How to remove space (margin) that between QMainWindow and mdiArea?

Question

I have a QMainWindow and I have inserted mdiArea in the main window.
You must first see the following image:

As you seen the gray area is mdiArea and the spaces(margin) that between main window and mdiArea are what I want to remove it.
I have used setContentsMargins function, but does not do anything.

How can I remove these spaces ?

I want to be like the following image:

Answer 1

QMainWindow has a central widget that has a layout. The layout you use on QMainWindow is should be used upon its central widget instead. Hence you have to call

QWidget* QMainWindow::centralWidget() first,

so you can get the central widget first and then use

void QLayout::setContentsMargins(int left, int top, int right, int bottom)

to adjust its layout. The diagram below comes from Qt's documentation.

Creating a main window without a central widget is not supported. You must have a central widget even if it's just a placeholder.

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For example, you could have the following in QMainWindow's constructor:

centralWidget()->layout()->setContentsMargins(0, 0, 0, 0);
statusBar()->hide();
ui->mainToolBar->hide();

The status bar and the tool bar have been hidden, in order to remove as much blank space as possible.