MDI应用程序就是在主窗口里创建多个同类型的MDI子窗口，这些MDI子窗口在主窗口里显示，并享受主窗口上的工具栏和菜单等操作功能，主窗口上的操作都针对当前活动的MDI子窗口进行。

图示

代码示例

QWMainWindow.h

#ifndef QWMAINWINDOW_H
#define QWMAINWINDOW_H

#include <QMainWindow>
#include <QMdiSubWindow>
namespace Ui
{
    class QWMainWindow;
}

class QWMainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit QWMainWindow(QWidget* parent = nullptr);
    ~QWMainWindow();
public slots :
    void newDoc();
    void openDoc();
    void cutDoc();
    void copyDoc();
    void pasteDoc();
    void fontDoc();
    void cascadeDemo();
    void titleDemo();
    void viewTabDemo();
    void viewSubDemo();
    void closeDemo();
private slots:
    void on_mdiArea_subWindowActivated(QMdiSubWindow* arg1);

private:
    Ui::QWMainWindow* ui;
    QAction* cutAct;
    QAction* copyAct ;
    QAction* pasteAct;
    QAction* fontAct;
    QAction* cascadeAct;
    QAction* titleAct ;
    QAction* viewTabAct;
    QAction* viewSuAct;
    QAction* closeAct;
};

#endif // QWMAINWINDOW_H

QWMainWindow.cpp

#include "QWMainWindow.h"
#include "ui_QWMainWindow.h"
#include "QFormDoc.h"
#include <QMdiSubWindow>
#include <QDir>
#include <QFileDialog>
QWMainWindow::QWMainWindow(QWidget* parent)
    : QMainWindow(parent)
    , ui(new Ui::QWMainWindow)
{
    ui->setupUi(this);
    this->setCentralWidget(ui->mdiArea);
    // this->setWindowState(Qt::WindowMaximized);
    ui->toolBar->setToolButtonStyle(Qt::ToolButtonTextOnly);
    QAction* newDocAct = new QAction(tr("New Doc"), this);
    connect(newDocAct, SIGNAL(triggered()), this, SLOT(newDoc()));
    QAction* openDocAct = new QAction(tr("Open Doc"), this);
    connect(openDocAct, SIGNAL(triggered()), this, SLOT(openDoc()));
    cutAct = new QAction(tr("Cut"), this);
    connect(cutAct, SIGNAL(triggered()), this, SLOT(cutDoc()));
    copyAct = new QAction(tr("Copy"), this);
    connect(copyAct, SIGNAL(triggered()), this, SLOT(copyDoc()));
    pasteAct = new QAction(tr("paste"), this);
    connect(pasteAct, SIGNAL(triggered()), this, SLOT(pasteDoc()));
    fontAct = new QAction(tr("Font"), this);
    connect(fontAct, SIGNAL(triggered()), this, SLOT(fontDoc()));
    cascadeAct = new QAction(tr("Cascade"), this);
    connect(cascadeAct, SIGNAL(triggered()), this, SLOT(cascadeDemo()));
    titleAct = new QAction(tr("Title"), this);
    connect(titleAct, SIGNAL(triggered()), this, SLOT(titleDemo()));
    viewTabAct = new QAction(tr("MDI Tab"), this);
    connect(viewTabAct,  &QAction::triggered, this, &QWMainWindow::viewTabDemo);
    viewSuAct = new QAction(tr("MDI Sub"), this);
    connect(viewSuAct,  &QAction::triggered, this, &QWMainWindow::viewSubDemo);
    closeAct = new QAction(tr("Close"), this);
    connect(closeAct, SIGNAL(triggered()), this, SLOT(closeDemo()));
    ui->toolBar->addAction(newDocAct);
    ui->toolBar->addAction(openDocAct);
    ui->toolBar->addAction(cutAct);
    ui->toolBar->addAction(copyAct);
    ui->toolBar->addAction(pasteAct);
    ui->toolBar->addAction(fontAct);
    ui->toolBar->addAction(cascadeAct);
    ui->toolBar->addAction(titleAct);
    ui->toolBar->addAction(viewTabAct);
    ui->toolBar->addAction(viewSuAct);
    ui->toolBar->addAction(closeAct);
    cutAct->setEnabled(false);
    copyAct->setEnabled(false);
    pasteAct->setEnabled(false);
    fontAct->setEnabled(false);
}

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

void QWMainWindow::newDoc()
{
    QFormDoc* formDoc = new QFormDoc(this);
    ui->mdiArea->addSubWindow(formDoc);
    formDoc->show();
    cutAct->setEnabled(true);
    copyAct->setEnabled(true);
    pasteAct->setEnabled(true);
    fontAct->setEnabled(true);
}

void QWMainWindow::openDoc()
{
    bool needNew = false;
    QFormDoc* formDoc;
    if(ui->mdiArea->subWindowList().count() > 0) {
        formDoc = (QFormDoc*)ui->mdiArea->activeSubWindow()->widget();
        needNew = formDoc->isFileOpened();
    } else {
        needNew = true;
    }
    QString curPath = QDir::currentPath();
    QString fileName = QFileDialog::getOpenFileName(this, u8"打开文件", curPath);
    if(fileName.isEmpty()) {
        return;
    }
    if(needNew) {
        formDoc = new QFormDoc(this);
        ui->mdiArea->addSubWindow(formDoc);
    }
    formDoc->loadFromFile(fileName);
    formDoc->show();
    cutAct->setEnabled(true);
    copyAct->setEnabled(true);
    pasteAct->setEnabled(true);
    fontAct->setEnabled(true);
}

void QWMainWindow::cutDoc()
{
    QFormDoc* formDoc = (QFormDoc*)ui->mdiArea->activeSubWindow()->widget();
    formDoc->textCut();
}

void QWMainWindow::copyDoc()
{
    QFormDoc* formDoc = (QFormDoc*)ui->mdiArea->activeSubWindow()->widget();
    formDoc->textCopy();
}

void QWMainWindow::pasteDoc()
{
    QFormDoc* formDoc = (QFormDoc*)ui->mdiArea->activeSubWindow()->widget();
    formDoc->textPaste();
}

void QWMainWindow::fontDoc()
{
    QFormDoc* formDoc = (QFormDoc*)ui->mdiArea->activeSubWindow()->widget();
    formDoc->setEditFont();
}

void QWMainWindow::cascadeDemo()
{
    ui->mdiArea->cascadeSubWindows();
}

void QWMainWindow::titleDemo()
{
    ui->mdiArea->tileSubWindows();
}

void QWMainWindow::viewTabDemo()
{
    ui->mdiArea->setViewMode(QMdiArea::TabbedView);//Tab多页模式
    ui->mdiArea->setTabsClosable(true);
    cascadeAct->setEnabled(false);
    titleAct->setEnabled(false);
}

void QWMainWindow::viewSubDemo()
{
    ui->mdiArea->setViewMode(QMdiArea::SubWindowView);//子窗口模式
    cascadeAct->setEnabled(true);
    titleAct->setEnabled(true);
}


void QWMainWindow::closeDemo()
{
    ui->mdiArea->closeAllSubWindows();
}

void QWMainWindow::on_mdiArea_subWindowActivated(QMdiSubWindow* arg1)
{
    if(ui->mdiArea->subWindowList().count() == 0) {
        cutAct->setEnabled(false);
        copyAct->setEnabled(false);
        pasteAct->setEnabled(false);
        fontAct->setEnabled(false);
        ui->statusbar->clearMessage();
    } else {
        QFormDoc* formDoc = static_cast<QFormDoc*>(ui->mdiArea->activeSubWindow()->widget());
        ui->statusbar->showMessage(formDoc->currentFileName());
    }
}