Q: How to use 'GetItemData()' and 'SetItemData()'?
A: A tree control is just a visual representation of some hierarchical data structure. You use 'SetItemData()' and 'GetItemData()' to link each tree item to a node if this data structure.
Firstly you have to define a class or a structure that holds the data for each node. For example if your tree is supposed to show a file system, the structure you define will reflect all the properties of a file, like name, size, timestamp, whether it is a directory or not, access rights and so on. The tree will display only the name. Each of the trees items will be linked to an instance of such a structure.
'SetItemData()' allows you to attach a 'DWORD' to each item and 'GetItemData()' allows you to get that 'DWORD' back. A plain 'DWORD' isn't of much use, but luckily under Windows a 'DWORD' and a pointer have the same size, so you can cast forth and back between them.
When you add an item to the tree, you attach a pointer to a 'node_data' structure to that item:
When you handle an operation on some item of the tree (for example selection) you retrieve the node using 'GetItemData()':
HTREEITEM hItem = m_tree.InsertItem(/*...*/);
node_data *node = new node_data();
// Fill up the new node
node->member = value; //...
// Attach the node to the item
m_tree.SetItemData(hItem, (DWORD) node);
// 'hItem' is a valid iten handle
node_data *node = (node_data *) m_tree.GetItemData(hItem);
// Retrieve, set or take decisions according to the nodes
if(node->member == some_value)
value = node->member;
node->member = value;