郵箱：yoshiro_gl@21cn.com
發現大多數人對VB中應用指針不太了解，作一些說明。
VB的指針挺簡單的，用著也很方便，其實對象變量就可以看成是指針，當你用Set A=Obj時，A就是指向Obj的地址。不用API就可以，當然用API可以實現更為高級的結構。
給一個例子，一個用VB實現的雙向循環鏈表。有鏈表的生成，刪除和結點的插入。
先定義一個結點類，類名為Node,代碼為:
Option Explicit
Public pNext As Node
Public pPrev As Node
Public data As Single
Private Sub Class_Initialize()
Set pNext = Nothing
Set pPrev = Nothing
End Sub
Private Sub Class_Terminate()
Set pNext = Nothing
Set pPrev = Nothing
End Sub
再添加一個窗體，窗體上添加兩個列表框，list1和list2，窗體的代碼為：
Option Explicit
Private pHead As Object
Private pV As Object
Private Sub Form_Load()
Dim i As Integer
Set pHead = New Node
Call CreateLinkList
Call InsertNode(pHead, 503)
Call InsertNode(pHead, 1.875)
Call InsertNode(pHead, -3.675)
For i = 1 To 100
Call InsertNode(pHead, -1 * i)
Next
Call PrintList
Call DeleteList
End Sub
Public Sub CreateLinkList()
Dim p As Node
Dim nLoop As Integer
Static pLast As Node
pHead.data = 0
Set pLast = pHead
For nLoop = 1 To 501
Set p = New Node
p.data = nLoop
Set pLast.pNext = p
Set p.pPrev = pLast
Set pLast = p
Next
Set pLast = Nothing
Set p.pNext = pHead
Set pHead.pPrev = p
Exit Sub
End Sub
Public Sub PrintList()
List1.AddItem "Forwards"
Set pV = pHead
Do
List1.AddItem pV.data
Set pV = pV.pNext
Loop While Not pV Is pHead
List2.AddItem "Backwards"
Set pV = pHead.pPrev
Do
List2.AddItem pV.data
Set pV = pV.pPrev
Loop While Not pV Is pHead.pPrev
End Sub
Public Sub DeleteList()
Dim p As Node
Set pV = pHead
Do
Set pV = pV.pNext
Set p = pV.pPrev
If Not p Is Nothing Then
Set p.pNext = Nothing
Set p.pPrev = Nothing
End If
Set p = Nothing
Loop While Not pV.pNext Is Nothing
Set pV = Nothing
Set pHead = Nothing
End Sub
Public Sub InsertNode(head As Node, data As Single)
Dim p As New Node, q As Node, prev As Node
p.data = data
Set q = head
Set prev = head.pPrev
While ((q.data < p.data) And Not q.pNext Is head)
Set q = q.pNext
Set prev = prev.pNext
Wend
If Not q.pNext Is head Then
Set p.pNext = q
Set p.pPrev = prev
Set prev.pNext = p
Set q.pPrev = p
If q Is head Then
Set head = p
End If
Else
Set p.pNext = head
Set p.pPrev = q
Set head.pPrev = p
Set q.pNext = p
End If
End Sub
一個雙向循環鏈表就形成了，List1中是正向遍歷的結果，List2中是反向遍歷的結果。類的構造器Class_Initialize()過程，類的析構Class_Termainate()過程，結點內存的分配和回收都由類自身完成，還有多態，pHead As Object；Set pHead = New Node；Set pHead.pPrev = p；指向基類的指針指向了子類，并調用了子類的屬性，是不是挺像C++的代碼？
鏈表有了，二叉樹，由臨接表構成的圖等數據結構都很容易實現了吧，實際上用VB能構造很復雜的數據結構，上面的代碼只是簡單的示例，實際可以做的更完善。
另外，VB6也能夠生成真實的地址。三種未正式公布的VBA方法VarPtr,ObjPtr,和StrPtr（實際上是指向運行DLL同一入口的三個不同的類型庫別名）就可以用來建立指針，使用address=ObjPtr(Obj)就可以獲得對象的地址，Obj為需要地址的對象，而Address為一個long型變量，其中放置了對象的地址，使用VarPtr(產生變量的地址和UDT），StrPtr（產生字符串的地址）和ObjPtr（產生對象的地址）可以構造真實的，非常復雜的數據結構。
上面三個方法并沒有在Microsoft的正式文檔資料中公布（包括MSDN），但查看VB6的基本動態運行庫MSVBVM60.DLL可以發現這三個方法：
[entry(0x60000006),hidden]
long __stdcall VarPtr([in]void* Ptr);
[entry(0x60000007),hidden]
long __stdcall StrPtr([in]BSTR Ptr);
[entry(0x60000008),hidden]
long __stdcall ObjPtr([in]IUnknown* Ptr);
類似這樣的隱藏方法還有不少，實際上VB6的功能是相當強大的，但大家又真正了解VB6多少呢？
延伸閱讀
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