In Qt the title of the main window can be set from everywhere and thankfully to the QApplication class we gain access to all created widgets by using the method findChildren() or topLevelWidgets(). The difference between both is that findChildren() returns a list only with widgets which reside inside another widget and in contrast topLevelWidgets() returns a list with parentless widgets which are always independent windows or dialogs.

With this background knowledge topLevelWidgets() is the way to go because findChildren only finds… children.

As a showing example here is a code snippet:

foreach(QWidget *widget, QApplication::topLevelWidgets()) {
  if(widget->objectName() == "MainWindow")
    qobject_cast<QMainWindow*>(widget)->setWindowTitle("-_-");
}

In the foreach loop we walk through the whole list of top-level widgets and because we want to change the title of the main window, we check the widget’s object name. In this example the main window’s object name is ‘MainWindow’ which is the default in Qt Designer.

Object names are not necessarily unique, multiple objects can have the same object name and thus the wrong widget could be chosen. MainWindow kind of implies there should only be one, so this simple check should be fine in most cases.

An alternative to the class QApplication is the macro qApp which is an instance of that class.

#define qApp (static_cast<QApplication *>(QCoreApplication::instance()))

Because QApplication implements the singleton design pattern only one instance can exist at most. If we are sure that there are no other objects of the type QMainWindow in the top-level widget list we could also check if it inherits from that class.

In place of the code above this would be fine then as well:

foreach(QWidget *widget, qApp->topLevelWidgets()) {
  if(widget->inherits("QMainWindow"))
    qobject_cast<QMainWindow*>(widget)->setWindowTitle("-_-");
}

In the following code sample a singleton is implemented with a static instance getter which can be called in any other class.

class MyClass{

public:

    static MyClass *instance(){
        if(!m_instance)
            m_instance = new MyClass;
        return m_instance;
    }

private:

    MyClass(){}
    ~MyClass(){}

    static MyClass *m_instance;
};

MyClass *MyClass::m_instance = NULL;

class SomeOtherClass{
    void doSomething(){
        MyClass *c = MyClass::instance();
    }
};

In SomeOtherClass the method doSomething() grabs the instance of MyClass and keeps it in a local pointer variable. Now if the asterisk * is forgotten, the variable c no longer defines a pointer and in that case ‘=’ is no assignment but the copy constructor used to initialize the local variable declaration while the compiler complains this with a error message telling you that the copy constructor is private.

If in your Qt application the compiler outputs similar messages as follows, check if you’ve forgotten the asterisk.

/usr/include/qt4/QtCore/qobject.h: In copy constructor 'MyClass::MyClass(const MyClass&)':
/usr/include/qt4/QtCore/qobject.h:309: error: 'QObject::QObject(const QObject&)' is private
src/MyClass.h:17: error: within this context
/usr/include/qt4/QtGui/qstandarditemmodel.h:424: error: 'QStandardItemModel::QStandardItemModel(const QStandardItemModel&)' is private
src/MyClass.h:17: error: within this context
/usr/include/qt4/QtGui/qstandarditemmodel.h:424: error: 'QStandardItemModel::QStandardItemModel(const QStandardItemModel&)' is private
src/MyClass.h:17: error: within this context
/usr/include/qt4/QtGui/qstandarditemmodel.h:424: error: 'QStandardItemModel::QStandardItemModel(const QStandardItemModel&)' is private
src/MyClass.h:17: error: within this context

Note: the singleton implementation above has some drawbacks like not being thread-safe. Here it’s only used as a showing example.