qt - Why does QComboBox ignore its fixed vertical size policy? -

i have widget 2 stacked layers. first looks this, fine:

the second looks this, wrong.

here complete code produces widget , prints size policies.

#include <qtwidgets>  int main (int argc, char ** argv) {     qapplication app (argc, argv);      qwidget w;     auto stack = new qstackedwidget ();     qwidget * labelbox = new qwidget ();     qwidget * combobox = new qcombobox ();     qwidget * label = new qlabel ("label");     qwidget * button = new qpushbutton ("button");     qwidget * date = new qdatetimeedit ();      auto gl = new qgridlayout (& w);     auto hl = new qhboxlayout (labelbox);      hl -> addwidget (label);     hl -> addwidget (button);      stack -> addwidget (labelbox);     stack -> addwidget (combobox);     stack -> setcurrentindex (1); // 0 layer, 1 bad      gl -> addwidget (date);     gl -> addwidget (stack, 0, 1);      w .setminimumsize (300, 200);     w .show ();      (auto x : {(qwidget *) stack, labelbox, combobox, label, button, date})     {         qwarning ()             << x -> metaobject () -> classname ()             << x -> sizepolicy ();     }     return app .exec (); } 

the stdout follows:

qstackedwidget qsizepolicy(horizontalpolicy = 5, verticalpolicy = 5) qwidget        qsizepolicy(horizontalpolicy = 5, verticalpolicy = 5) qcombobox      qsizepolicy(horizontalpolicy = 5, verticalpolicy = 0) qlabel         qsizepolicy(horizontalpolicy = 1, verticalpolicy = 5) qpushbutton    qsizepolicy(horizontalpolicy = 1, verticalpolicy = 0) qdatetimeedit  qsizepolicy(horizontalpolicy = 1, verticalpolicy = 0) 

the documentation these values is:

qsizepolicy::fixed      = 0 qwidget::sizehint() acceptable alternative, widget can never grow or shrink (e.g. vertical direction of push button). qsizepolicy::growflag   = 1 widget can grow beyond size hint if necessary. qsizepolicy::expandflag = 2 widget should space possible. qsizepolicy::shrinkflag = 4 widget can shrink below size hint if necessary. qsizepolicy::ignoreflag = 8 widget's size hint ignored. widget space possible. 

so looks vertical size policies fixed except qstackedwidget, qwidget contains label , button, , qlabel, have growflag|shrinkflag.

why qcombobox grow vertically, despite fixed policy? why not happen qpushbutton or qdatetimeedit?

this happens qt 4.8 , qt 5.1 on ubuntu 15.04.

you assigning combo box directly qstackedwidget haven't added qlayout first. assign layout, add layout qwidget , add qwidget qstackedwidget.

example:

qwidget * combowidget = new qwidget (); qwidget * combobox = new qcombobox ();  auto vl = new qvboxlayout (combowidget);  vl->addwidget(combobox); stack->addwidget (combowidget);