Creating a new widget and apply it on another widget

[seux]

Hello everyone,
imagine that i have something like this in a class

//...
private:
     QWidget *widget;

//...
private:
     QWidget *widget;

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and in one of the public functions i want to add other widgets like maybe a QPushButton and a QVBoxLayout. How can i do that?
For the QPushButton i tried it this way:

QPushButton *btn = new QPushButton("Hello");
widget->layout()->addWidget(btn);

QPushButton *btn = new QPushButton("Hello");
widget->layout()->addWidget(btn);

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But the widget has currently no layout, so i cannot try it out...
Is it possible to do it this way, and how can i apply a layout to the widget?