title: When and where is _DEBUG defined?
body: <p>I have the following sample code in one of my header files:   </p>
  
  <pre><code>#ifdef _DEBUG
  #define _DEBUG_NEW_REDEFINE_NEW 0
  #include "debug_new.h"
  #else
  #define DEBUG_NEW new
  #endif
  </code></pre>
  
  <p>The application which includes this header file is compiled using gcc compiler with <code>-DDEBUG</code> option.</p>
  
  <p>Question:</p>
  
  <p>Is <code>_DEBUG</code> is defined because of the <code>-DDEBUG</code> option?</p>