QT 格局顶行显示

QT 布局顶行显示
代码如下，感觉没有什么问题，可是label为什么不顶行显示，总是从底部开始呢，请问下为什么？
#include "widget.h"
#include <QHBoxLayout>
#include <QLabel>
#include <QDesktopWidget>
#include <QApplication>
#include <QMessageBox>
#include <QTimer>
#include <QGridLayout>
Widget::Widget(QWidget *parent)
  : QWidget(parent)
{
  setWindowFlags(Qt::FramelessWindowHint); //设置无windows标题

  setFixedSize(160,40);

  QDesktopWidget* desktop = QApplication::desktop();
  move((desktop->width() - this->width())/2, (desktop->height() - this->height())/2);



  timer=new QTimer;
  connect(timer,SIGNAL(timeout()),this,SLOT(closewidget()));
  timer->start(5000);


  grpWidget=new QGroupBox;
  QHBoxLayout *rowGrpWin=new QHBoxLayout;
  rowGrpWin->setContentsMargins(0,0,0,0);
  rowGrpWin->setMargin(0);
  label=new QLabel(QObject::tr("set success!")); //就是想把label显示出来，外面套个groupbox
  rowGrpWin->addWidget(label);
  grpWidget->setLayout(rowGrpWin);



  QGridLayout *rowWin=new QGridLayout;
  rowWin->setContentsMargins(0,0,0,0);
  rowWin->addWidget(grpWidget,0,0);
  setLayout(rowWin);
}

Widget::~Widget()
{
   
}
void Widget::closewidget()
{
  timer->stop();
  close();
}


请教下，为什么label不是顶行显示呢，谢谢了

------解决方案--------------------
void setAlignment ( Qt::AlignTop )