I want that widgets contained in QMainWindow occupy all the available space when application starts.

So in QDockWidget 'first' : I want that the vertical layout occupies all the space in the QDockWidget ;

In QDockWidget 'third', I want that the splitter with the two QTableViews occupies all the space in the QDockWidget ;

And finally, I want that the central QTableView occupies all the available space between the left and right dock widgets.

With QtDesigner, I can't achieve this goal. So how could I do that by code please ?

Here is the look the mainwindow has for the moment :

CtdMainWindow.h :

#include <QMainWindow>

#include "QtMainWindowWithDocks.h"

QT_BEGIN_NAMESPACE
namespace Ui {
    class MainWindow; // This is the "main window" from the UI XML file
}
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    //Q_OBJECT

public:
    explicit MainWindow(QWidget* parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow* ui;  // Pointer to the UI generated by the XML file and the uic program

};

CtdMainWindow.cpp :

#include "QtMainWindowWithDocks.h"
#include "CtdMainWindow.h"

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

MainWindow::~MainWindow()
{
    delete ui;
}

QtMainWindowWithDocks.h :

/********************************************************************************
** Form generated from reading UI file 'QtMainWindowWithDocks.ui'
**
** Created by: Qt User Interface Compiler version 5.15.13
**
** WARNING! All changes made in this file will be lost when recompiling UI file!
********************************************************************************/

#ifndef QTMAINWINDOWWITHDOCKS_H
#define QTMAINWINDOWWITHDOCKS_H

#include <QtCore/QVariant>
#include <QtWidgets/QApplication>
#include <QtWidgets/QDockWidget>
#include <QtWidgets/QHeaderView>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QMenuBar>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QSplitter>
#include <QtWidgets/QStatusBar>
#include <QtWidgets/QTableView>
#include <QtWidgets/QVBoxLayout>
#include <QtWidgets/QWidget>

QT_BEGIN_NAMESPACE

class Ui_MainWindow
{
public:
    QWidget *centralwidget;
    QTableView *tableView;
    QMenuBar *menubar;
    QStatusBar *statusbar;
    QDockWidget *dockWidget;
    QWidget *dockWidgetContents;
    QWidget *widget;
    QVBoxLayout *verticalLayout;
    QPushButton *pushButton;
    QPushButton *pushButton_2;
    QPushButton *pushButton_3;
    QDockWidget *dockWidget_2;
    QWidget *dockWidgetContents_2;
    QDockWidget *dockWidget_3;
    QWidget *dockWidgetContents_3;
    QSplitter *splitter;
    QTableView *tableView_2;
    QTableView *tableView_3;

    void setupUi(QMainWindow *MainWindow)
    {
        if (MainWindow->objectName().isEmpty())
            MainWindow->setObjectName(QString::fromUtf8("MainWindow"));
        MainWindow->resize(936, 767);
        centralwidget = new QWidget(MainWindow);
        centralwidget->setObjectName(QString::fromUtf8("centralwidget"));
        tableView = new QTableView(centralwidget);
        tableView->setObjectName(QString::fromUtf8("tableView"));
        tableView->setGeometry(QRect(30, 40, 521, 661));
        MainWindow->setCentralWidget(centralwidget);
        menubar = new QMenuBar(MainWindow);
        menubar->setObjectName(QString::fromUtf8("menubar"));
        menubar->setGeometry(QRect(0, 0, 936, 21));
        MainWindow->setMenuBar(menubar);
        statusbar = new QStatusBar(MainWindow);
        statusbar->setObjectName(QString::fromUtf8("statusbar"));
        MainWindow->setStatusBar(statusbar);
        dockWidget = new QDockWidget(MainWindow);
        dockWidget->setObjectName(QString::fromUtf8("dockWidget"));
        dockWidgetContents = new QWidget();
        dockWidgetContents->setObjectName(QString::fromUtf8("dockWidgetContents"));
        widget = new QWidget(dockWidgetContents);
        widget->setObjectName(QString::fromUtf8("widget"));
        widget->setGeometry(QRect(20, 40, 77, 83));
        verticalLayout = new QVBoxLayout(widget);
        verticalLayout->setObjectName(QString::fromUtf8("verticalLayout"));
        verticalLayout->setContentsMargins(0, 0, 0, 0);
        pushButton = new QPushButton(widget);
        pushButton->setObjectName(QString::fromUtf8("pushButton"));

        verticalLayout->addWidget(pushButton);

        pushButton_2 = new QPushButton(widget);
        pushButton_2->setObjectName(QString::fromUtf8("pushButton_2"));

        verticalLayout->addWidget(pushButton_2);

        pushButton_3 = new QPushButton(widget);
        pushButton_3->setObjectName(QString::fromUtf8("pushButton_3"));

        verticalLayout->addWidget(pushButton_3);

        dockWidget->setWidget(dockWidgetContents);
        MainWindow->addDockWidget(Qt::LeftDockWidgetArea, dockWidget);
        dockWidget_2 = new QDockWidget(MainWindow);
        dockWidget_2->setObjectName(QString::fromUtf8("dockWidget_2"));
        dockWidgetContents_2 = new QWidget();
        dockWidgetContents_2->setObjectName(QString::fromUtf8("dockWidgetContents_2"));
        dockWidget_2->setWidget(dockWidgetContents_2);
        MainWindow->addDockWidget(Qt::LeftDockWidgetArea, dockWidget_2);
        dockWidget_3 = new QDockWidget(MainWindow);
        dockWidget_3->setObjectName(QString::fromUtf8("dockWidget_3"));
        dockWidgetContents_3 = new QWidget();
        dockWidgetContents_3->setObjectName(QString::fromUtf8("dockWidgetContents_3"));
        splitter = new QSplitter(dockWidgetContents_3);
        splitter->setObjectName(QString::fromUtf8("splitter"));
        splitter->setGeometry(QRect(10, 0, 141, 691));
        splitter->setOrientation(Qt::Vertical);
        tableView_2 = new QTableView(splitter);
        tableView_2->setObjectName(QString::fromUtf8("tableView_2"));
        splitter->addWidget(tableView_2);
        tableView_3 = new QTableView(splitter);
        tableView_3->setObjectName(QString::fromUtf8("tableView_3"));
        splitter->addWidget(tableView_3);
        dockWidget_3->setWidget(dockWidgetContents_3);
        MainWindow->addDockWidget(Qt::RightDockWidgetArea, dockWidget_3);

        retranslateUi(MainWindow);

        QMetaObject::connectSlotsByName(MainWindow);
    } // setupUi

    void retranslateUi(QMainWindow *MainWindow)
    {
        MainWindow->setWindowTitle(QCoreApplication::translate("MainWindow", "MainWindow", nullptr));
        dockWidget->setWindowTitle(QCoreApplication::translate("MainWindow", "first", nullptr));
        pushButton->setText(QCoreApplication::translate("MainWindow", "PushButton", nullptr));
        pushButton_2->setText(QCoreApplication::translate("MainWindow", "PushButton", nullptr));
        pushButton_3->setText(QCoreApplication::translate("MainWindow", "PushButton", nullptr));
        dockWidget_2->setWindowTitle(QCoreApplication::translate("MainWindow", "second", nullptr));
        dockWidget_3->setWindowTitle(QCoreApplication::translate("MainWindow", "third", nullptr));
    } // retranslateUi

};

namespace Ui {
    class MainWindow: public Ui_MainWindow {};
} // namespace Ui

QT_END_NAMESPACE

#endif // QTMAINWINDOWWITHDOCKS_H

and main.cpp :

#include "CtdMainWindow.h"

int main(int argc, char** argv)
{
    QApplication app(argc, argv);

    MainWindow window;
    window.setVisible(true);

    return app.exec();
}