QSizePolicy true meaning, documentation and thoughts

Question

I have been now two years working deeply with Qt layout system. After this time i encountered thosand of problems with SizePolicies and Layouts. I normally found out solutions, but never really understood what i was doing.

Now i was taking some time to try to understand correctly and build a 101 GUIDE for them and never fail. I found out this piece of information in the documentation about QSizePolicy of a QWidget:

This property holds the default layout behavior of the widget

If there is a QLayout that manages this widget's children, the size policy specified by that layout is used. If there is no such QLayout, the result of this function is used.

I thought that if you had a QLabel, for example, and you set the policies to Horizontal Expanding, Vertical Fixed, THE LABEL itself changed that way. But it doesn't, at all.

When reading that i see that it talks about ITS CHILDREN inside the LAYOUT. So what does it mean, then?. Nothing is inside the label, is that why it does not work?.

WHat about inserting a label inside a QFrame, and telling the frame to be Expanding... Will the QFrame expand or stretch (depending on the rest of brother widgets in the same Layout) or will the label expand or stretch, not the QFrame?

What a mess...

What about Stretching?. If you set stretching 10 when you add the widget:

layout->addwidget(label, 10, Qt::AlignHCenter);

It doesn't work either.

Stretch 0 when you add the widget means : Take the policies i told you. Default, depends on the type of widget. Button-like widgets have expanding-fixed. Box-like Expanding-Expanding...

Stretch 10 means: grow maximum.

Am i right? Well. When having a QFrame and a QLabel inside, setting Expanding, and 10 to strech to the label DOES NOT WORK.

Answer 1

I don't understand all of your questions, so I will only answer to those that I think i do.

• I thought that if you had a QLabel, for example, and you set the policies to Horizontal Expanding, Vertical Fixed, THE LABEL itself changed that way. But it doesn't, at all.

I don't know what that means. What is the exact behavior you're expecting and what is actually happening? Is this label in a layout? Are there any other widgets in this layout?

• WHat about inserting a label inside a QFrame, and telling the frame to be Expanding...

If you set a QSizePolicy to your QFrame object and set a layout to it, this size policy might be ignored. Docs: If there is a QLayout that manages this widget's children, the size policy specified by that layout is used. If there is no such QLayout, the result of this function is used. This means that the widget's layout should manage the size of the widget. Note that it will still respect the minimum/maximum width/height values.

• Stretch 10 means: grow maximum.

What makes you think that?. The stretch factor is dependent on other widgets inside the layout. Docs: Stretch factors are used to change how much space widgets are given in proportion to one another.

• When having a QFrame and a QLabel inside, setting Expanding, and 10 to strech to the label DOES NOT WORK.

Does not work how? If you set stretch for QFrame to 1 and for QLabel to 10, your QLabel object should always be 10 times as wide/high(depending of your layout type) as your QFrame object. If your QFrame has a layout and it contains children, then this might not work as it would depend on the childrens size policies.