GetProp/SetProp verses cbWndExtra Bytes

[Fred Harris]

This question is kind of related to Chris Holbrook's current thread about SetProp/GetProp, but I'm wondering what might the advantages be of that technique for associating data with a window, as opposed to storing data (or pointers to data) in the cbWndExtra bytes of the window structure? Typically, I do the later, as for example in my little tid bit I put in Source Code the other day...

http://www.powerbasic.com/support/pbforums/showthread.php?t=36717

The only advantage that comes to my mind is that if one is writting a custom control that others may use, one might want to keep the cbWndExtra bytes free for users?

[Peter Jinks]

Hi Fred,

I say, use whatever works for you. Personally I tend to go with Properties, but that's partly because I do a lot of code in VB and very rarely have control over the extra bytes.

But to answer your question, according to MSDN:

Advantages of Using Window Properties
Window properties are typically used to associate data with a subclassed window or a window in a multiple document interface (MDI) application. In either case, it is not convenient to use the extra bytes specified in the CreateWindow function or class structure for the following two reasons:

• An application might not know how many extra bytes are available or how the space is being used. By using window properties, the application can associate data with a window without accessing the extra bytes.
• An application must access the extra bytes by using offsets. However, window properties are accessed by their string identifiers, not by offsets.

Regards,

Pete.

[Chris Holbrook]

I find the string identifiers somehow comforting. Maybe I wrote too many lines of COBOL. However, given the awkwardness of handling properties and this atom stuff I may revert to cbwndextra.

Of course, with DDT I could also use DIALOG GET/SET USER, but only 8 x 32 bit values

[Kev Peel]

There is no written rule, but in my mind, the class bytes are for custom/superclassed controls and SetProp and SetWindowLong/GWL_USERDATA used by application-specific code for all windows. I make a point not to use these latter methods in custom controls or library code.

In reality, if your data is large you only need one 32-bit value, as it could be used for a pointer to a larger structure.

[Chris Holbrook]

If I was designing DDT features I would be tempted to include a wrapper for EnumProps and the EnumPropProc to filter out inaccessible properties, maybe along the lines of DIR$.

[Chris Boss]

My impression is that the extra window bytes is better and faster than using properties. The extra window bytes are not for users (except for the GWL_USER area). They are for the window class itself. The window class determines how the bytes are used and not users.

I haven't done any benchmarking, but I would guess that the window bytes are faster in access then properties. Properties have to be searched for in some kind of list in Windows. Using string names for properties is going to be a bit slower than using atoms. I prefer to use atoms.

Properties are more user oriented in that you can add properties for any window. The extra window bytes are class defined so you can't just use them for your own user, unless you create the window class yourself (which is what they are intended for).

[Fred Harris]

Thanks for the input Peter, Chris & Kev. Been thinking about that for some time. I tend to settle on one technique and kind of wear it out. That's what I do with extra window bytes. What I'm always doing is making myself little remmed out tables I copy from place to place in my apps to keep track of what I put there, e.g.,

'Offset   What's There
'========================
'0  -  3    ptrHandles
'4  -  7    ptrSizes
'8  -  11   ptrScrollInfo
'etc

I guess if I used window properties I'd be copying around string atom names instead.

I recall studying code generated by Paul Squires FileFly and noted he made major use of properties for window data. I suppose I was concerned there might be some untoward consequences of which I was unaware in my continued use of cbWndExtra bytes.

[Chris Holbrook]

I recall studying code generated by Paul Squires FileFly and noted he made major use of properties for window data.

Having run Peter Jinks' code to solve my original problem it appears that properties may be set by others (Windows? DDT?) as well as the developer. As one wouldn't know who set the properties when removing them in bulk, it would be best to remove them by name. It would be useful to know how Paul Squires does this, also to know whether DDT and or Windows is responsible for the other properties.

[jcfuller]

This is the way I do it using props for control info in rc Dialogs.

In WM_INITDIALOG I call EnumChildWindows

In the call back Function I allocate memory for a UDT that has all
my control info on my own Heap.


IF SetProp(hWnd,"CtlInfo",HeapAlloc(ghHeap,%HEAP_ZERO_MEMORY,LEN(ControlInfoType))) = 0 THEN
FUNCTION = 0
EXIT FUNCTION
END IF

Where ControlInfoType is the UDT of info I want to store

Then

lpCtlInfo = GetProp(hWnd,"CtlInfo")

where lpCtlInfo is a ControlInfoType PTR


Add my Info:

@lpCtlInfo.szClassName = SysAllocStringByteLen(szClassName,LEN(szClassName))

....

In the Dialogs WM_DESTROY I again enumchildWindows to another call back to free the memory.

DIM lpCtlInfo         AS ControlInfoType PTR

lpCtlInfo = GetProp(hWnd,"CtlInfo")

RemoveProp hWnd,"CtlInfo"

    IF @lpCtlInfo.hFont THEN
      IF ISFALSE @lpCtlInfo.StkFont THEN
        DeleteObject @lpCtlInfo.hFont
      END IF
    END IF
    IF @lpCtlInfo.hBkBrush THEN
      DeleteObject @lpCtlInfo.hBkBrush
    END IF
    IF @lpCtlInfo.szTag THEN
      SysFreeString @lpCtlInfo.szTag
    END IF
    IF @lpCtlInfo.szClassName THEN
      SysFreeString  @lpCtlInfo.szClassName
    END IF

   HeapFree ghHeap,0&,lpCtlInfo

James

[Chris Holbrook]

In the call back Function I allocate memory for a UDT that has all my control info on my own Heap.

Well, that was my instinct, too (see my initial post in other thread), to create a list of properties which I could use to remove all properties with a single call, but I thought there might be a neater way with Windows. At least I have all the tools and knowledge available to do it that way, whereas I do not know how to tell if a given property is one created by my app itself or created by WIndows, or maybe DDT, without using a lookup of windows I have created.

[jcfuller]

Well, that was my instinct, too (see my initial post in other thread), to create a list of properties which I could use to remove all properties with a single call, but I thought there might be a neater way with Windows. At least I have all the tools and knowledge available to do it that way, whereas I do not know how to tell if a given property is one created by my app itself or created by WIndows, or maybe DDT, without using a lookup of windows I have created.

No list just one "CtrlInfo" per (control) Window. That prop holds the PTR to my ControlInfo UDT. I doubt anyone is using that name for MY controls.

James

[Chris Holbrook]

No list just one "CtrlInfo" per (control) Window

You're right. It is more compact, and no more trouble than using getprop/setprop, and avoids EnumProps. That's quite a good outcome. Thanks!

[Chris Holbrook]

... developing James's theme but avoiding defining a UDT leads me to a set of functions with an underlying dynamic array or list which can be searched on the identifier string attribute. I'm assuming that all values are DWORDS but the approach could be modified for mixed types.

    ....element of the array or list (or split over three arrays would be easiest)
    Id                AS STRING
    Value             AS DWORD
    Destructor        AS DWORD ' index to destructor method

'--------------------------------------------------------------------------
' Add a property to the list
' retval is zero if an error has occurred, else = dwval
FUNCTION MyPropSet ( hD AS DWORD, id AS STRING, dwval AS DWORD, DestroyAction AS DWORD) AS LONG
END FUNCTION
'--------------------------------------------------------------------------
' get the value of a property from the list
' retval is property value
FUNCTION MyPropGet ( hD AS DWORD, id AS STRING) AS DWORD
END FUNCTION
'--------------------------------------------------------------------------
' returns 1 if the property has been defined, 0 if not
FUNCTION MyPropIs ( hD AS DWORD, id AS STRING) AS LONG
END FUNCTION
'--------------------------------------------------------------------------
' 2nd param is propertyname or "*" for all properties in current dialog
FUNCTION MyPropDestroy ( hD AS DWORD, id AS STRING) AS LONG
END FUNCTION
'--------------------------------------------------------------------------
' a debugging fn to return all property ids and values for a dialog
FUNCTION MyPropS ( hd AS DWORD) AS STRING
END FUNCTION

[Chris Holbrook]

fleshed out a bit and in the Source Code Forum