title: Correct way of using a constant pointer in QT signals
body: <p>I want to connect signals and slots and pass a constant pointer as an argument, like so:</p>
  
  <p>ClassA.hpp</p>
  
  <pre><code>using another_type_id_t = std::string; 
  using type_ids_t = std::set&lt;abc::another_type_id_t&gt;;
  
  Q_DECLARE_METATYPE(type_ids_t *);
  
  signals:
  FuncA(results::type_ids_t const *a);
  </code></pre>
  
  <p>ClassB.hpp</p>
  
  <pre><code>slots:
  FuncA(results::type_ids_t const *a)
  </code></pre>
  
  <p>ClassB.cpp</p>
  
  <pre><code>qRegisterMetaType&lt;results::type_ids_t *&gt;("results::type_ids_t *");
  connect(classAInstance, SIGNAL(FuncA(results::type_ids_t *)),
      this, SLOT(FuncA(results::type_ids_t *));
  </code></pre>
  
  <p>The connect function fails. I cannot use the more modern style of connect because the two classes are not being compiled together. They are aware of each other at runtime. Other connections between them work, but not this one for whatever reason.</p>
  
  <p>What am I doing wrong here?</p>