title: How to initialize a data member of unknown type to a default
body: <p>So I have a class</p>
  
  <pre><code>template&lt;typename edge_val_type&gt;
  class Graph {
         struct edge {
           int index_;
           edge_val_type v_;
           edge(int index, edge_val_type v) : index_(index), v_(v) {}
  }
  </code></pre>
  
  <p>How do I initialize <code>v_</code> to some default value? Since at this point the program has no idea what <code>edge_val_type</code> is.</p>