title: XCode: Initializer element is not a compile-time constant
body: <p>I am writing a program in C using XCode. I don't use C much, usually I use C++. I've never used XCode before.</p>
  
  <p>Compile error is quite simple, the following lines of code are not being treated as compile time constants by the compiler.</p>
  
  <pre><code>const double PI = 4.0 * atan(1.0);
  const double TAU = 8.0 * atan(1.0);
  </code></pre>
  
  <p>I'm sure this is allowed in C++ 11, although I can't be certain, since I last used it some months ago.</p>
  
  <p>My guess is that the XCode compiler / the C standard does not allow constants to be computed in this way.</p>
  
  <p>Is there an alternative I can use? I don't much fancy the "define" alternative...</p>
  
  <pre><code>#define PI 4.0 * atan(1.0);
  </code></pre>
  
  <p>As this will (may?) cause unnecessary run-time overhead.</p>