Re: question about CComBSTR / _bstr_t subtlety

From:
"Igor Tandetnik" <itandetnik@mvps.org>
Newsgroups:
microsoft.public.vc.atl
Date:
Mon, 11 Dec 2006 12:12:33 -0500
Message-ID:
<Ot9HNeUHHHA.4804@TK2MSFTNGP03.phx.gbl>
Jason S <jmsachs@gmail.com> 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()
{
 g(f());
}

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
copy-constructor.

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
bottlenecks.
--
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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