_bstr_t help needed

**Ita_c** · 11-04-2005

Hi all ,
could anyone please help ,
I am keeping getting app crashes with my app
I suspect the following code
Could anyone have a look and help ,
In addition , could any one explain what are the diffrences between the _bstr_t constructor , and if I have to free them ?

Thanks very much

Code:

BOOL CMSInterface::ConvertBSTRtoCharWithoutTerminator(BSTR i_sStr,char* o_Str,long *o_Length)
{
	bool retVal = true;
	_bstr_t bstrObj(i_sStr, FALSE);
	*o_Length = bstrObj.length();
	
	long lLen = WideCharToMultiByte(CP_ACP, 0, i_sStr,(*o_Length-2), o_Str, *o_Length, NULL, NULL);

	
	if(lLen)
	{
		*o_Length-= 2;
		retVal = true;
	}
	else
	{
		*o_Length =0;
		retVal = false;
	}
	
	return retVal;

}

**Ancient Dragon** · 11-04-2005

when you use the _bstr_t constructor with the second argument FALSE, the _bstr_t class takes ownership of the BSTR object passed as the first argument. That means you can not laster use SysFreeString() to free up the memory because _bstr_t will do that when the class is destroyed. My hunch is that the function that called ConvertBSTRtoCharWithoutTerminator() is calling SystFreeString() on i_sStr, and that will crash because it has already been freed when ConvertBSTRtoCharWithoutTerminator returns.

One of two possible solutinos:
1. do not use the constructor that takes ownership of the string. Use one of the other constructors that will duplicate the string.

Code:

_bstr_t b(i_sStr);

or
2. Do not use _bstr_t class at all, but use the BSTR directly.

Code:

// get the number of bytes allocated to the BSTR
*o_Length = *(long *)(i_sStr-2);
// or get the string length, which may or may not be the same
*o_Length = wcslen(i_sStr);

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