converting string (BSTR) to char [ ], VB to C++

Posted on 2003-03-26
Medium Priority
Last Modified: 2007-12-19
Hello experts,

I am new to COM programming, and I have encountered a problem which I am having troubles to resolve.

So I have a dll which has an external function that can be called from vb, and it expects
a string parameter (BSTR),

STDMETHODIMP MyClass::myMethod(BSTR input)

After I get the BSTR input from VB, I want to convert this string into unsigned char array[]
type to use for a private function.
Can anyone gimme a hand with this? an Example to do this would be prefect! thanks

Question by:WacKy
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LVL 86

Expert Comment

ID: 8211086
You could

#include <comdef.h>

STDMETHODIMP MyClass::myMethod(BSTR input)
 __bstr_t _bstr ( input);
 char* psz = ( char*) _bstr; // will call 'operator char*( ) const' and convert to char*


Expert Comment

ID: 8211326
Here's how I did it when I had to figure a way
probably not the most efficient method in the world, but it worked

for (DWORD c=0;c<SysStringLen(bstrResponse);c++){
 strResponse[c] = (char)bstrResponse[c];
bstrResponse is a BSTR variable
strResponse is defined as char strResponse[256]

Author Comment

ID: 8212052
Thanks to both jkr and ckaneta,
both examples worked fine, except I needed a char[] not a char*.

I changed my external function however, to:

STDMETHODIMP MyClass::myMethod(BSTR input, BSTR *output)
//get BSTR input, convert to unsinged char[], call
myFunction(unsigned char[]), convert from array[] to BSTR*

unsigned char[] MyClass::myFunction(unsigned char[])
//myFunction gets this Array, do some manipulation, and returns array[]

So can you guys show me how to convert from unsigned char[] to BSTR* so I can output this to VB? Thanks much
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LVL 86

Accepted Solution

jkr earned 200 total points
ID: 8212101
Sure. Just use

char* psz = "test";
__bstr_t _bstr ( psz );
BSTR bstr = _bstr.copy();

Expert Comment

ID: 8212430
It seems that you are using ATL or MFC. If this is true, the shortest way is:

STDMETHODIMP MyClass::myMethod(BSTR input)
    unsigned char *array;
    array = (unsigned char*)OLE2A(input);


Your array is allocated on stack. You do not need to free anything.
LVL 11

Expert Comment

ID: 8212685
Except the OLE2A isn't supposed to be used that way.  Microsoft had a long INFO: thing on it which I can't seem to locate.  Anyway, if I remember correctly the code with OLE2A does work, but only because the converted string doesn't get overwritten (which can happen in some circumstances).

The reccomended way to do it is with CW2A as described at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclib/html/_atl_String_Conversion_Macros.asp


Author Comment

ID: 8212884
thanks to all who's trying to help me.
jkr, I tried your last example, and I get a warning saying
"forcing value to bool true or false"

unsigned char array[256];
for (DWORD c=0; c<SysStringLen(input); c++)
     array[c] = (unsigned char)input[c];

unsigned char* psz = myFunction(array); //myFunction
//returns unsigned char *
_bstr_t _bstr(psz);
BSTR bstr = _bstr.copy();
*output= bstr;

//and my output string becomes -1 as a result of this, I lost all my collection of data inside the array?

Expert Comment

ID: 8214059
Try using wcstombs().  As long as it's not expected to be an an out parameter.

Author Comment

ID: 8214742
thanks to everyone, but I think I should give points to jkr. =)

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