title: binary tree inorder traversal in C++
body: <p>I'm trying to print out a binary tree I've build using inorder traversal, but I'm having trouble on defining how to pass values to the recursive function. Here is the error I'm getting:</p>
  
  <blockquote>
    <p>1>methods.obj : error LNK2001: unresolved external symbol "public:
    void __thiscall morsecode::in_order(struct letter *)"
    (?in_order@morsecode@@QAEXPAUletter@@@Z)</p>
  </blockquote>
  
  <p>Here's my tree from my header file:</p>
  
  <pre><code>struct letter
  {
      string let;
      string morse;
      letter *left;
      letter *right;
  };
  </code></pre>
  
  <p>Method from source file: </p>
  
  <pre><code>void in_order(struct letter *P)
      {
          if(P==NULL) return;
          in_order(P-&gt;left);
          cout&lt;&lt;"letter: "&lt;&lt;P-&gt;let&lt;&lt;endl;
          in_order(P-&gt;right);
      }
  </code></pre>
  
  <p>Am I missing something important here?</p>