QAbstractListModel.match() causes ASSERT failure in QList<T>::operator[]: "index out of range"

Question

I'm searching for the Index of an item(if it's existent in the model) with QAbstractListModel.match().

QModelIndex childIndex = m_DataSourceModel.match(m_DataSourceModel.index(0,0),Qt::UserRole,QVariant::fromValue(messageID),1,Qt::MatchRecursive)[0];

When the item is not found, this error occures:

ASSERT failure in QList<T>::operator[]: "index out of range", file C:/Qt/5.10.0/mingw53_32/include/QtCore/qlist.h, line 549

The manual says: "The list that is returned may be empty." and afterwards the QModelIndex should be checked with QModelIndex.isValid()

So why is the program crashing when nothing matches before I can check the index?

Answer 1

As indicated by the docs match, you can return an empty list so before accessing you must verify that you have at least the necessary number of elements:

QModelIndexList indexes = m_DataSourceModel.match(m_DataSourceModel.index(0, 0),
                                                  Qt::UserRole, 
                                                  QVariant::fromValue(messageID),
                                                  1, 
                                                  Qt::MatchRecursive);
if(!indexes.empty()){ 
    QModelIndex childIndex = indexes.first();
    // or QModelIndex childIndex = indexes[0];
}