问题:

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?

回答1:

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.

回答2:

SysStringLen() should only be used on BSTRs allocated by SysAllocString() family functions. Using it as you do will lead to undefined behavior - program can crash or produce unexpected results. Better yet use wrapper classes - ATL::CComBSTR or _bstr_t.