指向存储为集合/字典项VBA的数组的指针

Question

对于变量数组,其中每个元素都是一个双数组,我可以执行以下操作:

Public Declare PtrSafe Sub CopyMemoryArray Lib "kernel32" Alias "RtlMoveMemory" (ByRef Destination() As Any, ByRef Source As Any, ByVal Length As Long)

Sub test()
    Dim vntArr() as Variant
    Dim A() as Double
    Dim B() as Double

    Redim vntArr(1 to 10)
    Redim A(1 to 100, 1 to 200)
    vntArr(1) = A
    CopyMemoryArray B, ByVal VarPtr(vntArr(1)) + 8, PTR_LENGTH '4 or 8
    'Do something
    ZeroMemoryArray B, PTR_LENGTH
End Sub

然后A和B将指向内存中的相同块.(设置W = vntArr(1)会创建一个副本.对于非常大的数组,我想避免这种情况.)

我正在尝试做同样的事情,但是有了收藏:

Sub test()
    Dim col as Collection
    Dim A() as Double
    Dim B() as Double

    Set col = New Collection
    col.Add A, "A"
    CopyMemoryArray B, ByVal VarPtr(col("A")) + 8, PTR_LENGTH '4 or 8
    'Do something
    ZeroMemoryArray B, PTR_LENGTH
End Sub

这种工作,但由于某种原因,col("A")返回的安全数组结构(包含在Variant数据类型中,类似于上面的变量数组)只包含一些外部属性,如维数和暗边界,但是指向pvData本身的指针是空的,因此CopyMemoryArray调用导致崩溃.(设置B = col("A")工作正常.)与Scripting.Dictionary相同的情况.

有谁知道这里发生了什么？

---

编辑

#If Win64 Then
    Public Const PTR_LENGTH As Long = 8
#Else
    Public Const PTR_LENGTH As Long = 4
#End If

Public Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (ByRef Destination As Any, ByRef Source As Any, ByVal Length As Long)

Private Const VT_BYREF As Long = &H4000&
Private Const S_OK As Long = &H0&

Private Function pArrPtr(ByRef arr As Variant) As LongPtr
    Dim vt As Integer

    CopyMemory vt, arr, 2
    If (vt And vbArray) <> vbArray Then
        Err.Raise 5, , "Variant must contain an array"
    End If
    If (vt And VT_BYREF) = VT_BYREF Then
        CopyMemory pArrPtr, ByVal VarPtr(arr) + 8, PTR_LENGTH
        CopyMemory pArrPtr, ByVal pArrPtr, PTR_LENGTH
    Else
        CopyMemory pArrPtr, ByVal VarPtr(arr) + 8, PTR_LENGTH
    End If
End Function

Private Function GetPointerToData(ByRef arr As Variant) As LongPtr
    Dim pvDataOffset As Long
    #If Win64 Then
        pvDataOffset = 16 '4 extra unused bytes on 64bit machines
    #Else
        pvDataOffset = 12
    #End If
    CopyMemory GetPointerToData, ByVal pArrPtr(arr) + pvDataOffset, PTR_LENGTH
End Function

Sub CollectionWorks()
    Dim A(1 To 100, 1 To 50) As Double

    A(3, 1) = 42

    Dim c As Collection
    Set c = New Collection

    c.Add A, "A"

    Dim ActualPointer As LongPtr
    ActualPointer = GetPointerToData(c("A"))

    Dim r As Double
    CopyMemory r, ByVal ActualPointer + (0 + 2) * 8, 8

    MsgBox r  'Displays 42
End Sub

Answer 1

VB旨在隐藏复杂性.通常,这会产生非常简单直观的代码,有时却不会.

A VARIANT可以包含非VARIANT数据数组没有问题,例如正确Double的数组.但是当你试图从VB访问这个数组时,你没有得到Double像它实际存储的原始blob,你得到它包装在一个临时的Variant,在访问时构造,特别是不要惊讶你的事实声明的数组As Variant突然产生一个值As Double.您可以在此示例中看到:

Sub NoRawDoubles()
  Dim A(1 To 100, 1 To 50) As Double
  Dim A_wrapper As Variant

  A_wrapper = A

  Debug.Print VarPtr(A(1, 1)), VarPtr(A_wrapper(1, 1))
  Debug.Print VarPtr(A(3, 3)), VarPtr(A_wrapper(3, 3))
  Debug.Print VarPtr(A(5, 5)), VarPtr(A_wrapper(5, 5))
End Sub

在我的电脑上,结果是:

88202488      1635820 
88204104      1635820 
88205720      1635820

来自A的元素实际上是不同的,并且位于存储器中它们应该在数组中,并且每个元素的大小为8个字节,而"元素" A_wrapper实际上是相同的"元素" - 该数字重复三次是该地址的临时的Variant,大小为16个字节,用于保存数组元素,编译器决定重用它.

这就是为什么以这种方式返回的数组元素不能用于指针算术.

集合本身不会为此问题添加任何内容.事实上,Collection必须将它存储的数据包装成Variant混乱的数据.将数组存储在任何其他地方的Variant中时会发生这种情况.

要获得适合于指针算术的实际展开数据指针,您需要查询SAFEARRAY*指针Variant,从中可以存储一个或两个间接级别的指针,并从那里获取数据指针.

在前面的示例的基础上,天真的非x64兼容代码将是:

Private Declare Function GetMem2 Lib "msvbvm60" (ByVal pSrc As Long, ByVal pDst As Long) As Long  ' Replace with CopyMemory if feel bad about it
Private Declare Function GetMem4 Lib "msvbvm60" (ByVal pSrc As Long, ByVal pDst As Long) As Long  ' Replace with CopyMemory if feel bad about it

Private Const VT_BYREF As Long = &H4000&

Private Function pArrPtr(ByRef arr As Variant) As Long  'Warning: returns *SAFEARRAY, not **SAFEARRAY
  'VarType lies to you, hiding important differences. Manual VarType here.
  Dim vt As Integer
  GetMem2 ByVal VarPtr(arr), ByVal VarPtr(vt)

  If (vt And vbArray) <> vbArray Then
    Err.Raise 5, , "Variant must contain an array"
  End If


  'see https://msdn.microsoft.com/en-us/library/windows/desktop/ms221627%28v=vs.85%29.aspx
  If (vt And VT_BYREF) = VT_BYREF Then
    'By-ref variant array. Contains **pparray at offset 8
    GetMem4 ByVal VarPtr(arr) + 8, ByVal VarPtr(pArrPtr)  'pArrPtr = arr->pparray;
    GetMem4 ByVal pArrPtr, ByVal VarPtr(pArrPtr)          'pArrPtr = *pArrPtr;
  Else
    'Non-by-ref variant array. Contains *parray at offset 8
    GetMem4 ByVal VarPtr(arr) + 8, ByVal VarPtr(pArrPtr)  'pArrPtr = arr->parray;
  End If

End Function

Private Function GetPointerToData(ByRef arr As Variant) As Long
  GetMem4 pArrPtr(arr) + 12, VarPtr(GetPointerToData)
End Function

然后可以使用以下非x64兼容方式:

Sub CollectionWorks()
  Dim A(1 To 100, 1 To 50) As Double

  A(3, 1) = 42

  Dim c As Collection
  Set c = New Collection

  c.Add A, "A"

  Dim ActualPointer As Long
  ActualPointer = GetPointerToData(c("A"))

  Dim r As Double
  GetMem4 ActualPointer + (0 + 2) * 8, VarPtr(r)
  GetMem4 ActualPointer + (0 + 2) * 8 + 4, VarPtr(r) + 4

  MsgBox r  'Displays 42
End Sub

请注意,我不确定c("A")每次返回相同的实际数据,而不是随意制作副本,因此可能不建议以这种方式缓存指针,并且最好先将结果保存c("A")到变量中,然后GetPointerToData取消那个.

显然,这应该被重写使用LongPtr和CopyMemory,和我可能会做,明天,但你的想法.

Answer 2

如果将两个基本变量都视为 Variant，那就更容易了。

Option Explicit

#If Vba7 Then
    Private Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
    Private Declare PtrSafe Sub FillMemory Lib "kernel32" Alias "RtlFillMemory" (Destination As Any, ByVal Length As Long, ByVal Fill As Byte)
#Else
    Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
    Private Declare Sub FillMemory Lib "kernel32" Alias "RtlFillMemory" (Destination As Any, ByVal Length As Long, ByVal Fill As Byte)
#End If


Sub test()
    Dim col As Variant
    Dim B As Variant
    Dim A() As Double

    ReDim A(1 To 100, 1 To 200)
    A(1, 1) = 42
    Set col = New Collection
    col.Add A, "A"
    Debug.Print col("A")(1, 1)

    CopyMemory B, col, 16
    Debug.Print B("A")(1, 1)

    FillMemory B, 16, 0
End Sub

另请参阅这些有用的链接

通过引用的部分数组

在 VBA 中复制数组引用

如何在 Excel VBA 中对数组进行切片？

http://bytecomb.com/vba-reference/