Often there are situations when you need to get data having only the address (for example, in WndProc, HookProc). Usually, simply copy the data via CopyMemory the structure after changing data and copy it back. If the structure is large, it will be a waste of resources to copy into structure and back. In languages such as C ++ is all done easily with the help of pointers, written something like newpos = (WINDOWPOS *) lparam. Nominally VB6 does not work with pointers, but there are a few workarounds.
PublicDeclareFunction GetMem4 Lib "msvbvm60" (src As Any, Dst As Any) AsLong
PublicDeclareFunction ArrPtr Lib "msvbvm60" Alias "VarPtr" (src() As Any) AsLong
For a start I will give the necessary declarations:
Code:
PrivateType Vector
X AsSingle
Y AsSingle
EndType
PrivateType TestRec
Name AsString
Value AsLong
Position As Vector
Money AsDouble
EndType
PrivateSub Form_Load()
Dim tr As TestRec
Test tr
EndSub
PrivateFunction Test(Pointer As TestRec, OptionalByVal nu AsLong)
Dim q As TestRec, z As TestRec
q.Name = "The trick"
q.Position.X = 5: q.Position.Y = 15
q.Value = 12345: q.Money = 3.14
z.Name = "Visual Basic 6.0"
z.Position.X = 99: z.Position.Y = 105
z.Value = 7643: z.Money = 36.6
GetMem4 VarPtr(q), ByVal VarPtr(nu) - 4 ' Set pointer to q (Pointer = &q)
PrintRec Pointer
GetMem4 VarPtr(z), ByVal VarPtr(nu) - 4 ' Set pointer to z (Pointer = &z)
PrintRec Pointer
EndFunction
PrivateSub PrintRec(Pt As TestRec)
Debug.Print"----------------"
Debug.Print"Name = "& Pt.Name
Debug.Print"Value = "& Pt.Value
Debug.Print"Money = "& Pt.Money
Debug.Print"Position.X = "& Pt.Position.X
Debug.Print"Position.Y = "& Pt.Position.Y
EndSub
You can also create a pointer by using arrays. The idea is to create 2 arrays one element each, which will store the address of a variable, and the other will refer to the data. The first will always be Long, a second type of data desired. This is useful for example if you want to pass on lists, etc. It's no secret that the array in VB is simply an SafeArray. In the data structure of this array contains a lot of useful information, and a pointer to the data. What we do, we create two arrays:
- 1st (with address) refers to a pointer to the second data array. As a result, changing the values in the first array, 2nd automatically refer to the desired data.*
- 2nd is directly the data pointed to by the first.*
Also, after all the manipulations necessary to return all the pointers back to VB properly clear the memory.* For all manipulations I created auxiliary functions and structure for data recovery.* Address SafeArray is available through Not Not Arr, but IDE after such manipulations are glitches with floating point:
Code:
PublicType PtDatExample of use:
Prv1 AsLong
Prv2 AsLong
EndType
' Create the pointer. 1st param is pointer, 2nd address.
PublicFunction PtGet(Pointer() AsLong, ByVal VarAddr AsLong) As PtDat
Dim i AsLong
i = GetSA(ArrPtr(Pointer)) + &HC
GetMem4 ByVal i, PtGet.Prv1
GetMem4 VarAddr + &HC, ByVal i
PtGet.Prv2 = Pointer(0)
EndFunction
' Release pointer
PublicSub PtRelease(Pointer() AsLong, prev As PtDat)
Pointer(0) = prev.Prv2
GetMem4 prev.Prv1, ByVal GetSA(ArrPtr(Pointer)) + &HC
EndSub
' Obtaint address of SafeArray (same Not Not)
PublicFunction GetSA(ByVal addr AsLong) AsLong
GetMem4 ByVal addr, GetSA
EndFunction
Code:
PrivateSub Form_Load()
Dim pt() AsLong, var() As TestRec, prev As PtDat ' Pointer, references data, release data.
Dim q As TestRec, z As TestRec ' The structures, which we refer
ReDim pt(0): ReDim var(0)
q.Name = "The trick"
q.Position.X = 5: q.Position.Y = 15
q.Value = 12345: q.Money = 3.14
z.Name = "Visual Basic 6.0"
z.Position.X = 99: z.Position.Y = 105
z.Value = 7643: z.Money = 36.6
prev = PtGet(pt, GetSA(ArrPtr(var))) ' Create "pointer"
pt(0) = VarPtr(q) ' Refer to q (pt = &q)
PrintRec var(0)
pt(0) = VarPtr(z) ' Refer to z (pt = &z)
PrintRec var(0)
PtRelease pt, prev ' Release
EndSub
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