Bogus Error Messages with Qt .ui Files

Name your Qt Forms correctly and you will save lots of debugging time.

Bogus errors together with their messages can have a large number of reasons – full hard drives being one of the classics. When it comes to programming and especially C++, the possibilities for cryptic, meaningless and misleading error message are infinite.

A nice one bit us at one of our customers the other day. The message was something like

QLayout can only have instances of QWidget as parent

and it appeared as standard error output during program start-up. Needless to say that the whole thing crashed with a segmentation fault after that. The only change that was made was a header file that was added to the Qt files list in the CMakeLists.txt file.  The Qt class in this header file was just in its beginnings and had not yet any QLayouts, or QWidgets in it. Even the  C++ standard measure of cleaning and recompiling everything didn’t help.

So how is it possible that an additional Qt header file that has not references to QLayout and QWidget can cause such an error message?

As all of you experienced C/C++ developers know, for the compiler, a code file is not only the stuff that it contains directly but also what is #included! The offending header file included a generated ui description file which you get when you design your windows – or Forms in Qt terminology – with the Qt designer and use the Compile-Time-Form-Processing-approach to incorporate the form into the code base.

But how can that effect anything?

The Qt designer saves the forms into .ui files. From that, the so-called User Interface Compiler (uic) generates a header file containing a C++ class together with inlined code that creates the form. Form components like line edits, or push buttons are generated as instance attributes. The name of the class is generated from the name of the form. You can even use namespaces.  By naming it e.g. myproject::BestFormEverDesigned the generated class is named BestFormEverDesigned   is put into namespace myproject.

So far, so nice, handy and easy to use.

When you create a new form in qt designer, the default name is Form. Maybe you can guess already where this leads to…

Two forms for which the respective developers forgot to set a proper name, existed in the same sub project and had been compiled and linked into the same shared library. The compiler has no chance to detect this, because it sees only one

class Form

at a time. The linker happily links all of this together since it thinks that all Forms are created equal. And then at run-time … Boom!

I will have to look into a little Jenkins helper which breaks the build when a Form form is checked in…

Responsive Qt GUIs – Threading with Qt

Qt4 used to have only primitive threading support. Starting with version 4.4 new classes and functions makes your threading life a lot easier. So in case you haven’t come around to look at those features, do it now!, it’s worth it.

If you have used Qt4 for some time now, specifically since pre 4.4 versions, you may or may not aware of the latest developments in the threading part of the library. This post shall be a reminder in case you didn’t follow the versions in detail or just didn’t get around to look closer and/or update.

In pre 4.4 versions, the only way to do threading was to use class QThread. Subclass QThread, implement the run method, and there you had your thread. This looks fine at first, but, taking the title of the post as example, it can get annoying very fast. Sometimes you have just few lines of code you want to keep away from the GUI thread because, e.g. they could potentially block on some communication socket. Subclassing QThread for every small little work package is not something you want to do, so I guess many users just wrote their own thread pool or the like.

Starting with version4.4. Qt gained two major threading features, for which, IMHO, the Qt people do not a very good job of advertising. The first is QThreadPool together with QRunnable. All Java programmers, which use java.lang.Runnable since the beginning, may have their laugh now, I’ll wait…

The second new threading feature is the QtConcurrent namespace (from the Qt documentation):

The QtConcurrent namespace provides high-level APIs that make it possible to write multi-threaded programs without using low-level threading primitives such as mutexes, read-write locks, wait conditions, or semaphores

Sounds great! What else?

QtConcurrent includes functional programming style APIs for parallel list prosessing, including a MapReduce and FilterReduce implementation for shared-memory (non-distributed) systems, and classes for managing asynchronous computations in GUI applications.

This is really great stuff. Functions like QtConcurrent::run together with QFuture<T> and QFutureWatcher<T> can simplify your threading code significantly. So, if you haven’t got around to look at those new classes by now, I can only advise you to do it immediately. Allocate a refactoring slot in your next Sprint to replace all those old QThread sub-classes by shiny new QRunnables or QtConcurrent functions. It’s worth it!

Let’s get back to the responsive GUIs example. In his Qt Quarterly article, Witold Wysota describes in detail every technical possibility to keep your GUI responsive. It’s a very good article which provides a lot of insights. He starts with manual event processing and mentions the QtConcurrent features only at the very end of the article. I suggest the following order of threading-solutions-to-consider:

  1. QtConcurrent functions
  2. QThreadPool + QRunnable
  3. rest

Stay responsive!

Non-trivial Custom Data in QActions

If you want to implement dynamic context menus with non-trivial custom data in your QActions, the Qt4 documentation is not very helpful. The article describes some solutions to this task.

Sometimes I get very frustrated with the online Qt4 documentation. Sure, the API docs are massive but for many parts they provide only very basic information. Unfortunately, many Qt books, too, often stop exactly at the point where it gets interesting.

One example for this are context menus. The API docs just show you how menus in general are created and how they are connected to the application: Basically, all menus are instances of QMenu which are filled with instances of QAction. QActions are used as representation of any kind of action than can be triggered from the GUI.

The standard method to connect QActions to the GUI controlling code is to use one of their signals, e.g. triggered(). This signal can be connected to a slot of your own class where you can then execute the corresponding action. This works fine as long as you have a limited set of actions that you all know at coding time. For example, a menu in your tool bar which contains actions Undo/Redo/Cut/Copy/Paste can be created very easily.

But there are use cases where you do not know in advance how many actions there will be in your menus. For example, in an application that provides a GUI for composing a complex data structure you may want to provide the user assisting context menus for adding new data parts depending on what parts already exist. Suddenly, you have to connect many actions to one slot and then you somehow have to know which QAction the user actually clicked.

Btw, let’s all recall the Command Pattern for a moment… ok, now on to some solutions.

Method 0 – QAction::setData: The QAction class provides method setData(), which can be used to store custom data in a QAction instance using QVariant as data wrapper. If you then use QMenu’s triggered signal, which gives you a pointer to the QAction that was clicked, you can extract your data from the QAction. I find this a little bit ugly since you have to wrap your data into QVariant which can get messy, if you want to provide more than one data element

Method 1 – Enhancing QAction::triggered(): By sub-classing QAction you can provide your own triggered() signal which you can enhance with all parameters you need in your slot.

class MyAction : public QAction
    MyAction(QString someActionInfo)
      : someActionInfo_(someActionInfo)
      connect(this, SIGNAL(triggered()),
              this, SLOT(onTriggered()));
    void triggered(QString someActionInfo);
  private slots:
    void onTriggered() {
      emit triggered(someActionInfo_);
    QString someActionInfo_;

This is nice and easy but limited to what data types can be transported via signal/slot parameters.

Method 2 – QSignalMapper: From the Qt4 docs on QSignalMapper:

This class collects a set of parameterless signals, and re-emits them with integer, string or widget parameters corresponding to the object that sent the signal.

… which is basically the same as we did in method 1.

Method 3 – Separate domain specific action classes: By the time the context menu is created you add QActions to the menu using QMenu’s addAction methods. Then you create instances of separate Command-like classes (as in Command Pattern) and connect them with the QAction’s triggered() signal:

// Command-like custom action class. No GUI related stuff here!
class MySpecialAction : public QObject
    MySpecialAction(QObject* parent, &lt;all necessary parameters to execute&gt;);

  public slots:
    void execute();

// create context menu
QAction* specialAction =
  menu-&gt;addAction(&quot;Special Action Nr. 1&quot;);
MySpecialAction* mySpecialAction =
  new MySpecialAction(specialAction, ...);
connect(specialAction, SIGNAL(triggered()),
        mySpecialAction, SLOT(execute()));

As you can see, QAction specialAction is parent of mySpecialAction, thereby taking ownership of mySpecialAction. This is my preferred approach because it is the most flexible in terms of what custom data can be stored in the command. Furthermore, the part that contains the execution code – MySpecialAction – has nothing at all to do with GUI stuff and can easily be used in other parts of the system, e.g. non-GUI system interfaces.

How have you solved this problem?

Wrestling with Qt’s Model/View API – Filtering in Tree Models

Qt4’s model/view API can be kind of a challenge sometimes. Well, prepare for a even harder fight when sorting and filtering come into play.

As I described in one of my last posts, Qt4’s model/view API can be kind of a challenge sometimes. Well, prepare for a even harder fight when sorting and filtering come into play.

Let’s say you finally managed to get the data into your model and to provide correct implementations of the required methods in order for the attached views to display it properly. One of your next assignments after that is very likely something like implementing some kind of sorting and filtering of the model data. Qt provides a simple-at-first-sight proxy architecture for this with API class QSortFilterProxyModel as main ingredient.

Small preliminary side note: Last time I checked it was good OO practice to have only one responsibility for a given class. And wasn’t that even more important for good API design? Well, let’s not distract us with such minor details.

With my model implementation, none of the standard filtering mechanisms, like setting a filter regexp, were applicable, so I had to override method

QVariant filterAcceptsRow ( int source_row, const QModelIndex& sourceParent ) const

in order to make it work. Well, the rows disappeared as they should, but unfortunately so did all the columns except the first one. So what to do now? One small part of the documentation of QSortFilterProxyModel made me a little uneasy:

“… This simple proxy mechanism may need to be overridden for source models with more complex behavior; for example, if the source model provides a custom hasChildren() implementation you should also provide one in the proxy model.”

What on earth should I do with that? “… may need to be overridden“? “… for example.. hasChildren()…” Why can’t they just say clearly what methods must be overridden in which cases???

After a lot more trial and error I found that for whatever reason,

int columnCount ( const QModelIndex& parent ) const

had to be overridden in order for the columns to reappear. The implementation looks like what I had thought the proxy model would do already:

int MyFilter::columnCount ( const QModelIndex& parent ) const
   return sourceModel()->columnCount(parent);

So beware of QSortFilterProxyModel! It’s not as easy to use as it looks, and with that kind of fuzzy documentation it is even harder.

Forced into switch/case – Qt’s Model/View API

During my life as a programmer I have more and more come to dislike switch/case statements. They tend to be hard to grasp and with languages like C/C++ they are often the source of hard-to-find errors. Compilers that have warnings about missing default statements or missing cases for enumerated values can help to mitigate the situation, but still, I try to avoid them whenever I can.

The same holds true for if-elseif cascades or lots of if-elses in one method. They are hard to read, hard to maintain, increase the Crap, etc.

If you share this kind of mindset I invite you implement to some custom models with Qt4’s Model/View API. The design of the Model/View classes is derived from the well-known MVC pattern which separates data (model), presentation (view) and application logic (controller). In Qt’s case, view and controller are combined, supposedly making it simpler to use.

The basic idea of Qt’s implementation of its Model/View design is that views communicate with models using so-called model indexes. Using a table as an example, a row/column pair of (3,4) would be a model index pointing to data element in row 3, column 4. When a view is to be displayed it asks the attached model for all sorts of information about the data.

There are a few model implementations for standard tasks like simple string lists (QStringListModel) or file system manipulation (QDirModel < Qt4.4, QFileSystemModel >= Qt4.4). But usually you have to roll your own. For that, you have to subclass one of the abstract model classes that suits your needs best and implement some crucial methods.

For example, model methods rowCount and columnCount are called by the view to obtain the range of data it has to display. It then uses, among others, the data method to query all the stuff it needs to display the data items. The data method has the following signature:

QVariant data ( const QModelIndex&amp; index, int role ) const

Seems easy to understand: parameter index determines the data item to display and with QVariant as return type it is possible to return a wide range of data types. Parameter role is used to query different aspects of the data items. Apart from Qt::DisplayRole, which usually triggers the model to return some text, there are quite a lot other roles. Let’s look at a few examples:

  • Qt::ToolTipRole can be used to define a tool tip about the data item
  • Qt::FontRole can be use to define specific fonts
  • Qt::BackgroundRole and Qt::ForegroundRole can be used to set corresponding colors

So the views call data repeatedly with all the different roles and your model implementation is supposed to handle those different calls correctly. Say you implement a table model with some rows and columns. The design of the data method is forcing you into something like this …

QVariant data ( const QModelIndex&amp; index, int role ) const  {
   if (!index.isValid()) {
      return QVariant();

   switch (role)
      case Qt::DisplayRole:
         switch (index.column())
            case 0:
               // return display data for column 0
            case 1:
               // return display data for column 1

      case Qt::ToolTipRole:
         switch (index.column())
            case 0:
               // return tool tip data for column 0
            case 1:
               // return tool tip data for column 1

… or equivalent if-else structures. What happens here? The design of the data method forces the implementation to “switch” over role and column in one method. But nested switch/case statements? AARGH!! With our mindset outlined in the beginning this is clearly unacceptable.

So what to do? Well, to tell the truth, I’m still working on the best™ solution to that but, anyway, here is a first easy improvement: handler methods. Define handler methods for each role you want to support and store them in a map. Like so:

#include &lt;QAbstractTableModel&gt;

class MyTableModel : public QAbstractTableModel

  typedef QVariant (MyTableModel::*RoleHandler) (const QModelIndex&amp; idx) const;
  typedef std::map&lt;int, RoleHandler&gt; RoleHandlerMap;

    enum Columns {
      NAME_COLUMN = 0,

    MyTableModel() {
      m_roleHandlerMap[Qt::DisplayRole] =
      m_roleHandlerMap[Qt::ToolTipRole] =

    QVariant displayRoleHandler(const QModelIndex&amp; idx) const {
      switch (idx.column()) {
        case NAME_COLUMN:
          // return name data

        case ADDRESS_COLUMN:
          // return address data

          Q_ASSERT(!&quot;Invalid column&quot;);
      return QVariant();

    QVariant tooltipRoleHandler(const QModelIndex&amp; idx) const {

    QVariant data(const QModelIndex&amp; idx, int role) const {
      // omitted: check for invalid model index

      if (m_roleHandlerMap.count(role) == 0) {
        return QVariant();

      RoleHandler roleHandler =
      return (this-&gt;*roleHandler)(idx);
    RoleHandlerMap m_roleHandlerMap;

The advantage of this approach is that the supported roles are very well communicated. We still have to switch over the columns, though.

I’m currently working on a better solution which splits the data calls up into more meaningful methods and kind of binds the columns to specific parts of the data items in order to get a more row-centric approach: one row = one element, columns = element attributes. I hope this will get me out of this switch/case/if/else nightmare.

What do you think about it? I mean, is it just me, or is an API that forces you into crappy code just not so well done?

How would you solve this?