Qt程序的自定义对话框不能显示解决思路

#include<QtGui>class MyDlg : public QDialog{public:        MyDlg(const QString &);};MyDlg::MyDlg(const QString &a){        QLabel *text=new QLabel(a);        QPushButton *okbutton=new QPushButton(tr("Ok"));        QPushButton *cancelbutton=new QPushButton(tr("Cancel"));        QHBoxLayout *buttonlayout=new QHBoxLayout;        buttonlayout->addWidget(okbutton);        buttonlayout->addWidget(cancelbutton);        QVBoxLayout *vboxlayout=new QVBoxLayout;        vboxlayout->addWidget(text);        vboxlayout->addLayout(buttonlayout);        setLayout(vboxlayout);}int main(int argc,char *argv[]){        QApplication app(argc,argv);        bool ok;        QFont font = QFontDialog::getFont(&ok, QFont("Times", 12));        if (ok)        {                MyDlg choose(QObject::tr("You have chosen a font"));                choose.show();        }        else        {                MyDlg choose(QObject::tr("default font"));                choose.show();        }        return app.exec();}