How to get a QTableView to fill 100% of the width?

If you are using Qt 5, QHeaderView::setResizeMode() is no longer available. Instead, you can use QHeaderView::setSectionResizeMode():

ui->tableView->horizontalHeader()->setSectionResizeMode(QHeaderView::Stretch);

Or just call it for every column:

for (int c = 0; c < ui->tableView->horizontalHeader()->count(); ++c)
{
    ui->tableView->horizontalHeader()->setSectionResizeMode(
        c, QHeaderView::Stretch);
}