Re: question about CComBSTR / _bstr_t subtlety

"Igor Tandetnik" <>
Mon, 11 Dec 2006 12:12:33 -0500
Jason S <> wrote:

I have a head-scratcher question about how CComBSTR / _bstr_t work in
a certain case.

Let's say I have these functions:

_bstr_t f()
 _bstr_t b;

 b = L"foo";
 b += L"bar";
 return b;

void g(BSTR b0)
 // do something with b0

void h()

If I call h(), how many times does the string "foobar" get copied
after its initial construction inside f()?

_bstr_t is reference-counted. The underlying BSTR is not copied by the

I guess I am confused
about how functions can return _bstr_t or CComBSTR as values w/o
causing a copy of the underlying string to be made.

CComBSTR does make a copy of the underlying string. What makes you
believe it doesn't?

A function can take _bstr_t& or CComBSTR& parameter by reference, and
place its output into the caller-supplied variable, if you are worried
about performance. As usual, before engaging in microoptimizations,
profile to make sure they do in fact contribute to performance
With best wishes,
    Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925

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