What do you mean by "fully loaded"?

You can trigger some activity when the program returns to the Qt event loop with a single-shot timer at the end of the constructor:
Qt Code: 
  1. #include <QtGui>
  2. #include <QDebug>
  3.  
  4. class MainWindow: public QMainWindow {
  5.     Q_OBJECT
  6.     QLabel *label1;
  7.     QLabel *label2;
  8. public:
  9.     MainWindow(QWidget *p = 0): QMainWindow(p) {
  10.         QWidget *central = new QWidget(this);
  11.         label1 = new QLabel("Label 1", this);
  12.         label2 = new QLabel("Label 2", this);
  13.  
  14.         QVBoxLayout *layout = new QVBoxLayout(central);
  15.         layout->addWidget(label1);
  16.         layout->addWidget(label2);
  17.  
  18.         central->setLayout(layout);
  19.         setCentralWidget(central);
  20.  
  21.         QTimer::singleShot(0, this, SLOT(doLater()));
  22.     }
  23. public slots:
  24.     void doLater() {
  25.         // Executes when event loop reached (after show in this case)
  26.         label2->setText("I've been changed");
  27.     }
  28.  
  29. private:
  30. };
  31.  
  32. int main(int argc, char *argv[])
  33. {
  34.     QApplication app(argc, argv);
  35.  
  36.     MainWindow m;
  37.     m.show();
  38.     return app.exec();
  39. }
  40. #include "main.moc"
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