setColumnWidth doesn't work for me. Here is my code:

treeExample::treeExample(QWidget *parent)
: QWidget(parent)
{
ui.setupUi(this);

QStandardItem *item;
model = new QStandardItemModel(0,3);

model->setHeaderData(0,Qt::Horizontal,QObject::tr("Date" ));
model->setHeaderData(1,Qt::Horizontal,QObject::tr("Time" ));
model->setHeaderData(2,Qt::Horizontal,QObject::tr("Proc" ));

item = new QStandardItem("First");
model->setItem(0, 0, item);

item = new QStandardItem("Sec");
model->setItem(0, 1, item);

item = new QStandardItem("Thi");
model->setItem(0, 2, item);

QTreeView *tree = ui.treeView;

tree->setModel(model);
tree->setColumnWidth(0,100);
tree->setColumnWidth(1,20);

tree->show();

}


What is wrong?
Thanks