VB6中的LSet语句和RSet语句详解

在VB6里，有几个语句长得特别像，容易搞混：Let、Set、LSet、RSet。先看个全家福，心里有个底。

Let是给一般变量赋值的，语法长这样：

[Let] varname = expression

Set呢，是给对象赋值的，把变量指向对象，同时增加对象的引用计数。说到引用计数，很多人也是一脸懵。

那LSet是什么？乍一看像是Let和Set的结合体，其实完全不是。LSet里的L是Left的缩写，对应的还有RSet。怎么知道的？文档上写得清楚：

LSet Statement
Left aligns a string within a string variable, or copies a variable of one user-defined type to another variable of a different user-defined type.

RSet Statement
Right aligns a string within a string variable.

Sub Main()
    Dim url As String
    Dim s As String
    
    Let url = "https://www.jb51.net"
    s = String$(20, "*")
    
    LSet s = url
    Debug.Print s
    
    RSet s = url
    Debug.Print s
End Sub

https://www.jb51.net    
    https://www.jb51.net

不过LSet的另一个功能可就厉害了——可以把一个用户定义类型变量复制到另一个用户自定义类型变量。这又是什么意思？

举个例子：IP地址大家都懂吧？比如ping百度返回的IP是61.135.169.125，这格式是给人看的。计算机内部其实拿32位整数来表示IP。怎么用VB把 xxx.xxx.xxx.xxx 格式的IP转成32位整数？Google一番后，你可能写出这样的代码：

Sub Main()
    Debug.Print IPToLong("61.135.169.125")
End Sub

Private Function IPToLong(IPStr As String) As Long
    Dim Str() As String, HEXStr As String, TempStr As String
    Dim x As Long
    
    Str = Split(IPStr, ".")
    HEXStr = ""
    For x = 0 To UBound(Str)
        TempStr = Hex(Str(x))
        HEXStr = HEXStr & String(2 - Len(TempStr), "0") & TempStr
    Next x
    IPToLong = CLng("&H" & HEXStr)
End Function

Private Type myBytes
    B1 As Byte
    B2 As Byte
    B3 As Byte
    B4 As Byte
End Type

Private Type myLong
    Val As Long
End Type

Public Function IP2Long(ip As String) As Long
    Dim a() As String
    Dim b As myBytes
    Dim l As myLong
    
    a = Split(ip, ".")
    '注意Little-Endian
    b.B1 = CByte(a(3))
    b.B2 = CByte(a(2))
    b.B3 = CByte(a(1))
    b.B4 = CByte(a(0))
    LSet l = b
    IP2Long = l.Val
End Function

00401A0E   lea     eax, dword ptr [ebp-0x20]   ; 变量b的地址
00401A11   push    eax
00401A12   lea     eax, dword ptr [ebp-0x14]   ; 变量l的地址
00401A15   push    eax
00401A16   push    0x4
00401A18   call    __vbaCopyBytes               ; jmp to MSVBVM60.__vbaCopyBytes

72A1A0F3 >  mov     ecx, dword ptr [esp+0x4]
72A1A0F7    push    esi
72A1A0F8    mov     esi, dword ptr [esp+0x10]
72A1A0FC    push    edi
72A1A0FD    mov     edi, dword ptr [esp+0x10]
72A1A101    mov     eax, ecx
72A1A103    mov     edx, edi
72A1A105    shr     ecx, 0x2
72A1A108    rep     movs dword ptr es:[edi], dword ptr [esi]
72A1A10A    mov     ecx, eax
72A1A10C    mov     eax, edx
72A1A10E    and     ecx, 0x3
72A1A111    rep     movs byte ptr es:[edi], byte ptr [esi]
72A1A113    pop     edi
72A1A114    pop     esi
72A1A115    retn    0xC

Warning   Using LSet to copy a variable of one user-defined type into a variable of a different user-defined type is not recommended. Copying data of one data type into space reserved for a different data type can cause unpredictable results.

When you copy a variable from one user-defined type to another, the binary data from one variable is copied into the memory space of the other, without regard for the data types specified for the elements.