title: Objective-C preprocessor directive for 'if not'
body: <p>I understand how to use a preprocessor directive like this:</p>
  
  <pre><code>#if SOME_VARIABLE
      // Do something
  #else
      // Do something else
  #endif
  </code></pre>
  
  <p>But what if I only want to do something IF NOT SOME_VARIABLE.</p>
  
  <p>Obviously I still could do this:</p>
  
  <pre><code>#if SOME_VARIABLE
  
  #else
      // Do something else
  #endif
  </code></pre>
  
  <p>. . . leaving the if empty, But is there a way to do:</p>
  
  <pre><code>#if not SOME_VARIABLE
     // Do something
  #endif
  </code></pre>
  
  <p>Apple documentation <a href="https://developer.apple.com/library/mac/#documentation/DeveloperTools/gcc-4.2.1/cpp/Conditional-Syntax.html#Conditional-Syntax">here</a> suggests not, but this seems like a very basic need.</p>
  
  <p>Basically I want to do the preprocessor equivalent of:</p>
  
  <pre><code>if(!SOME_VARIABLE)(
  {
     // Do Something
  }
  </code></pre>