A lightning-fast StringBuilder
I like this, a lot. It's.. brilliant. Like .NET String objects, VBA strings are immutable, which means like in .NET, when "the quick brown fox" gets appended with "jumps over" and then "the lazy dog", it's 4 strings that have been generated, and thus the first one got copied 3 times; a VBA StringBuilder class is therefore definitely welcome to any VBA toolkit!

This is some serious code you've got here. Let's take a look.

So you've called the class clsStringBuilder. I know where you're coming from, but there's no real reason for this "cls" Hungarian prefix - I'd remove it, and call the class StringBuilder.


'******
'* v2 *
'******



Don't bother with that. I know version control is natively near-impossible with VBA, but there is no need to "version" code in comments nonetheless; do you actually maintain the version number? Why bother? Just remove it, it's useless clutter.


Private Const DEFAULT_CAPACITY As Long = &amp;H10



Why not 16? Hexadecimal 10 is 16 right? I think it would be clearer to use a decimal notation. Actually that capacity is potentially confusing, especially given the hex notation. Is &amp;H10 the number of bytes? Characters?


ReDim m_stringBuffer(0 To (DEFAULT_CAPACITY * 2) - 1) 'Each unicode character is 2 bytes



Ah, characters then. How about calling the constant DEFAULT_CHARACTER_CAPACITY? Nah, too long.. and I personally don't like the YELLCASE, I'd just call it InitialCharacterCapacity, but I've seen other people use ALL CAPS for constants - as long as you're consistent, it works :)

BTW that's a good comment you have there, but I wouldn't bother specifying "unicode" characters; it gets confusing when the IDE itself only supports ANSI strings!

I don't like prefixes and abbreviated names, so m_currLen would become currentLength and m_stringBuffer would become stringBufffer, or just buffer.

Actually since currentLength is in bytes, I'd call it currentByteLength, so as to avoid question marks when it comes to this:


Public Property Get Length() As Long
    Length = m_currLen / 2
End Property






Public Function Append(strString As String) As clsStringBuilder



strString, really? I doesn't get any more Hungarian than that! Also you should know that parameters are passed ByRef by default - I'd change the signature to this:

Public Function Append(ByVal value As String) As StringBuilder


The error handling isn't optimal - if anything blows up, you're bringing up the IDE for the end user to scratch their head and debug your code! That's not production-ready:


derp:
    Stop
    Resume



"derp" means nothing to me - I like following a "template" like this:

Public Sub Foo()
    On Error GoTo CleanFail

    'implementation

CleanExit:
    Exit Sub

CleanFail:
    'handle error
    Resume CleanExit
End Sub




You might also want to make sure Expand doesn't actually shrink the buffer. I think. ;)

Lastly, I'm not sure I understand why toString isn't following the convention and named in PascalCase like every public method - ToString would be better-looking.

Good job!