOR="#000077"><B>Listing 17.1. Using #define.</B></FONT></P>
<PRE><FONT COLOR="#0066FF">1: #define DemoVersion
2: #define DOS_VERSION 5
3: #include <iostream.h>
4:
5:
6: int main()
7: {
8:
9: cout << "Checking on the definitions of DemoVersion, DOS_VERSION Â _and WINDOWS_VERSION...\n";
10:
11: #ifdef DemoVersion
12: cout << "DemoVersion defined.\n";
13: #else
14: cout << "DemoVersion not defined.\n";
15: #endif
16:
17: #ifndef DOS_VERSION
18: cout << "DOS_VERSION not defined!\n";
19: #else
20: cout << "DOS_VERSION defined as: " << DOS_VERSION << endl;
21: #endif
22:
23: #ifdef WINDOWS_VERSION
24: cout << "WINDOWS_VERSION defined!\n";
25: #else
26: cout << "WINDOWS_VERSION was not defined.\n";
27: #endif
28:
29: cout << "Done.\n";
30: return 0;
<TT>31: }</TT></FONT>
<FONT COLOR="#0066FF">
Output: Checking on the definitions of DemoVersion, DOS_VERSION
 _and WINDOWS_VERSION...\n";
DemoVersion defined.
DOS_VERSION defined as: 5
WINDOWS_VERSION was not defined.
Done.
</FONT></PRE>
<P><FONT COLOR="#000077"><B>Analysis:</B></FONT><B> </B>On lines 1 and 2, <TT>DemoVersion</TT>
and <TT>DOS_VERSION</TT> are defined, with <TT>DOS_VERSION</TT> defined with the
string <TT>5</TT>. On line 11, the definition of <TT>DemoVersion</TT> is tested,
and because <TT>DemoVersion</TT> is defined (albeit with no value), the test is true
and the string on line 12 is printed.<BR>
On line 17 is the test that <TT>DOS_VERSION</TT> is not defined. Because <TT>DOS_VERSION</TT>
is defined, this test fails and execution jumps to line 20. Here the string <TT>5</TT>
is substituted for the word <TT>DOS_VERSION</TT>; this is seen by the compiler as</P>
<PRE><FONT COLOR="#0066FF">cout << "DOS_VERSION defined as: " << 5 << endl;</FONT></PRE>
<P>Note that the first word <TT>DOS_VERSION</TT> is not substituted because it is
in a quoted string. The second <TT>DOS_VERSION</TT> is substituted, however, and
thus the compiler sees <TT>5</TT> as if you had typed <TT>5</TT> there.</P>
<P>Finally, on line 23, the program tests for <TT>WINDOWS_VERSION</TT>. Because you
did not define <TT>WINDOWS_VERSION</TT>, the test fails and the message on line 24
is printed.
<H3 ALIGN="CENTER"><A NAME="Heading10"></A><FONT COLOR="#000077">Inclusion and Inclusion
Guards</FONT></H3>
<P>You will create projects with many different files. You will probably organize
your directories so that each class has its own header file (HPP) with the class
declaration, and its own implementation file (CPP) with the source code for the class
methods.</P>
<P>Your <TT>main()</TT> function will be in its own CPP file, and all the CPP files
will be compiled into OBJ files, which will then be linked together into a single
program by the linker.</P>
<P>Because your programs will use methods from many classes, many header files will
be included in each file. Also, header files often need to include one another. For
example, the header file for a derived class's declaration must include the header
file for its base class.</P>
<P>Imagine that the <TT>Animal</TT> class is declared in the file <TT>ANIMAL.HPP</TT>.
The <TT>Dog</TT> class (which derives from <TT>Animal</TT>) must include the file
<TT>ANIMAL.HPP</TT> in <TT>DOG.HPP</TT>, or <TT>Dog</TT> will not be able to derive
from <TT>Animal</TT>. The <TT>Cat</TT> header also includes <TT>ANIMAL.HPP</TT> for
the same reason.</P>
<P>If you create a method that uses both a <TT>Cat</TT> and a <TT>Dog</TT>, you will
be in danger of including <TT>ANIMAL.HPP</TT> twice. This will generate a compile-time
error, because it is not legal to declare a class (<TT>Animal</TT>) twice, even though
the declarations are identical. You can solve this problem with inclusion guards.
At the top of your <TT>ANIMAL</TT> header file, you write these lines:</P>
<PRE><FONT COLOR="#0066FF">#ifndef ANIMAL_HPP
#define ANIMAL_HPP
... // the whole file goes here
#endif
</FONT></PRE>
<P>This says, if you haven't defined the term <TT>ANIMAL_HPP</TT>, go ahead and define
it now. Between the <TT>#define</TT> statement and the closing <TT>#endif</TT> are
the entire contents of the file.</P>
<P>The first time your program includes this file, it reads the first line and the
test evaluates to <TT>TRUE</TT>; that is, you have not yet defined <TT>ANIMAL_HPP</TT>.
So, it goes ahead and defines it and then includes the entire file.</P>
<P>The second time your program includes the <TT>ANIMAL.HPP</TT> file, it reads the
first line and the test evaluates to <TT>FALSE</TT>; <TT>ANIMAL.HPP</TT> has been
defined. It therefore skips to the next <TT>#else</TT> (there isn't one) or the next
<TT>#endif</TT> (at the end of the file). Thus, it skips the entire contents of the
file, and the class is not declared twice.</P>
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