title: How can I make doxygen create full inheritance diagrams across multiple projects?
body: <p>When using doxygen to generate C++ inheritance diagrams, I've noticed that the diagrams can be incomplete.</p>
  
  <p>If several projects are documented using <a href="http://www.doxygen.nl/manual/external.html" rel="noreferrer">tag files</a> to allow cross-referencing, doxygen will <strong>successfully show all base classes</strong> that exist in other tag files, but it will <strong>not show derived classes</strong> if they are in other tag files.  The derived classes shown are always only the classes that exist in the current project.</p>
  
  <p><em>Example project 1 code:</em></p>
  
  <pre><code>class A {};
  
  class B : public A {};
  
  class C : public A {};
  </code></pre>
  
  <p><em>Example project 2 code:</em></p>
  
  <pre><code>#include &lt;proj1.h&gt;
  
  class D : public A {};
  
  class E : public A {};
  
  class F : public E {};
  </code></pre>
  
  <p>An inheritance diagram of <code>A</code> from project 1, using a tag file from project 2 - <strong>where is <code>D</code>, <code>E</code> &amp; <code>F</code>?</strong></p>
  
  <p><a href="https://i.stack.imgur.com/SaKlR.png" rel="noreferrer"><img src="https://i.stack.imgur.com/SaKlR.png" alt="enter image description here"></a></p>
  
  <p>An inheritance diagram of <code>E</code> from project 2, using a tag file from project 1 - <strong>parent class <code>A</code> is shown</strong>.</p>
  
  <p><a href="https://i.stack.imgur.com/Orsyz.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Orsyz.png" alt="enter image description here"></a></p>
  
  <p>Is there a setting to tell doxygen to generate complete inheritance graphs across projects when tag files are used?  </p>