VB6 UDT 中固定长度字符串与可变长度字符串的处理方式有何不同？
在 VB6 中，我可以在 UDT 中使用固定或可变长度字符串：
Public Type MyRecord
VariableLengthString As String
FixedLengthString As String * 80
End Type
我希望 FixedLengthString 在 UDT 中分配 160 个字节，如果我有这些 UDT 的数组，则固定长度字符串数据是内联的。
VariableLengthString 发生了什么？是在堆上分配的吗？
因此，如果我这样做：
Dim record1 As MyRecord
Dim record2 As MyRecord
record1.VariableLengthString = "a"
record2 = record1
...显然 record2.VariableLengthString 将是 "a"。但是，如果我这样做：
record2.VariableLengthString = "b"
...那么 record1.VariableLengthString 的值是多少？
更新：结果发现结果仍然是"a"，也就是说字符串被复制了。那么不是在堆上吗？
我的测试代码：
record1.VariableLengthString = "a"
record2 = record1
? record2.VariableLengthString
a
record2.VariableLengthString = "b"
? record1.VariableLengthString
a
In VB6, I can use either fixed or variable length strings in a UDT:
Public Type MyRecord
VariableLengthString As String
FixedLengthString As String * 80
End Type
I would expect FixedLengthString to allocate 160 bytes in the UDT, and if I had an array of these UDT's, the fixed length string data is inline.
What's going on with VariableLengthString? Is it allocated on the heap?
Therefore, if I do this:
Dim record1 As MyRecord
Dim record2 As MyRecord
record1.VariableLengthString = "a"
record2 = record1
... obviously record2.VariableLengthString will be "a". But, if I do this:
record2.VariableLengthString = "b"
... then what is the value of record1.VariableLengthString?
Update: It turns out that the result is still "a", which means the string is copied. So is it not on the heap?
My test code:
record1.VariableLengthString = "a"
record2 = record1
? record2.VariableLengthString
a
record2.VariableLengthString = "b"
? record1.VariableLengthString
a
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基本上，“作为字符串”在 VB6 中分配 BSTR。 OLE（VB6 的基础技术）以特殊方式处理 BSTR，包括维护缓存以使字符串操作更快。 MSDN 中的这篇有关字符串操作的文章对此进行了更详细的解释。 UDT 指向 BSTR 结构，该结构基本上是一个 32 位长度字段，后跟字符串的字符。它支持 Unicode。分配给 BSTR 的空间是 4 个字节 + 存储字符的字节数。所以它是可变的，不像固定长度字符串那样固定。
Basically "as string" allocates a BSTR in VB6. OLE (the technology underlying VB6) handles BSTRs in special ways including maintaining a cache to make string manipulation faster. This article on String Manipulation in MSDN explains it more detail. The UDT points to a BSTR stucture that is basically a 32bit length field followed by the characters of the string. It is Unicode aware. The space allocated to a BSTR is 4 bytes + the number of bytes to store the characters. So it is variable not fixed like Fixed Length String.