=====Preprocessor Conditionals=====

<code>
#if, #ifdef, #ifndef, #else, #elif, #endif
</code>

These six preprocessor commands give simple logic control to the compiler. As a file is being
compiled, you can use these commands to cause certain lines of code to be
included or not included.
<code>
     #if expression
</code>

If the value of expression is true, then the code that immediately follows the
command will be compiled.
<code>
     #ifdef macro
</code>

If the macro has been defined by a [[define|#define]] statement, then the code immediately
following the command will be compiled.
<code>
     #ifndef macro
</code>

If the macro has not been defined by a [[define|#define]] statement, then the code
immediately following the command will be compiled.

A few side notes: The command #elif is simply a horribly truncated way to say
"elseif" and works like you think it would. You can also throw in a "defined"
or "!defined" after an #if to get added functionality.

Here's an example of all these:
<code>
     #ifdef DEBUG
       cout << "This is the test version, i=" << i << endl;
     #else
       cout << "This is the production version!" << endl;
     #endif
</code>

You might notice how that second example could make debugging a lot easier than
inserting and removing a million "cout"s in your code.

Related topics: [[define|#define]]