lt value of zero if it is omitted. The class template has a protected
      data member called <code class="computeroutput"><span class="identifier">member</span></code> that
      the derived class can use for later base classes (or itself).
    </p>
<p>
      If the appropriate features of C++11 are present, there will be a single constructor
      template. It implements <span class="emphasis"><em>perfect forwarding</em></span> to the best
      constructor call of <code class="computeroutput"><span clas