QWidget及其子类都可有右键菜单，因为QWidget有以下两个与右键菜单有关的函数：

Qt::ContextMenuPolicy contextMenuPolicy () const

void setContextMenuPolicy ( Qt::ContextMenuPolicy policy )

Qt::ContextMenuPolicy枚举类型包括：Qt::DefaultContextMenu, Qt::NoContextMenu, Qt::PreventContextMenu, Qt::ActionsContextMenu, and Qt::CustomContextMenu。

使用方式如下：

1）默认是Qt::DefaultContextMenu。
它是利用右键菜单事件contextMenuEvent()来处理（which means the contextMenuEvent() handler is called)。就是要重写contextMenuEvent( QContextMenuEvent * event )函数。

实例：

void CGuiMainwindow::contextMenuEvent(QContextMenuEvent* e)
{
QMenu *menu = new QMenu();
menu->addSeparator();
menu->addSeparator();
menu->addAction(Act_Maxsize);
menu->addSeparator();
menu->addSeparator();
menu->addAction(Act_Normal);
menu->addSeparator();
menu->addSeparator();
menu->exec(e->globalPos());
delete menu;
}

注意，里面的action是需要自己实现编辑好的，可以在action editor内先进行编辑。

2）使用Qt::CustomContextMenu。
它是发出QWidget::customContextMenuRequested信号，注意仅仅只是发信号，意味着要自己写显示右键菜单的slot。
这个信号是QWidget唯一与右键菜单有关的信号（也是自有的唯一信号），同时也是很容易被忽略的signal：

void customContextMenuRequested ( const QPoint & pos )

该信号的发出条件是：用户请求contextMenu（常规就是鼠标右击啦）且同时被击的widget其contextMenuPolicy又是Qt::CustomContextMenu。
注意：pos是该widget接收右键菜单事件的位置，一般是在该部件的坐标系中。但是对于QAbstratScrollArea及其子类例外，是对应着其视口viewport()的坐标系。如常用的QTableView、QHeaderView就是QAbstratScrollArea的子类。
因为仅发信号，所以需自己写显示右键菜单的slot来响应，例如一个表格（QTableView类型）表头的显示右键菜单槽：
datatable->horizontalHeader()->setContextMenuPolicy(Qt::CustomContextMenu);
connect(datatable->horizontalHeader(), SIGNAL(customContextMenuRequested(const QPoint&)), 
        this, SLOT(show_contextmenu(const QPoint&)));//this是datatable所在窗口
QMenu *cmenu = NULL;
show_contextmenu(const QPoint& pos)
{
    if(cmenu)//保证同时只存在一个menu，及时释放内存
    {
        delete cmenu;
        cmenu = NULL;
    }
    QMenu cmenu = new QMenu(datatable->horizontalHeader());
    
    QAction *ascendSortAction = cmenu->addAction("升序");
    QAction *descendSortAction = cmenu->addAction("降序");
    QAction *filterAction = cmenu->addAction("过滤");
    QAction *reshowAction = cmenu->addAction("重载");
    
    connect(ascendSortAction, SIGNAL(triggered(bool)), this, SLOT(sort_ascend()));
    connect(descendSortAction, SIGNAL(triggered(bool)), this, SLOT(sort_descend()));
    connect(filterAction, SIGNAL(triggered(bool)), this, SLOT(show_filter_dlg()));
    connect(reshowAction, SIGNAL(triggered(bool)), this, SLOT(reshow_data()));
    
    cmenu->exec(QCursor::pos());//在当前鼠标位置显示
    //cmenu->exec(pos)是在viewport显示
}

也可先做好cmenu，好处是始终使用一个：
    QMenu cmenu = new QMenu(datatable->horizontalHeader());
    
    QAction *ascendSortAction = cmenu->addAction("升序");
    QAction *descendSortAction = cmenu->addAction("降序");
    QAction *filterAction = cmenu->addAction("过滤");
    QAction *reshowAction = cmenu->addAction("重载");
    
    connect(ascendSortAction, SIGNAL(triggered(bool)), this, SLOT(sort_ascend()));
    connect(descendSortAction, SIGNAL(triggered(bool)), this, SLOT(sort_descend()));
    connect(filterAction, SIGNAL(triggered(bool)), this, SLOT(show_filter_dlg()));
    connect(reshowAction, SIGNAL(triggered(bool)), this, SLOT(reshow_data()));
show_contextmenu(const QPoint& pos)
{
    if(cmenu)
    {
        cmenu->exec(QCursor::pos());
    }
}

3）使用Qt::ActionsContextMenu。
把部件的所有action即QWidget::actions()作为context menu显示出来。
还是上面的例子，要在表格（QTableView类型）表头显示右键菜单：
        QAction *ascendSortAction = new QAction("升序", this);
        QAction *descendSortAction = new QAction("降序", this);
        QAction *filterAction = new QAction("过滤", this);
        QAction *unfilterAction = new QAction("取消过滤", this);
    
        connect(ascendSortAction, SIGNAL(triggered(bool)), this, SLOT(sort_ascend()));
        connect(descendSortAction, SIGNAL(triggered(bool)), this, SLOT(sort_descend()));
        connect(filterAction, SIGNAL(triggered(bool)), this, SLOT(filter_table()));
        connect(unfilterAction, SIGNAL(triggered(bool)), this, SLOT(unfilter_table()));
    
        datatable->horizontalHeader()->addAction(ascendSortAction);
        datatable->horizontalHeader()->addAction(descendSortAction);
        datatable->horizontalHeader()->addAction(filterAction);
        datatable->horizontalHeader()->addAction(unfilterAction);
         
        datatable->horizontalHeader()->setContextMenuPolicy(Qt::ActionsContextMenu);

另外两个就是不显示context menu了：
Qt::NoContextMenu
    the widget does not feature a context menu, context menu handling is deferred to the widget's parent.
    
Qt::PreventContextMenu
    the widget does not feature a context menu, and in contrast to NoContextMenu, the handling is not deferred to the widget's parent. This means that all right mouse button events are guaranteed to be delivered to the widget itself through mousePressEvent(), and mouseReleaseEvent().

补充：
    使用Qt::ActionsContextMenu比较简洁，但是如果需要根据当前菜单弹出的位置来定义不同菜单，或者像上个例子，在表格（QTableView类型）表头显示右键菜单时，我需要知道是哪一列表头被点击，从而在后来调用sort_ascend()排序函数时能够根据不同列进行不同排序策略，那么Qt::ActionsContextMenu就做不到了。
    这种需要捕捉弹出位置的情况只好用Qt::ActionsContextMenu了，customContextMenuRequested ( const QPoint & pos )信号返回点击位置pos（在表头视口坐标系中位置），然后表头即可调用logicalIndexAt(pos)函数得到被点击section对应的index即被点击部分的列号，然后存下来可供后面action激活的排序槽使用。
show_contextmenu(const QPoint& pos)
{
    //get related column of headerview
    contextmenu_column = datatable->horizontalHeader()->logicalIndexAt(pos);

    //show contextmenu
    if(cmenu)
    {
        cmenu->exec(QCursor::pos());
    }
}

4）二级菜单实例

准备工作一：定义和实现需要的action，可以用代码编写，也可以用creator进行制作。代码编写如下

头文件定义：

QMenu              *popupMenu;                      /*popupMenu*/  //主菜单
    QMenu              *twoPictures;                    /*two pictures*/  // 二级菜单
    QMenu              *onePictures;                    /*one pictures*/ // 二级菜单
    QAction            *firstChannel;                   /*channel 1*/
    QAction            *secondChannel;                  /*channel 2*/
    QAction            *thirdChannel;                   /*channel 3*/
    QAction            *forthChannel;                   /*channel 4*/
    QAction            *frontSection;
    QAction            *backSection;
    QActionGroup       *channels;     //用来实现子菜单选项互斥
    QActionGroup       *sections;

       cpp文件实现：

void layout3::createActions(void)
{
    firstChannel = new QAction(tr("first channel"), this);  //创建新的菜单项
    firstChannel->setCheckable(true);                            //属性是可选的
    connect(firstChannel, SIGNAL(triggered()), this, SLOT(firstChannelSlot()));       //该菜单项的连接信号和槽

    secondChannel = new QAction(tr("second channel"), this);
    secondChannel->setCheckable(true);
    connect(secondChannel, SIGNAL(triggered()), this, SLOT(secondChannelSlot()));

    thirdChannel = new QAction(tr("third channel"), this);
    thirdChannel->setCheckable(true);
    connect(thirdChannel, SIGNAL(triggered()), this, SLOT(thirdChannelSlot()));

    forthChannel = new QAction(tr("forth channel"), this);
    forthChannel->setCheckable(true);
    connect(forthChannel, SIGNAL(triggered()), this, SLOT(forthChannelSlot()));

    frontSection = new QAction(tr("front section"), this);
    frontSection->setCheckable(true);
    connect(frontSection, SIGNAL(triggered()), this, SLOT(frontSectionSlot()));

    backSection = new QAction(tr("back section"), this);
    backSection->setCheckable(true);
    connect(backSection, SIGNAL(triggered()), this, SLOT(backSectionSlot()));

    channels = new QActionGroup(this);        //创建菜单项组，里面的菜单项为互斥
    channels->addAction(firstChannel);         //添加菜单项到组里
    channels->addAction(secondChannel);
    channels->addAction(thirdChannel);
    channels->addAction(forthChannel);
    firstChannel->setChecked(true);          //设置默认的菜单组的菜单项状态，firstChannel被选中

    sections = new QActionGroup(this);   //同上
    sections->addAction(frontSection);
    sections->addAction(backSection);
    frontSection->setChecked(true);

}

      准备工作二：编写相应的action对应的槽函数

/*
 * 槽函数准备为
 */
void layout3::firstChannelSlot(void)
{
}

void layout3::secondChannelSlot(void)
{
}

void layout3::thirdChannelSlot(void)
{
}

void layout3::forthChannelSlot(void){
}

void layout3::frontSectionSlot(void)
{}

void layout3::backSectionSlot(void)
{}

 最后进行菜单的编辑，重写 void contextMenuEvent ( QContextMenuEvent *event);

void layout3::contextMenuEvent(QContextMenuEvent* e)
{

    popupMenu = new QMenu();  //创建主菜单

    onePictures = popupMenu->addMenu("one pictures");   //在主菜单中创建子菜单one pictures
    onePictures->addAction(firstChannel);               //把action项放入子菜单中
    onePictures->addAction(secondChannel);
    onePictures->addAction(thirdChannel);
    onePictures->addAction(forthChannel);

    popupMenu->addSeparator();
    popupMenu->addSeparator();

    twoPictures = popupMenu->addMenu("two pictures");  //在主菜单中创建子菜单two pictures
    twoPictures->addAction(frontSection);
    twoPictures->addAction(backSection);

    popupMenu->addSeparator();
    popupMenu->addSeparator();

    popupMenu->addAction(ui->actionAbout);              //把action项放入主菜单中
    popupMenu->addSeparator();
    popupMenu->addSeparator();
    popupMenu->addAction(ui->actionBuild);
    popupMenu->addSeparator();
    popupMenu->addSeparator();

    popupMenu->exec(e->globalPos());                    //选择菜单弹出的位置
    delete popupMenu;
    popupMenu = NULL;

}

结果如下：