vb6 - 你如何让 VB6 用 +infinity、-infinity 和 NaN 初始化 double ？

Question

VB6 似乎并没有让将 +infinity、-infinity 和 NaN 存储到双变量中那么容易。如果可以的话，这会有所帮助，这样我就可以在复数的上下文中与这些值进行比较。如何？

Answer 1

一些不同的事情。正如您从 Pax 的示例中看到的，您实际上只需要查找 IEEE 754 标准，然后将您的字节插入正确的位置。我要给你的唯一警告是 MicroSoft has deprecated RtlMoveMemory因为它有可能产生溢出类型的安全问题。作为替代方案，您可以在“纯”VB 中使用用户定义类型和 LSet 进行一些仔细的强制转换来完成此操作。 (另请注意，有两种类型的 NaN。)

Option Explicit

Public Enum abIEEE754SpecialValues
    abInfinityPos
    abInfinityNeg
    abNaNQuiet
    abNaNSignalling
    abDoubleMax
    abDoubleMin
End Enum

Private Type TypedDouble
    value As Double
End Type

Private Type ByteDouble
    value(7) As Byte
End Type

Public Sub Example()
    MsgBox GetIEEE754SpecialValue(abDoubleMax)
End Sub

Public Function GetIEEE754SpecialValue(ByVal value As abIEEE754SpecialValues) As Double
    Dim dblRtnVal As Double
    Select Case value
    Case abIEEE754SpecialValues.abInfinityPos
        dblRtnVal = BuildDouble(byt6:=240, byt7:=127)
    Case abIEEE754SpecialValues.abInfinityNeg
        dblRtnVal = BuildDouble(byt6:=240, byt7:=255)
    Case abIEEE754SpecialValues.abNaNQuiet
        dblRtnVal = BuildDouble(byt6:=255, byt7:=255)
    Case abIEEE754SpecialValues.abNaNSignalling
        dblRtnVal = BuildDouble(byt6:=248, byt7:=255)
    Case abIEEE754SpecialValues.abDoubleMax
        dblRtnVal = BuildDouble(255, 255, 255, 255, 255, 255, 239, 127)
    Case abIEEE754SpecialValues.abDoubleMin
        dblRtnVal = BuildDouble(255, 255, 255, 255, 255, 255, 239, 255)
    End Select
    GetIEEE754SpecialValue = dblRtnVal
End Function

Public Function BuildDouble( _
    Optional byt0 As Byte = 0, _
    Optional byt1 As Byte = 0, _
    Optional byt2 As Byte = 0, _
    Optional byt3 As Byte = 0, _
    Optional byt4 As Byte = 0, _
    Optional byt5 As Byte = 0, _
    Optional byt6 As Byte = 0, _
    Optional byt7 As Byte = 0 _
    ) As Double
    Dim bdTmp As ByteDouble, tdRtnVal As TypedDouble
    bdTmp.value(0) = byt0
    bdTmp.value(1) = byt1
    bdTmp.value(2) = byt2
    bdTmp.value(3) = byt3
    bdTmp.value(4) = byt4
    bdTmp.value(5) = byt5
    bdTmp.value(6) = byt6
    bdTmp.value(7) = byt7
    LSet tdRtnVal = bdTmp
    BuildDouble = tdRtnVal.value
End Function

最后一个旁注，您也可以通过这种方式获得 NaN:

Public Function GetNaN() As Double
    On Error Resume Next
    GetNaN = 0 / 0
End Function