Re: How to return string through output VARIANT from C++ COM object

From: Igor Tandetnik (itandetnik_at_mvps.org)
Date: 07/26/04 

Date: Mon, 26 Jul 2004 18:35:05 -0400

"Kim Gräsman" <kim@mvps.org> wrote in message
news:uo5FR7ncEHA.2408@tk2msftngp13.phx.gbl
>> "If the variant to be cleared is a COM object that is passed by
>> reference, the vt field of the pvarg parameter is VT_DISPATCH |
>> VT_BYREF or VT_UNKNOWN | VT_BYREF. In this case, VariantClear does
>> not release the object. Because the variant being cleared is a
>> pointer to a reference to an object, VariantClear has no way to
>> determine if it is necessary to release the object. It is therefore
>> the responsibility of the caller to release the object or not, as
>> appropriate."
>
> Gah! I had no idea... I thought VariantClear was smarter than that.

It is plenty smart. A VT_BYREF variant does not own the resource it
holds a reference to. Hence VariantClear is not supposed to release it.
You can legally have

VARIANT var;
var.vt = VT_BSTR;
var.bstrVal = SysAllocString(OLESTR("blah"));

VARIANT varRef;
varRef.vt = VT_BSTR | VT_BYREF;
varRef.pbstrVal = &var.bstrVal;

VariantClear(&varRef); // does not relase the string
VariantClear(&var); // releases the string

> You would have to dereference the VARIANT then, before clearing its
> contents.

Illegal. The server is not supposed to touch the referred-to data, as
opposed to the data directly contained in the VARIANT. Straight
VariantClear implements correct semantics. 

-- 
With best wishes,
    Igor Tandetnik
"For every complex problem, there is a solution that is simple, neat,
and wrong." H.L. Mencken

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