How to convert BSTR to std::string in Visual Studio C++ 2010? - c++

Question

I am working on a COM dll. I wish to convert a BSTR to a std::string to pass to a method that takes a const reference parameter.
It seems that using _com_util::ConvertBSTRToString() to get the char* equivalent of the BSTR is an appropriate way to do so. However, the API documentation is sparse, and the implementation is potentially buggy:
http://msdn.microsoft.com/en-us/library/ewezf1f6(v=vs.100).aspx
http://www.codeproject.com/Articles/1969/BUG-in-_com_util-ConvertStringToBSTR-and-_com_util
Example:
#include <comutil.h>
#include <string>
void Example(const std::string& Str) {}
int main()
{
BSTR BStr = SysAllocString("Test");
char* CharStr = _com_util::ConvertBSTRToString(BStr);
if(CharStr != NULL)
{
std::string StdStr(CharStr);
Example(StdStr);
delete[] CharStr;
}
SysFreeString(BStr);
}
What are the pros and cons of alternatives to using ConvertBSTRToString(), preferrably based on standard methods and classes?

Answer 1

You can do this yourself. I prefer to convert into the target std::string if possible. If not, use a temp-value override.
// convert a BSTR to a std::string.
std::string& BstrToStdString(const BSTR bstr, std::string& dst, int cp = CP_UTF8)
{
if (!bstr)
{
// define NULL functionality. I just clear the target.
dst.clear();
return dst;
}
// request content length in single-chars through a terminating
// nullchar in the BSTR. note: BSTR's support imbedded nullchars,
// so this will only convert through the first nullchar.
int res = WideCharToMultiByte(cp, 0, bstr, -1, NULL, 0, NULL, NULL);
if (res > 0)
{
dst.resize(res);
WideCharToMultiByte(cp, 0, bstr, -1, &dst[0], res, NULL, NULL);
}
else
{ // no content. clear target
dst.clear();
}
return dst;
}
// conversion with temp.
std::string BstrToStdString(BSTR bstr, int cp = CP_UTF8)
{
std::string str;
BstrToStdString(bstr, str, cp);
return str;
}
Invoke as:
BSTR bstr = SysAllocString(L"Test Data String")
std::string str;
// convert directly into str-allocated buffer.
BstrToStdString(bstr, str);
// or by-temp-val conversion
std::string str2 = BstrToStdString(bstr);
// release BSTR when finished
SysFreeString(bstr);
Something like that, anyway.

Answer 2

Easy way
BSTR => CStringW => CW2A => std::string.