My blog has been moved to

Wednesday, March 15, 2006

Resize Pop-up List of A Combo Box

You want to be able to resize the pop-up list of a combo box. Normally, this pop-up list has always the same width as the combo box. However, in certain cases the width of the combo box needs to be fixed. But if the pop-up list does not expand itself, then only part of the items on the list is visible as illustrated here:

In this particular situation, the look of the pop-up list will be better if it can be resized independently from the combo box itself.

QComboBox unfortunately does not provide public functions to access its pop-up list. Also, the pop-up list widget itself is not accessible. It is possible to create a subclass of QComboBox and reuse its private members (in header file qcombobox_p.h), but this would have required a quite amount of effort.

By looking at the source of QComboBox, we find out that the pop-up widget is QComboBoxPrivateContainer. It is also constructed as a child widget of the combo box. Using run-time introspection feature of Qt, it is very easy to find this widget and change its size:

void resizeComboBoxPopup(QComboBox* comboBox, int width, int height)
  if(!comboBox) return;
  // force the construction of QComboBoxViewContainer

  // iterate to find QComboBoxPrivateContainer
  QWidget* popup = 0;
  QObjectList objects = comboBox->children();
  for(int i = 0; i < objects.size(); ++i)
      QObject* obj =;
          popup = qobject_cast<QWidget*>(obj);
    popup->resize(width, height);

Of course, this cheap trick is only a workaround and only works as long as the implementation of QComboBox does not change too much. It can also be considered as a crime to Qt. So, kids, don't try this at home...


Thomas Matelich said...

you could save yourself a few cycles with a break in your if in the for loop.

if(obj->inherits(...)) { popup = ...; break; }

Unknown said...

What about:

Q3ComboBox cb = ...
Q3ListBox * lb;
lb = combobox->listBox();
// adjust size of lb here

In Qt Creator, you can promote QComboBox to Q3ComboBox. I did not try it yet but seems clean.

Anonymous said... Simple way to do it.