title: Do I check the existence of a macro function with or without the parens?
body: <p>Before I define a macro function, I can check that it doesn't already exist<br>
  (this avoids overwriting a previous definition).  </p>
  
  <p>I can implement the check and definition like this:  </p>
  
  <pre><code>#ifndef MACRO(X)
  #define MACRO(X) FUNCTION(X)
  #endif
  </code></pre>
  
  <p>Or like this:</p>
  
  <pre><code>#ifndef MACRO
  #define MACRO(X) FUNCTION(X)
  #endif
  </code></pre>
  
  <p>Both appear to work when the function is already defined.<br>
  So, which is correct?  Which is preferred?</p>