Is this safe to use?

[mark smit]

Hello,


Is it safe to use these to store a pointer?


Pointer = GetWindowLong(hWnd, %GWL_USERDATA)
SetWindowLong(hWnd, %GWL_USERDATA, Pointer)

Just wondering...


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Cheers

[Semen Matusovski]

mark --
ddt uses %gwl_userdata.
see, for example,


[this message has been edited by semen matusovski (edited october 31, 2000).]

[Tom Hanlin]

Sure, with the usual caveats. There is no point in storing a pointer
to a data item that may move or disappear, for example.

...eh, provided you're not using DDT, I guess. Good catch!

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Tom Hanlin
PowerBASIC Staff

[Steve Hutchesson]

Mark,

======================================

MSDN
~~~~
GWL_USERDATA
Retrieves the 32-bit value associated with the window. Each window has a
corresponding 32-bit value intended for use by the application that
created the window.

======================================

This says you can use it for whatever you like but the normal
recommendation from MSDN is as follows.

======================================

Remarks
Reserve extra window memory by specifying a nonzero value in the
cbWndExtra member of the WNDCLASSEX structure used with the
RegisterClassEx function.

======================================

There is no problems using pointers in the 40 bytes of memory that you can
allocate in the extra window memory, probably the pick of the crop in memory
allocation functions is the older GlobalAlloc() as it handles small amounts
of memory well.

The result of either VarPtr() or StrPtr() is a 32 bit variable so it will fit
into any of the 4 byte long allocations in the extra memory fine.

Regards,

[email protected]

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[mark smit]

So let me get this straight, as long as I DONT use DDT then it is
completely safe to use this userdata area. I am writing a custom
control that uses a structure/type to store instance related data
about the control. I dont use any DDT so this should be ok then?



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Cheers

[Steve Hutchesson]

Mark,

If you create the window using the CreateWindowEx() function, then
you should be able to use this value if you want to.

Regards,

[email protected]

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[Edwin Knoppert]

Hopefully DDT will NOT use this UserData in the next release.
It should be used by the programmer, not the engine.

Sorry.

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[email protected]

[Steve Hutchesson]

Mark,

Something I should have remembered is an old trick in custom controls
that do not use a title bar if you need additional memory on an instance
basis, the memory allocated to a window for text is usually not used so
you can use that as well if you are running out of room with the extra
memory allocated in the WNDCLASSEX structure.

It is a bit more messing around to access it as it was not directly
designed to be used in this manner but it gives you an extra byte sequence
to use that can be loaded any way you like.

Regards,

[email protected]

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