Question

If I am to inherit from a class, would I have to define all of its virtual and pure virtual functions?

For example, I have a derived class that is inheriting from QAbstractItemModel. QAbstractItemModel has the following pure virtual functions. If my derived class is not going to use the index() and parent() method, would I need to implement it?

//qabstractitemmodel.h
virtual QModelIndex index(int row, int column,
                              const QModelIndex &parent = QModelIndex()) const = 0;
virtual QModelIndex parent(const QModelIndex &child) const = 0;
virtual int rowCount(const QModelIndex &parent = QModelIndex()) const = 0;
virtual int columnCount(const QModelIndex &parent = QModelIndex()) const = 0;
virtual QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const = 0;

Answer 1

Yes. Declaring a method as pure virtual (‘ = 0’) means that any concrete subclass (which can be instantiated) has to implement them.