Error in custom delegate

I copied qt designers' SheetDelegate class with some modification to show sheets like what's in designers' widgetbox. But the compiler says "StencilView.cpp:21: undefined reference to `SheetDelegate::SheetDelegate(QObject*)' ".

Modified SheetDelegate.h

Code:

#ifndef SHEET_DELEGATE_H
#define SHEET_DELEGATE_H
 
#include <QStyledItemDelegate>
 
class SheetDelegate: public QStyledItemDelegate
{
    Q_OBJECT
public:
    SheetDelegate(QObject *parent = 0);
 
    virtual void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const;
    virtual QSize sizeHint(const QStyleOptionViewItem &opt, const QModelIndex &index) const;
};
 
#endif // SHEET_DELEGATE_H

Modified SheetDelegate.cpp

Code:

#include <QtGui/QStyle>
#include <QtGui/QPainter>
 
 
SheetDelegate::SheetDelegate(QObject *parent)
    : QStyledItemDelegate(parent)
{
}
 
void SheetDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    if (index.model()->parent(index).isValid()) {
        // this is a top-level item.
        QStyleOptionButton buttonOption;
 
        buttonOption.state = option.state;
#ifdef Q_WS_MAC
        buttonOption.state |= QStyle::State_Raised;
#endif
        buttonOption.state &= ~QStyle::State_HasFocus;
 
        buttonOption.rect = option.rect;
        buttonOption.palette = option.palette;
        buttonOption.features = QStyleOptionButton::None;
        QStyle::drawControl(QStyle::CE_PushButton, &buttonOption, painter, parent);
 
        QStyleOption branchOption;
        static const int i = 9; // ### hardcoded in qcommonstyle.cpp
        QRect r = option.rect;
        branchOption.rect = QRect(r.left() + i/2, r.top() + (r.height() - i)/2, i, i);
        branchOption.palette = option.palette;
        branchOption.state = QStyle::State_Children;
 
        if (parent->isExpanded(index))
            branchOption.state |= QStyle::State_Open;
 
        QStyle::drawPrimitive(QStyle::PE_IndicatorBranch, &branchOption, painter, parent);
 
        // draw text
        QRect textrect = QRect(r.left() + i*2, r.top(), r.width() - ((5*i)/2), r.height());
        QString text = elidedText(option.fontMetrics, textrect.width(), Qt::ElideMiddle, 
            index.model()->data(index, Qt::DisplayRole).toString());
        QStyle::drawItemText(painter, textrect, Qt::AlignCenter,
            option.palette, parent->isEnabled(), text);
 
    } else {
        QStyledItemDelegate::paint(painter, option, index);
    }
}
 
QSize SheetDelegate::sizeHint(const QStyleOptionViewItem &opt, const QModelIndex &index) const
{
    QStyleOptionViewItem option = opt;
    QSize sz = QStyledItemDelegate::sizeHint(opt, index) + QSize(2, 2);
    return sz;
}

The StencilView class is defined as a QWidget class where shows the result.

StencilView.h

Code:

#ifndef STENCILVIEW_H
#define STENCILVIEW_H
 
#include "SheetDelegate.h"
 
#include <QWidget>
 
class StencilSelector;
class QFileSystemModel;
class QSortFilterProxyModel;
class QTreeView;
class QLineEdit;
class QPushButton;
class SheetDelegate;
class StencilView : public QWidget
{
    Q_OBJECT
 
public:
    StencilView(StencilSelector *parent);
 
private slots:
    void reapplyFilter();
 
private:
    QTreeView *treeView;
    SheetDelegate *delegate;
    QFileSystemModel *sourceModel;
    QSortFilterProxyModel *proxyModel;
    QLineEdit *filterLineEdit;
    QPushButton *moreButton;
};
 
#endif

StencilView.cpp

Code:

#include <QtGui>
 
#include "StencilView.h"
#include "StencilSelector.h"
 
StencilView::StencilView(StencilSelector *parent): QWidget(parent)
{
    sourceModel = new QFileSystemModel;
    sourceModel->setReadOnly(true);
    sourceModel->setFilter(QDir::AllEntries);
    //sourceModel->setNameFilters(QStringList::QStringList("*.odg"));
    sourceModel->setNameFilterDisables(false);
    sourceModel->setRootPath("/home/yue/stencils_test");
    //QModelIndex index = sourceModel->rootPath();
 
    proxyModel = new QSortFilterProxyModel;
    proxyModel->setSourceModel(sourceModel);
    proxyModel->setFilterKeyColumn(-1);
    proxyModel->setDynamicSortFilter(true);
 
    delegate = new SheetDelegate(this);
    treeView = new QTreeView;
    treeView->setModel(proxyModel);
    treeView->setItemDelegate(delegate);
    /*treeView->header()->setStretchLastSection(true);
    treeView->header()->setSortIndicator(0, Qt::AscendingOrder);
    treeView->header()->setSortIndicatorShown(true);
    treeView->header()->setClickable(true);
    */
 
    //treeView->setRootIndex(proxyModel->mapFromSource(index));
 
    filterLineEdit = new QLineEdit;
 
    connect(filterLineEdit, SIGNAL(textChanged(const QString &)),
            this, SLOT(reapplyFilter()));
 
    moreButton = new QPushButton("...");
    QVBoxLayout *mainLayout = new QVBoxLayout;
    QHBoxLayout *interLayout = new QHBoxLayout;
 
    interLayout->addWidget(moreButton);
    interLayout->addWidget(filterLineEdit);
    mainLayout->addLayout(interLayout);
    mainLayout->addWidget(treeView);
    setLayout(mainLayout);
    //filterLineEdit->setText(tr("<Filter>"));
    //setWindowTitle(tr("Stencil View"));
}
 
void StencilView::reapplyFilter()
{
    QRegExp regExp(filterLineEdit->text(), Qt::CaseInsensitive, QRegExp::RegExp);
    proxyModel->setFilterRegExp(regExp);
}
 
#include <StencilView.moc>

Could anyone point out what's wrong there?