Include guards in c++ for main method
<p>I know the purpose and usage of include guards around the .h file in C++ </p>

<pre><code>  #ifndef FILENAME_H
  #define FILENAME_H
  #endif
</code></pre>

<p>However, I am trying to understand the usage for #ifdef TEST_FILENAME in the test stub or around the main() method used to test run the class.</p>

<pre><code>  #ifdef TEST_FILENAME
   void main()
   {
      /////////////////
   }
  #endif
</code></pre>
