Resize MDI subwindows

[ChrisW67]

If you want to resize the MdiSubWindow then you should call resize() on that object, not the widget it contains:

#include "MainWindow.h"
#include "ui_MainWindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    m_firstWindow = new FirstWindow;
    m_secondWindow = new SecondWindow;
    m_thirdWindow = new ThirdWindow;
 
    QMdiSubWindow *w1 = ui->mdiArea->addSubWindow( m_firstWindow );
    ui->mdiArea->addSubWindow( m_secondWindow );
    ui->mdiArea->addSubWindow( m_thirdWindow );
 
    w1->resize( 500, 500 );
}

#include "MainWindow.h"
#include "ui_MainWindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    m_firstWindow = new FirstWindow;
    m_secondWindow = new SecondWindow;
    m_thirdWindow = new ThirdWindow;

    QMdiSubWindow *w1 = ui->mdiArea->addSubWindow( m_firstWindow );
    ui->mdiArea->addSubWindow( m_secondWindow );
    ui->mdiArea->addSubWindow( m_thirdWindow );

    w1->resize( 500, 500 );
}

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If your widgets actually contained anything then the containing QMdiSubWindow would get a preferred size from the layout and you probably would not need to resize at all.