如何将项目添加到QT中的布局中

我正在开发QT 4.8.4中的应用程序，其中我进行以下操作：

i子类qgridlayout如下：

class Viewer : public QGridLayout
{
    Q_OBJECT
    ...................
private:
    // Objects
    /// Maximize button object
    ViewerGeneric* viewerGeneric;
    /// Maximize button object
    QPushButton* btnMaximize;
    /// Close button object
    QPushButton* btnClose;
    /// Connect button object
    QPushButton* btnConnect;
    /// Central viewer layout object
    QGridLayout* viewer;
    /// Indicates the row position in the main grid
    unsigned int row;
    /// Indicates the column position in the main grid
    unsigned int col;
};

然后在构造函数中我做这样的事情：

// Create the objects
btnMaximize = new QPushButton("max");
btnClose = new QPushButton("close");
btnConnect = new QPushButton("connect");
// Add the horizontal toolbar
QHBoxLayout* toolbar = new QHBoxLayout();
toolbar->setSizeConstraint(QLayout::SetMinimumSize);
toolbar->addItem(new QSpacerItem(0, 0, 
                     QSizePolicy::Expanding, QSizePolicy::Minimum));
toolbar->addWidget(btnMaximize);
toolbar->addWidget(btnClose);
// Add the 'Connect' button
viewer = new QGridLayout();
viewer->addWidget(btnConnect);
// Add the widgets
this->addItem(toolbar, 0, 0);
this->addItem(viewer, 0, 0, 2);

但是，最后，当我在主窗口中显示查看器类时，窗口为完全空白！希望任何人都能帮助我。谢谢。

欢呼，