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

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?

Garbage in, garbage out. You can write your own converter with WideCharToMultiByte() and do something more reasonable when you get a one megabyte string. Like throwing an exception. — Hans Passant
To clarify, is this a question about a poorly documented MS comsupp-lib API? Or are you asking how to do this yourself? (or both?) — WhozCraig
@WhozCraig: I suppose both, so perhaps I should split them into two separate questions, and I'll edit this one to focus on my goal rather than my method. — user1472525
Note that, in that codeproject page, the author rants about a function allocating "twice as much memory as [it] needs," because they allocate wcslen(str)*2. There seems to be a misunderstanding as to just how wide wide-characters are. The article should be taken with some amount of salt, probably twice as much as usual. — ssube

WhozCraig WhozCraig · Accepted Answer · 2012-12-05T15:40:22

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.

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

2 Answers