BSTR and SysAllockStringByteLen() in C++

Question

I'm new to C++, so this may be a noobish question; I have the following function:

#define SAFECOPYLEN(dest, src, maxlen)                               \
{                                                                    \
    strncpy_s(dest, maxlen, src, _TRUNCATE);                          \
    dest[maxlen-1] = '\0';                                            \
}

short _stdcall CreateCustomer(char* AccountNo)
{
    char tmpAccountNumber[9];
    SAFECOPYLEN(tmpAccountNumber, AccountNo, 9);
    BSTR strAccountNumber = SysAllocStringByteLen(tmpAccountNUmber, 9);

    //Continue with other stuff here.
}

When I debug through this code, I pass in the account number "A101683" for example. When it does the SysAllocStringByteLen() part, the account number becomes a combination of Chinese symbols...

Anyone that can shed some light on this?

Updated with SAFECOPYLEN definition. – Mr. Smith Sep 16 '09 at 08:26 — Mr. Smith, Sep 16 '09 at 08:26

1800 INFORMATION · Accepted Answer · 2009-09-16T09:12:59.750

6

SysAllocStringByteLen is meant for when you are creating a BSTR containing binary data, not actual strings - no ANSI to unicode conversion is performed. This explains why the debugger shows the string as containing apparently chinese symbols, it is trying to interpret the ANSI string copied into the BSTR as unicode. You should probably use SysAllocString instead - ~~this will convert the string correctly to unicode~~ you must pass it a unicode string. If you are working with actual text, this is the function you should be using.

edited Sep 16 '09 at 09:12

answered Sep 16 '09 at 08:17

1800 INFORMATION

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2

General idea is right, but SysAllocString() will not perform ANSI-to-Unicode conversion. You'll need to do it yourself: http://stackoverflow.com/questions/606075/how-to-convert-char-to-bstr/606122#606122 – sharptooth Sep 16 '09 at 08:20
Yes, I'm working with actual text, not binary data. Do you mean I should SysAllocString() instead of SysAllocStringByteLen() in this case? – Mr. Smith Sep 16 '09 at 08:41
1

Yes, to get a BSTR you need to call either SysAllocString() or SysAllocStringLen() and do the ANSI-to-Unicode conversion as in the linked answer. – sharptooth Sep 16 '09 at 08:49
I've updated the answer to be correct - that was more or less a think-o - I so rarely work with actual ANSI strings these days – 1800 INFORMATION Sep 16 '09 at 09:14

score 0 · Answer 2 · answered Sep 16 '09 at 08:10

0

First of all, there's a problem with the line containing SAFECOPYLEN. It's missing ')' and it's not clear what it's supposed to do.

The second issue is that you're not using AccountNo anywhere in this code. tmpAccountNumber is on the stack, and could have anything in it.

answered Sep 16 '09 at 08:10

Douglas Mayle

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I retyped only the parts of the function relevant to my question. I made a mistake, it's updated now. – Mr. Smith Sep 16 '09 at 08:22

score 0 · Answer 3 · answered Sep 16 '09 at 08:20

BSTR are double-byte character arrays so you can't just copy a char* array into it. Instead of passing it "A12123" try L"A12323".

short _stdcall CreateCustomer(wchar_t* AccountNo)
{
wchar_t tmpAccountNumber[9];
wcscpy(tmpAccountNumber[9], AccountNo);
BSTR strAccountNumber = SysAllocStringByteLen(tmpAccountNUmber, 9);

//Continue with other stuff here.
}

BSTR and SysAllockStringByteLen() in C++

3 Answers3