title: Function Implementation in Separate File
body: <p>What is the correct syntax for having function implementation in a separate file?  For example:</p>
  
  <p><strong>foo.h</strong></p>
  
  <pre><code>int Multiply(const int Number);
  </code></pre>
  
  <p><strong>foo.cpp</strong></p>
  
  <pre><code>#include "foo.h"
  
  int Multiply(const int Number)
  {
      return Number * 2;
  }
  </code></pre>
  
  <p>I see this used a lot, but when I try it I get an error having to do with a missing <code>main()</code> function.  I get the error even when I try to compile working code.</p>