title: Error: macro names must be identifiers using #ifdef 0
body: <p>I have the source code of an application written in C++ and I just want to comment something using:</p>
  
  <pre><code>#ifdef 0
  ...
  #endif
  </code></pre>
  
  <p>And I get this error</p>
  
  <blockquote>
    <p>error: macro names must be identifiers</p>
  </blockquote>
  
  <p>Why is this happening?</p>