Re: interfacing between BSTR, LPCTSTR, CComBSTR, _bstr_t

"Igor Tandetnik" <>
Tue, 11 Sep 2007 13:30:13 -0400
Jason S <> wrote:

I notice that CComBSTR can be used with the COM method but _bstr_t
cannot, e.g.:
CComBSTR b1;
_bstr_t b2;
pComObject->Something(&b1); // works
pComObject->Something(&b2); // won't work

Make it


Also CComBSTR has Attach() and Detach() members to allow you to
control ownership w/o having to duplicate memory. But _bstr_t does
not. :(

It does as of VC7.1. I don't have VC6 handy to check at the moment.

I also notice that _bstr_t has the facilities to convert to a char *
if necessary, but CComBSTR does not.

ATL way is to use explicit conversion macros:


I therefore end up doing stuff like this a lot:
CComBSTR btemp; // short term storage
_bstr_t b; // long term storage, usually a member of a class
b = btemp; // store it for long term use.

If you insist on doing it this way, a more efficient sequence would be

CComBSTR btemp; // short term storage
_bstr_t b(btemp.Detach(), false);

This avoids any unnecessary copies.

Is there any way to get a _bstr_t to use the [out] parameter of a COM
method without copying the string?

[p.s. I am using VC6, has this been improved in later editions of the

How about this little bit of self-help:

class bstr_out {
    bstr_out(_bstr_t& b) : ref(b), bstr(0) {}
    ~bstr_out() { ref = _bstr_t(bstr, false); }

    operator BSTR*() { return &bstr; }
    _bstr_t& ref;
    BSTR bstr;

Now you can do

_bstr_t b;

With best wishes,
    Igor Tandetnik

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