This class defines a generic tree interface for use by the Gtk::TreeView widget.
It is is designed to be usable with any appropriate data structure. The programmer just has to implement this interface on their own data type for it to be viewable by a Gtk::TreeView widget.
The model is represented as a hierarchical tree of strongly-typed, columned data. In other words, the model can be seen as a tree where every node has different values depending on which column is being queried. The type of data found in a column is determined by TreeModel::Column<> templates. The types are homogeneous per column across all nodes. It is important to note that this interface only provides a way of examining a model and observing changes. The implementation of each individual model decides how and if changes are made.
In order to make life simpler for programmers who do not need to write their own specialized model, two generic models are provided - the Gtk::TreeStore and the Gtk::ListStore. To use these, the developer simply pushes data into these models as necessary. These models provide the data structure as well as all appropriate tree interfaces. As a result, implementing drag and drop, sorting, and storing data is trivial. For the vast majority of trees and lists, these two models are sufficient.
Models are accessed on a node/column level of granularity. One can query for the value of a model at a certain node and a certain column on that node. There are two structures used to reference a particular node in a model. They are the Path and the iterator. Most of the interface consists of operations on a iterator.
A path is essentially a potential node. It is a location on a model that may or may not actually correspond to a node on a specific model. A Path can be converted into either an array of unsigned integers or a string. The string form is a list of numbers separated by a colon. Each number refers to the offset at that level. Thus, the path "0" refers to the root node and the path "2:4" refers to the fifth child of the third node.
By contrast, a iterator is a reference to a specific node on a specific model. It is a generic structure with an integer and three generic pointers. These are filled in by the model in a model-specific way. One can convert a path to an iterator by calling Gtk::TreeModel::get_iter(). These iterators are the primary way of accessing a model and are similar to the iterators used by Gtk::TextBuffer. The model interface defines a set of operations using them for navigating the model.
The lifecycle of an iterator can be a little confusing at first. Iterators are expected to always be valid for as long as the model is unchanged (and doesn't emit a signal). The model is considered to own all outstanding iterators and nothing needs to be done to free them from the user's point of view. Additionally, some models guarantee that an iterator is valid for as long as the node it refers to is valid (most notably the Gtk::TreeStore and Gtk::ListStore). Although generally uninteresting, as one always has to allow for the case where iterators do not persist beyond a signal, some very important performance enhancements were made in the sort model. As a result, the GTK_TREE_MODEL_ITERS_PERSIST flag was added to indicate this behavior.