QModelIndex parentIndex = model->index(0, 0); QModelIndex childIndex = parentIndex.child(1, 0); // Row 1, Column 0This code creates a QModelIndex parentIndex representing the first row and column of a model. Then, it uses the parentIndex.child() method to get the second row and first column of the parent index. The QModelIndex child method is useful for navigating complex data structures represented by QModelIndex objects, such as tree views or table models. The package library in this case is Qt, which provides the QModelIndex class and other classes for developing UI applications in C++.