From https://doc.qt.io/qt-5/model-view-programming.html

Basic concepts

In the model/view architecture, the model provides a standard interface that views and delegates use to access data. In Qt, the standard interface is defined by the QAbstractItemModel class. No matter how the items of data are stored in any underlying data structure, all subclasses of QAbstractItemModel represent the data as a hierarchical structure containing tables of items. Views use this convention to access items of data in the model, but they are not restricted in the way that they present this information to the user.

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Models also notify any attached views about changes to data through the signals and slots mechanism

Model indexes

To ensure that the representation of the data is kept separate from the way it is accessed, the concept of a model index is introduced. Each piece of information that can be obtained via a model is represented by a model index. Views and delegates use these indexes to request items of data to display.

As a result, only the model needs to know how to obtain data, and the type of data managed by the model can be defined fairly generally. Model indexes contain a pointer to the model that created them, and this prevents confusion when working with more than one model.

QAbstractItemModel *model = index.model(); //index is a realization of QModelIdex class

Model indexes provide temporary references to pieces of information, and can be used to retrieve or modify data via the model. Since models may reorganize their internal structures from time to time, model indexes may become invalid, and should not be stored . If a long-term reference to a piece of information is required, a persistent model index must be created. This provides a reference to the information that the model keeps up-to-date. Temporary model indexes are provided by the QModelIndex class, and persistent model indexes are provided by the QPersistentModelIndex class.

To obtain a model index that corresponds to an item of data, three properties must be specified to the model: a row number, a column number, and the model index of a parent item. The following sections describe and explain these properties in detail.

Rows and columns

In its most basic form, a model can be accessed as a simple table in which items are located by their row and column numbers. This does not mean that the underlying pieces of data are stored in an array structure; the use of row and column numbers is only a convention to allow components to communicate with each other. We can retrieve information about any given item by specifying its row and column numbers to the model, and we receive an index that represents the item:

QModelIndex index = model->index(row, column, ...);
 //model is a realization of Model class

Models that provide interfaces to simple, single level data structures like lists and tables do not need any other information to be provided but, as the above code indicates, we need to supply more information when obtaining a model index.

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Parents of items

The table-like interface to item data provided by models is ideal when using data in a table or list view; the row and column number system maps exactly to the way the views display items. However, structures such as tree views require the model to expose a more flexible interface to the items within. As a result, each item can also be the parent of another table of items, in much the same way that a top-level item in a tree view can contain another list of items.

When requesting an index for a model item, we must provide some information about the item's parent. Outside the model, the only way to refer to an item is through a model index, so a parent model index must also be given:

QModelIndex index = model->index(row, column, parent);

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Item roles

Items in a model can perform various roles for other components, allowing different kinds of data to be supplied for different situations. For example, Qt::DisplayRole is used to access a string that can be displayed as text in a view. Typically, items contain data for a number of different roles, and the standard roles are defined by Qt::ItemDataRole.

The role indicates to the model which type of data is being referred to. Views can display the roles in different ways, so it is important to supply appropriate information for each role. The Creating New Models section covers some specific uses of roles in more detail.

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Summary

• Model indexes give views and delegates information about the location of items provided by models in a way that is independent of any underlying data structures.
• Items are referred to by their row and column numbers, and by the model index of their parent items.
• Model indexes are constructed by models at the request of other components, such as views and delegates.
• If a valid model index is specified for the parent item when an index is requested using index(), the index returned refers to an item beneath that parent item in the model. The index obtained refers to a child of that item.
• If an invalid model index is specified for the parent item when an index is requested using index(), the index returned refers to a top-level item in the model.
• The role distinguishes between the different kinds of data associated with an item.

Model can obtain the Index, Index can also obtain the Model

Using model indexes

To demonstrate how data can be retrieved from a model, using model indexes, we set up a QFileSystemModel without a view and display the names of files and directories in a widget. Although this does not show a normal way of using a model, it demonstrates the conventions used by models when dealing with model indexes.

We construct a file system model in the following way:

QFileSystemModel *model = new QFileSystemModel;
QModelIndex parentIndex = model->index(QDir::currentPath());
int numRows = model->rowCount(parentIndex);

In this case, we set up a default QFileSystemModel, obtain a parent index using a specific implementation of index() provided by that model, and we count the number of rows in the model using the rowCount() function.

For simplicity, we are only interested in the items in the first column of the model. We examine each row in turn, obtaining a model index for the first item in each row, and read the data stored for that item in the model.
To obtain a model index, we specify the row number, column number (zero for the first column), and the appropriate model index for the parent of all the items that we want. The text stored in each item is retrieved using the model's data() function. We specify the model index and the DisplayRole to obtain data for the item in the form of a string.

for (int row = 0; row < numRows; ++row) {
        QModelIndex index = model->index(row, 0, parentIndex);
        QString text = model->data(index, Qt::DisplayRole).toString();
        // Display the text in a widget.
    }

The above example demonstrates the basic principles used to retrieve data from a model:

• The dimensions of a model can be found using rowCount() and columnCount(). These functions generally require a parent model index to be specified.
• Model indexes are used to access items in the model. The row, column, and parent model index are needed to specify the item.
• To access top-level items in a model, specify a null model index as the parent index with QModelIndex().
• Items contain data for different roles. To obtain the data for a particular role, both the model index and the role must be supplied to the model.

Further reading

New models can be created by implementing the standard interface provided by QAbstractItemModel. In the Creating New Models section, we demonstrate this by creating a convenient ready-to-use model for holding lists of strings.