TVITEM LPARAM to store string

Question

I've got a treeview listing files that are dropped upon it.

When I make a new treeview item, I'd like to store the address of the file as a string in that item, and retrieve it for various nefarious purposes at a later point in time.

Looking at the TVITEM structure in Microsoft docs, apparently LPARAM is the place to store a value:

lParam

Type: LPARAM

A value to associate with the item.

So, I have gone ahead and done that:

TVITEM tvi;
tvi.mask = TVIF_TEXT;
tvi.pszText = const_cast<char *> (str0.c_str());
tvi.cchTextMax = sizeof(tvi.pszText);
tvi.lParam = (LPARAM) foo;  // SETTING LPARAM HERE, foo IS A const char * 

TVINSERTSTRUCT tvis;
tvis.item = tvi;
tvis.hInsertAfter = 0;
tvis.hParent = hti0;

// Send message to update tree, and return tree item.
return TreeView_InsertItem(tvw_filelist_, &tvis);

Then, when I try to retrieve my value...

HTREEITEM htiSel = TreeView_GetSelection(tvw_filelist_);

TVITEM tvItem;
tvItem.hItem = htiSel;

TreeView_GetItem(tvw_filelist_, &tvItem);
const char * info = (const char *) tvItem.lParam;
MessageBox(NULL, info, "Alert", MB_OK);

...I just get garbage, indicating my pointer went out of scope or is taking a nap or something. The size of that pointer is always 4.

Is this the right way to do what I'm trying to do? If so, what's going on?

Well, it depends on what foo is. Your code seems a little odd - if foo is a pointer to some text, then you're storing the address of that pointer, as opposed to the address of the data it points to. (the pointer's value) — enhzflep, Nov 08 '12 at 08:34
Yeah, that was fluctuating around in my various experiments...thanks and sorry. Editing code now. Turns out that wasn't the main problem :) — Ben, Nov 08 '12 at 08:35

score 2 · Accepted Answer · edited May 23 '17 at 10:33

2

Of course, take the time to post a question after a long time trying to figure it out, and the answer shows up in seconds.

Turns out the TVITEM mask needs to include TVIF_PARAM, similar to this question.

If I change the above code to:

tvi.mask = TVIF_TEXT | TVIF_PARAM;

it works as expected.

~~I'm still not sure if this is the recommended use for LPARAM, though.~~

edited May 23 '17 at 10:33

Community

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answered Nov 08 '12 at 08:27

Ben

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Yeah, you an use it for anything you like that fits into 32 bits. If it's 4 chars or an int - great, just put em in. If it's anything larger, just store a pointer to the data there. LParam is for anything you please. I've used it in the past to hold structs that represent the material used to render a 3d ball, displayed as the image for each item in the list. This then allows me to use a custom sort function to sort on Hue, Sat or Lum - or of course anything else I like. :) glad you got it sorted. – enhzflep Nov 08 '12 at 08:43
I think you should "wrap" you text string into the structure, and store a pointer to it in `LPARAM` - instead of string itself. This can be useful in future then you decide to expand your custom data with some new params. – eraxillan Nov 08 '12 at 08:49
@Axilles - you can do that?!!!! Something to try out tomorrow (already 8 hours today, eyes bleeding). – Ben Nov 08 '12 at 08:51
@Steve Sure. See my answer below. – eraxillan Nov 08 '12 at 08:55
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@enhzflep: *"you [c]an use [`lParam`] for anything you like that fits into 32 bits"* - The `LPARAM` data type is pointer-sized, i.e. 32 bits on a 32-bit system, 64 bits on a 64-bit system. This is an important detail. As written, your comment suggests, that you could store a pointer into *lParam* on 32-bit systems only. – IInspectable Aug 25 '16 at 11:41
@IInspectable - thanks for the correction for everyone else's sake. I'd long since forgotten giving that piece of unclear/misleading advice. – enhzflep Aug 26 '16 at 13:02

score 2 · Answer 2 · answered Nov 08 '12 at 08:59

2

struct CustomTreeData
{
    LPSTR str;   // or even std::string to forget about memory managment
    // TODO: any other data you need
};
...
TVITEM tvi;
tvi.mask = TVIF_TEXT | TVIF_PARAM;

CustomTreeData* myDataPtr = new CustomTreeData;   // the memory should be free later
myDataPtr->str = stringWhatIWant;   // And don't forget to alloc memory for str!
tvi.lParam = (LPARAM) myDataPtr;

I don't check this code, but it should work. Happy coding :)

answered Nov 08 '12 at 08:59

eraxillan

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Sorry to accept my own answer when you were so helpful, but my answer answers the original question :) Thanks though +1 – Ben Nov 11 '12 at 05:30

TVITEM LPARAM to store string

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