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Can anybody please help me to know, what is the use of CListCtrl::GetItemData() in MFC, VC++ ?

I went through MSDN description but it was not clear enough.

If anybody can provide a brief explanation and some example it will be really great and would help a lot.

John Conde
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user3587879
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3 Answers3

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The item data allows you to associate any data you want to each item of your list control. This data is most of the time a pointer to some data structure.

For example in an email application you have a list control containing all received emails. Each item contains for example the sender, the subject and the date. The item data would contain a pointer to some data structure which contains the whole mail text amoung other information.

Jabberwocky
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The method GetItemData() is used in association with the method SetItemData() to allow a CListCtrl or a CTreeCtrl to have associated with items or nodes in the control some data.

The idea is that when populating the list or tree with nodes you can also assign a data item to the node using SetItemData() which can be retrieved by using GetItemData() when processing a selection event. The associated data may be something like a pointer to an object or an identifier of some sort which is the key to data stored in a collection such as a std::map.

The data type for the value retrieved with GetItemData() or assigned to an item with SetItemData() is a DWORD_PTR (see DWORD and DWORD_PTR on 64 bit machine for a discussion on DWORD_PTR).

Another coincidence is that the SendMessage() and PostMessage() Windows API function have the LPARAM parameter, e.g. SendMessage(HWND hWnd, UINT uiMsg, WPARAM wParam, LPARAM lParam)which is the same size as a LONG_PTR which is a long on x32 compile or __int64 on x64 compiles (see Microsoft Docs - Windows Data Types). And a DWORD_PTR is the same as a ULONG_PTR which just so happens to be a unsigned long on x32 compile or unsigned __int64 on x64 compiles.

This means that the DWORD_PTR value from GetItemDataa() can be used as the LPARAM value in a SendMessage() or PostMessage(). So you could use the SetItemData() method to associate a LPARAM type of identifier that the handler for a selection event for a CListCtrl or CTreeCtrl can then send to some other window or thread whose handle is known to communicate the selection event to some other part of the application.

Richard Chambers
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Solution for beginners

For example, you have an int / string /struct or something else

struct foo
{
   ...
};

foo myData;    

list.SetItemData(nItem, reinterpret_cast<DWORD_PTR>(&myData));   
...  
myData = *(reinterpret_cast<foo*>(list.GetItemData(nItem)));

Note that SetItemData accepts a reference to an object, the object must be global,otherwise it will be destroyed

nikki
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