title: Accessing active union member
body: <p>Is there a simple way to understand which <code>union</code> member is active?</p>
  <p>An example:</p>
  <pre><code>union Read_Value{
   char ch;
   float number;
   string str;
  }
  </code></pre>
  <p>Suppose a void function reads from an input file stream and initialise <code>Read_Value</code> on the base of the type of the variable read. Assuming I don't know how the function works, how can I understand which of the three member is active?</p>