注：文中所列代码质量不高，但不影响演示我的思路

实现思路说明

1. 实现DemoApplication
   相当于MFC中CWinAppEx的派生类，暂时没加什么功能。
   DemoApplication.h

   #pragma once#include <QtWidgets/QApplication>//相当于MFC中CWinAppEx的派生类，
   class DemoApplication : public QApplication
   {Q_OBJECTpublic:DemoApplication(int &argc, char **argv);~DemoApplication();};
   

   DemoApplication.cpp

   #include "DemoApplication.h"DemoApplication::DemoApplication(int &argc, char **argv): QApplication(argc, argv)
   {
   }DemoApplication::~DemoApplication()
   {
   }

2. 实现DemoDocument
   相当与MFC的CDocument。DemoDocument保存了当前所有视图的指针（此处实际是DeomChildWindow*，因为DeomChildWindow与DeomView时1对1关系，根据DeomChildWindow可以获得DemoView指针，简化设计，就这样处理了），实现了增加视图addView、移除视图removeView、获取视图数量getViewCount等函数。
   DemoDocument.h

   #pragma once#include <QObject>
   #include <list>class DemoChildWindow;//相当与MFC的CDocument
   class DemoDocument : public QObject
   {Q_OBJECTpublic:DemoDocument(QObject *parent = 0);~DemoDocument();void addView(DemoChildWindow* pChildWindow);void removeView(DemoChildWindow* pChildWindow);int getViewCount() const;unsigned int getId() { return m_nId; }signals:void closedDocument(DemoDocument* pDocument);private:static unsigned int allocId();private:static unsigned int s_NextId;unsigned int m_nId = 0;std::list<DemoChildWindow*> m_viewList; //视图列表
   };

   DemoDocument.cpp

   #include "DemoDocument.h"unsigned int DemoDocument::s_NextId = 0;DemoDocument::DemoDocument(QObject *parent): QObject(parent)
   {m_nId = allocId();
   }DemoDocument::~DemoDocument()
   {
   }unsigned int DemoDocument::allocId()
   {if (DemoDocument::s_NextId == std::numeric_limits<unsigned int>::max()){DemoDocument::s_NextId = 0;}return ++DemoDocument::s_NextId;
   }void DemoDocument::addView(DemoChildWindow* pChildWindow)
   {m_viewList.push_back(pChildWindow);
   }void DemoDocument::removeView(DemoChildWindow* pChildWindow)
   {auto it = std::find(m_viewList.begin(), m_viewList.end(), pChildWindow);if (it != m_viewList.end() ){m_viewList.erase(it);}if (m_viewList.size() == 0){emit closedDocument(this);}
   }int DemoDocument::getViewCount() const
   {return int(m_viewList.size());
   }

3. 实现DemoMainWindow
   相当于MFC中的CMainFrame，派生自CMDIFrameWndEx。此类new了一个QMdiArea对象，通过此对象实现多文档程序的通用功能，如切换窗口、层叠窗口、平铺窗口等。类DemoMainWindow直接管理了所有打开的文档，这点同MFC不一样，MFC是通过文档管理器、文档模版处理的，此处简化下，直接在DemoMainWindow中管理。此处我让DemoMainWindow负责新建、打开、关闭文档。
   DemoMainWindow.h

   #pragma once#include <QtWidgets/QMainWindow>
   #include <QMdiArea>
   #include <list>
   #include <memory>class DemoDocument;//相当于MFC中的CMainFrame，派生自CMDIFrameWndEx
   class DemoMainWindow : public QMainWindow
   {Q_OBJECTpublic:DemoMainWindow(QWidget *parent = Q_NULLPTR);virtual ~DemoMainWindow();DemoDocument* getActiveDocument() const;protected slots:void onSlotNewDocument();void onSlotClosedDocument(DemoDocument* pDocument );void onSlotNewWindow();private://创建文档的一个视图（DemoChildWindow-DeomView）void createNewWindow(DemoDocument* pDocument );private:QMdiArea* m_pMDIArea = nullptr;std::list<DemoDocument*> m_DocList; //文档列表
   };

   DemoMainWindow.cpp

   #include "DemoMainWindow.h"
   #include "DemoDocument.h"
   #include "DemoChildWindow.h"#include <QMdiSubWindow>
   #include <QMenuBar>DemoMainWindow::DemoMainWindow(QWidget *parent): QMainWindow(parent)
   {m_pMDIArea = new QMdiArea();this->setCentralWidget(m_pMDIArea);//void subWindowActivated(QMdiSubWindow * window)//菜单QMenu* pFileMenu = menuBar()->addMenu(QStringLiteral("文件"));QAction* pNewDocAction = new QAction(QStringLiteral("新建"), this);connect(pNewDocAction, SIGNAL(triggered()), this, SLOT(onSlotNewDocument()));pFileMenu->addAction(pNewDocAction);//窗口（实际需要动态添加到菜单栏中，即有视图窗口打开，就加入，否则就移除，此处暂未实现）QMenu* pWinMenu = menuBar()->addMenu(QStringLiteral("窗口"));QAction* pNewWinAction = new QAction(QStringLiteral("新建"), this);connect(pNewWinAction, SIGNAL(triggered()), this, SLOT(onSlotNewWindow()));pWinMenu->addAction(pNewWinAction);}DemoMainWindow::~DemoMainWindow()
   {for (auto pDoc : m_DocList){delete pDoc;}m_DocList.clear();
   }void DemoMainWindow::onSlotNewDocument()
   {auto pNewDoc = new DemoDocument();m_DocList.push_back(pNewDoc);createNewWindow(pNewDoc);connect(pNewDoc, SIGNAL(closedDocument(DemoDocument*)), this, SLOT(onSlotClosedDocument(DemoDocument*)));
   }void DemoMainWindow::onSlotClosedDocument(DemoDocument* pDocument)
   {auto it = std::find(m_DocList.begin(), m_DocList.end(), pDocument);if (it != m_DocList.end()){auto pDoc = *it;delete pDoc;m_DocList.erase(it);}
   }void DemoMainWindow::onSlotNewWindow()
   {auto pDocument = getActiveDocument();createNewWindow(pDocument);
   }//创建文档的一个视图（DemoChildWindow-DeomView）
   void DemoMainWindow::createNewWindow(DemoDocument* pDocument)
   {if (!pDocument){Q_ASSERT(false);return;}auto pChildWnd = new DemoChildWindow(pDocument);//自己new QMdiSubWindow时，必须设置Qt::WA_DeleteOnClose，参看文档//When you create your own subwindow, you must set the Qt::WA_DeleteOnClose widget //attribute if you want the window to be deleted when closed in the MDI area. //If not, the window will be hidden and the MDI area will not activate the next subwindow.//添加方式如下：
   //	QMdiSubWindow *pMdiSubWindow = new QMdiSubWindow;
   //	pMdiSubWindow->setWidget(pChildWnd);
   //	pMdiSubWindow->setAttribute(Qt::WA_DeleteOnClose);
   //	m_pMDIArea->addSubWindow(pMdiSubWindow);
   //	pMdiSubWindow->show();//这中方法更简单auto pMdiSubWindow = m_pMDIArea->addSubWindow(pChildWnd);pMdiSubWindow->setWindowTitle(QStringLiteral("文档%1:%2").arg(pDocument->getId()).arg(pDocument->getViewCount()));pMdiSubWindow->show();
   }DemoDocument* DemoMainWindow::getActiveDocument() const
   {auto pCurMdiSubWindow = m_pMDIArea->currentSubWindow();if (!pCurMdiSubWindow){return nullptr;}auto pChildWnd = dynamic_cast<DemoChildWindow*>(pCurMdiSubWindow->widget());if (!pChildWnd){Q_ASSERT(false);return nullptr;}return pChildWnd->GetDocument();
   }

4. 实现DemoChildWindow
   相当于MFC中的ChildFrm，派生自CMDIChildWndEx， 与文档是n-1关系, 与DemoView时1-1关系。此类负责创建DemoView，并且包含了文档对象的指针。MFC中创建ChildFrm以及CView没那么直接，通过消息触发创建了CView，具体参看MFC即可，我此处就简化处理创建的过程。
   DemoChildWindow.h

   #pragma once#include <QWidget>class DemoDocument;
   class DemoView;//相当于MFC中的ChildFrm，派生自CMDIChildWndEx， 与文档是n-1关系, 与DemoView时1-1关系，
   class DemoChildWindow : public QWidget
   {Q_OBJECTpublic:DemoChildWindow(DemoDocument* pDoc, QWidget* parent = 0, Qt::WindowFlags f = 0);~DemoChildWindow();DemoDocument* GetDocument() const;protected:virtual void closeEvent(QCloseEvent * event) override;private:DemoDocument* m_pDoc;DemoView* m_pView;
   };

   DemoChildWindow.cpp

   #include "DemoChildWindow.h"
   #include "DemoView.h"
   #include "DemoDocument.h"
   #include <QVBoxLayout>DemoChildWindow::DemoChildWindow(DemoDocument* pDoc, QWidget *parent, Qt::WindowFlags flags): QWidget(parent, flags)
   {m_pDoc = pDoc;m_pView = new DemoView();m_pDoc->addView(this);auto pVBoxLayout = new QVBoxLayout();pVBoxLayout->addWidget(m_pView);this->setLayout(pVBoxLayout);
   }DemoChildWindow::~DemoChildWindow()
   {
   }DemoDocument* DemoChildWindow::GetDocument() const
   {return m_pDoc;
   }void DemoChildWindow::closeEvent(QCloseEvent * event)
   {m_pDoc->removeView(this);return QWidget::closeEvent(event);
   }
   

5. 实现DemoView
   相当与MFC中的CView，在DemoView中仅显示了硬编码的文本字符串。
   DemoView.h

   #pragma once#include <QWidget>//相当与MFC中的CView
   class DemoView : public QWidget
   {Q_OBJECTpublic:DemoView(QWidget *parent = Q_NULLPTR);~DemoView();private://Ui::DemoView ui;
   };

   DemoView.cpp

   #include "DemoView.h"
   #include <QVBoxLayout>
   #include <QLabel>DemoView::DemoView(QWidget *parent): QWidget(parent)
   {auto pVBoxLayout = new QVBoxLayout();pVBoxLayout->addWidget(new QLabel(QStringLiteral("视图测试")));this->setLayout(pVBoxLayout);this->setMinimumSize(300, 200);
   }DemoView::~DemoView()
   {
   }

6. main函数

   #include "DemoApplication.h"
   #include "DemoMainWindow.h"int main(int argc, char *argv[])
   {DemoApplication a(argc, argv);DemoMainWindow w;w.show();return a.exec();
   }

运行演示