title: How to define the declared function - template struct member with unnamed template arguments?
body: <p>I have a template struct:</p>
  
  <pre><code>template&lt;typename T2, class, typename, class T3 = vec1&lt;T2&gt; &gt;
  struct vec2
  {
      template&lt;typename D2, class, typename, class D3&gt;
      void f14(int a);
  
      template&lt;typename D2, class, typename, class D3&gt;
      void f15(int a);
  };
  </code></pre>
  
  <p>And I want to define, for example function f15. I have no idea, what should I write on the place of the "??". I tried to solve this problem in a different way. But compiler kills my dream to compile my program. </p>
  
  <pre><code>template&lt;typename T2, class , typename , class T3&gt;
  template&lt;typename D2, class , typename , class D3&gt;
  void vec2&lt;T2,??,??,T3&gt;::f15(int a) {  }
  </code></pre>
  
  <p>How should I define the function f15?</p>
  
  <p>What is the scientific name for such constructions "class , typename ,"? 
  I googled a lot ("unnamed template parameter", etc...), but I didn't found anything about it. I guess, it alikes the function with unnamed arguments. I would like to read something about it - from C++ standart or from other sources, but I don't know where the necessary information is.</p>