C++: Convert wchar_t* to BSTR?

c++ string com bstr
Nick Heiner picture Nick Heiner · Jul 24, 2010 · Viewed 13.1k times · Source

I'm trying to convert a wchar_t * to BSTR. 

#include <iostream>
#include <atlstr.h>

using namespace std;

int main()
{
    wchar_t* pwsz = L"foo"; 

    BSTR bstr(pwsz);

    cout << SysStringLen(bstr) << endl;

    getchar();
}

This prints 0, which is less than what I'd hoped. What is the correct way to do this conversion?

Answer

CB Bailey picture CB Bailey · Jul 24, 2010

You need to use SysAllocString (and then SysFreeString).

BSTR bstr = SysAllocString(pwsz);

// ...

SysFreeString(bstr);

A BSTR is a managed string with the characters of the string prefixed by their length. SysAllocString allocates the correct amount of storage and set up the length and contents of the string correctly. With the BSTR correctly initialized, SysStringLen should return the correct length.

If you're using C++ you might want to consider using a RAII style class (or even Microsoft's _bstr_t) to ensure that you don't forget any SysFreeString calls.

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