title: ambiguous namespace issue after adding Ui form to Qt project
body: <p>I have been working on a c++/qt 5.15.2 project without using QtDesigner (all by coding). The project has a top level namespace &quot;future&quot;. I recently tried to use QtDesigner and added a form to the project. The form and related files were working fine on their own. But when I tried to add the form header file &quot;domclass.h&quot; to my previous header file &quot;Quote.h&quot; and tried to pass data between them, compiling error shows &quot;future: ambiguous symbol&quot;. And adding &quot;using namespace std;&quot; to &quot;domclass.h&quot; also gives &quot;future: ambiguous symbol&quot;. Here are the namespace settings in the files:
  <strong>in Quote.h:</strong> (compiling error if including &quot;domclass.h&quot;)</p>
  <pre><code>#include &quot;domclass.h&quot;
  using namespace std;
  namespace future
  { ... }
  </code></pre>
  <p><strong>in &quot;domclass.h&quot;</strong> (compiling error if &quot;using namespace std&quot;)</p>
  <pre><code>using namespace std;
  namespace Ui { class DOMClass; }
  class DOMClass : public QWidget
  {...};
  </code></pre>
  <p>I have tried to add the declaration of namespace &quot;future&quot; in &quot;domclass.h&quot; on top of &quot;namespace Ui&quot;, but it causes &quot;ui&quot; to be an undeclared identifier. Could someone please explain how to handle this namespace issue with Qt GUI forms? Thanks.</p>