Qt在mainwindow下代码添加控件不能显示的问题

以前在dialog或者widget下代码添加控件能显示，但是今天发现在mainwindow下居然不能显示，后来查了资料才知道，需要先定义一个widget，具体代码如下：

    QWidget *center = new QWidget(this);

    setCentralWidget(center);

    penColorComboBox = new QComboBox;

    createColorComboBox(penColorComboBox);

    penColorLabel = new QLabel(tr("pen color"));

    penColorLabel->setBuddy(penColorComboBox);

    penWidthspinbox = new QSpinBox;

    penWidthspinbox->setRange(0,20);

    penWidthspinbox->setSpecialValueText(tr("0 (cosmetic pen)"));

    penWidthLabel = new QLabel(tr("pen &width"));

    penWidthLabel->setBuddy(penWidthspinbox);

    createStyleComboBox();

    penStyleLabel = new QLabel(tr("&pen style"));

    penStyleLabel->setBuddy(penStyleComboBox);

    brushColorComboBox = new QComboBox();

    createColorComboBox(brushColorComboBox);

    brushColorLabel = new QLabel(tr("Brush Color"));

    brushColorLabel->setBuddy(brushColorComboBox);

    brushStyleComboBox = new QComboBox;

    brushStyleComboBox->addItem(tr("None"),Qt::NoBrush);

    brushStyleLabel = new QLabel(tr("&Brush Style"));

    brushStyleLabel->setBuddy(brushStyleComboBox);

    preLabel = new QLabel(this);

    connect(penColorComboBox,SIGNAL(currentIndexChanged(int)),this,SLOT(penchange()));

    connect(penWidthspinbox,SIGNAL(valueChanged(int)),this,SLOT(brushchange()));

    connect(penStyleComboBox,SIGNAL(currentIndexChanged(int)),this,SLOT(penchange()));

    connect(brushColorComboBox,SIGNAL(currentIndexChanged(int)),this,SLOT(brushchange()));

    connect(brushStyleComboBox,SIGNAL(currentIndexChanged(int)),this,SLOT(brushchange()));

    //connect(this,SIGNAL(penchange(QPen&)),;

    QGridLayout *mainlayout=new QGridLayout(center);

    mainlayout->addWidget(penColorLabel,0,0,Qt::AlignRight);

    mainlayout->addWidget(penColorComboBox,0,1);

    mainlayout->addWidget(penWidthLabel,1,0,Qt::AlignRight);

    mainlayout->addWidget(penWidthspinbox,1,1);

    mainlayout->addWidget(penStyleLabel,2,0,Qt::AlignRight);

    mainlayout->addWidget(penStyleComboBox,2,1);

    mainlayout->addWidget(brushColorLabel,3,0,Qt::AlignRight);

    mainlayout->addWidget(brushColorComboBox,3,1);

    mainlayout->addWidget(brushStyleLabel,4,0,Qt::AlignRight);

    mainlayout->addWidget(brushStyleComboBox,4,1);

    mainlayout->addWidget(preLabel,5,0,6,2);

    setLayout(mainlayout);