stdLambda stack issue

[sancarn]

Currently in evaluate:

            Case iType.oAccess
                Select Case op.subType
                    Case ISubType.argument
                        Dim iArgIndex As Long: iArgIndex = val(mid(op.value, 2)) + LBound(vLastArgs) - 1
                        If iArgIndex <= UBound(vLastArgs) Then
                            Call pushV(stack, stackPtr, vLastArgs(iArgIndex))
                        Else
                            Call Throw("Argument " & iArgIndex & " not supplied to Lambda.")
                        End If
                    Case Else
                        Call pushV(stack, stackPtr, stack(stackPtr - op.value))

Call pushV(stack, stackPtr, stack(stackPtr - op.value)) can sometimes error when the variable at stack(stackPtr - op.value) is an object. See explanation for details. As a temporary work around we can do the following:

 '@FIX bug where if stack(stackPtr-op.value) was an object, then array will become locked. Array locking occurs by compiler to try to protect
'instances when re-allocation would move the array, and thus corrupt the pointers. By copying the variant we redivert the compiler's efforts,
'but we might actually open ourselves to errors...
Dim vAccessVar: Call CopyVariant(vAccessVar, stack(stackPtr - op.value))
Call pushV(stack, stackPtr, vAccessVar)

This may still break so we need to consider a better solution. Our suggestion is to use a Collection (i.e. linked list) instead of a dynamic array. This should have fewer issues but might sacrifice on some performance.

---

Explanation:

Minimal examples where error occurs:

Sub Example()
    'Initialize the array with 3 elements
    Dim myArray() As string
    ReDim myArray(0)
    Set myArray(0) = "yop"
    Call Push(myArray, myArray(0))
End Sub

Sub Push(ByRef myArray() As string, ByRef element As string)
    'Lock the array to prevent deallocation of memory for the reference parameter
    ReDim Preserve myArray(UBound(myArray) + 1)
    If IsObject(element) Then
      Set myArray(UBound(myArray)) = element
    Else
      myArray(UBound(myArray)) = element
    End If
End Sub

The issue is that element, in this case is a reference to an element within myArray... The solution to above is change element to ByVal:

Sub Push(ByRef myArray() As string, ByVal element As string)

When extended to the variant case however:

Sub Push(ByRef myArray() As variant, ByVal element As variant)

Objects are also pointers... so

Sub Example()
    'Initialize the array with 3 elements
    Dim myArray() As Variant
    ReDim myArray(0)
    Set myArray(0) = CreateObject("scripting.dictionary")
    Call Push(myArray, myArray(0))
End Sub

Sub Push(ByRef myArray() As Variant, ByVal element As Variant)
    'Lock the array to prevent deallocation of memory for the reference parameter
    ReDim Preserve myArray(UBound(myArray) + 1)
    If IsObject(element) Then
      Set myArray(UBound(myArray)) = element
    Else
      myArray(UBound(myArray)) = element
    End If
End Sub

Also errors because we have a pointer directly to myArray(0) object. So the real work around is

Sub Example()
    'Initialize the array with 3 elements
    Dim myArray() As Variant
    ReDim myArray(0)
    Set myArray(0) = CreateObject("scripting.dictionary")
    Dim v: CopyVariant(v, myArray(0))
    Call Push(myArray, v)
End Sub