main.cpp

/*
 * http://ja.uwenku.com/question/p-xdsgakng-kv.html
 */
#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

mainwindow.cpp

/*
 * https://www.webdevqa.jp.net/ja/qt/コンテンツに合わせてウィンドウのサイズを変更する/1073740067/
 */
#include <QStandardItemModel>
#include <QVBoxLayout>
#include <QTableWidget>
#include <QHeaderView>
#include <QPushButton>
#include <QLabel>
#include <QDebug>

#include "mainwindow.h"
#include "SubWindowA.h"
#include "SubWindowB.h"


/*
 *
 */
MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent)
{
    auto mainLayout = new QVBoxLayout(this);
    {
        setLayout(mainLayout);

        auto button1 = new QPushButton("Dialog 1");
        {
            connect(button1, &QPushButton::clicked, this, &MainWindow::onButton1Clicked);
        }
        mainLayout->addWidget(button1);

        auto button2 = new QPushButton("Dialog 2");
        {
            connect(button2, &QPushButton::clicked, this, &MainWindow::onButton2Clicked);
        }
        mainLayout->addWidget(button2);

    }
}

MainWindow::~MainWindow()
{
    if (_windowA != nullptr)
    {
        _windowA->hide();
        _windowA->deleteLater();
    }

    if (_windowB != nullptr)
    {
        _windowB->hide();
        _windowB->deleteLater();
    }
}

void MainWindow::onButton1Clicked()
{
    _windowA = new SubWindowA;
//    _windowA->raise();
    _windowA->show();

    connect(_windowA, &SubWindowA::close, this, &MainWindow::closedWindowA);
//    this->hide();
}

void MainWindow::onButton2Clicked()
{
    _windowB = new SubWindowB;
    _windowB->show();
    _windowA->raise();
//    _windowA->activateWindow();

//    /*
//     * https://kojinjigyou.org/4119/
//     */
//    _windowB->lower();

//    /*
//     * WindowAの下に隠れるんじゃないのか？
//     */
//    _windowB->stackUnder(_windowA);

    connect(_windowB, &SubWindowB::close, this, &MainWindow::closedWindowB);
//    this->hide();
}


void MainWindow::closedWindowA()
{
    _windowA->hide();
//    this->show();
    delete _windowA;
    _windowA = nullptr;
}

void MainWindow::closedWindowB()
{
    _windowB->hide();
//    this->show();
    delete _windowB;
    _windowB = nullptr;
}

mainwindow.h

#pragma

#include <QWidget>

class SubWindowA;
class SubWindowB;

class MainWindow : public QWidget
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow() override;

private slots:
    void onButton1Clicked();
    void onButton2Clicked();

    void closedWindowA();
    void closedWindowB();

private:
    SubWindowA *_windowA;
    SubWindowB *_windowB;
};

SubWindowA.cpp

#include "SubWindowA.h"

#include "ui_window_a.h"


SubWindowA::SubWindowA(QWidget *parent)
    : QWidget(parent),
    form(new Ui::Form)
{
    form->setupUi(this);
}

void SubWindowA::closeEvent(QCloseEvent *event)
{
    emit close();
}

SubWindowA.h

#pragma once

#include <QWidget>

namespace Ui
{
    class Form;
};

class SubWindowA : public QWidget
{
Q_OBJECT
public:
    explicit SubWindowA(QWidget *parent = 0);
    ~SubWindowA() override = default;

signals:
    void close();

public slots:
    void closeEvent(QCloseEvent *event) override;

private:
    Ui::Form *form;
};

SubWindowB.cpp

#include "SubWindowB.h"

#include "ui_window_b.h"


SubWindowB::SubWindowB(QWidget *parent)
    : QDialog(parent),
      form(new Ui::FormB)
{
    form->setupUi(this);
}

void SubWindowB::closeEvent(QCloseEvent *event)
{
    emit close();
}

SubWindowB.h

#pragma once

#include <QDialog>

class QTableWidget;

namespace Ui
{
    class FormB;
};

class SubWindowB : public QDialog
{
Q_OBJECT
public:
    explicit SubWindowB(QWidget *parent = 0);
    ~SubWindowB() override = default;

signals:
    void close();

public slots:
    void closeEvent(QCloseEvent *event) override;

private:
    Ui::FormB *form;
};