title: Using "extern" keyword without #include "file.h"
body: <p>Right now i read Stephen Prata's book about C++, and learning about extern keyword and its usage. So i have a question. Can i type &quot;extern int var a;&quot; without including a file that defines and initializes this 'a' variable?</p>
  <pre class="lang-c prettyprint-override"><code>#include &lt;iostream&gt;
  //#include &quot;vars.h&quot; Not including the file with 'a' variable
  
  using namespace std;
  
  extern int a;
  
  int main()
  {   
      cout &lt;&lt; a &lt;&lt; endl;
  
      return 0;
  }
  </code></pre>