title: Use of constexpr in header file
body: <p>Can I have a definition like this in a header file?</p>
  <pre><code> constexpr double PI=3.14;
  </code></pre>
  <p>Is there any problem in having this in a header file that would be included to several cpp files?</p>
  <p>I am worried that since it says in standard that this constexpr has its own memory, putting it in header, and adding header to several cpp files, generate multiple copies of the same value in memory and some other nasty problems.</p>
  <p>I am using C++11</p>