usage help for _com_ptr_t smart pointers created with #import

"Jason S" <>
23 Aug 2006 10:18:50 -0700
I've been using #import a little bit, with smart pointers that I assign
only once with CreateInstance, e.g.

IFooPtr pfoo;
    HRESULT hr = pfoo.CreateInstance (CLSID_foo, NULL, CLSCTX_ALL);
    if (hr != S_OK)
        // handle accordingly

    // do stuff
catch (_com_error& e)
    // handle accordingly

It seems to work well.

But now I have a case where I need to assign smart pointers upon a
method call and can't quite figure out the right way to do this.
Say I have an object CFoo which has a member IBarPtr pBar, where
IBarPtr is an #import-generated smart pointer. pBar may or may not be
NULL. pBar may be assigned internally with IBarPtr::CreateInstance (via
some other action initiated by one of CFoo's methods), or it may be set
from a method call through CFoo's methods set_Bar(VARIANT v) and
get_Bar(VARIANT *pv).

get_Bar seems easy to implement, something like this:
    // should handle E_POINTER here

    if (pBar == NULL)
        return S_FALSE;
    // if pBar is empty, we leave the variant empty and return S_FALSE

    V_VT(pv) = VT_DISPATCH;
    hr = pBar->QueryInterface(V_DISPATCH(pv));
    return hr;

But I'm having trouble thinking what I need to do for set_Bar. I want
to make sure the following:

1) if the Variant contains VT_EMPTY, I want to release pBar and set it
to NULL, and return S_OK.
2) if the Variant contains something besides VT_EMPTY or VT_UNKNOWN or
VT_DISPATCH, I return an error and pBar is left alone. This is easy.
3) if the Variant does contain a VT_UNKNOWN or VT_DISPATCH, I want to
make sure it supports the IBar interface.
3a) if the Variant does support IBar, then pBar gets assigned a new
value from the variant, and the old value of pBar should be released
properly, and I return S_OK.
3b) if the Variant does not support IBar, then I want to return an
error and leave pBar alone.

Here's my attempt but I'm not sure it does the right thing. See my
questions marked with "???"

        case VT_EMPTY:
            // ??? is this the right way to free pBar?
            pBar = NULL;
            return S_OK;
        case VT_UNKNOWN:
        case VT_DISPATCH:
            try {
                pBar = V_UNKNOWN(&v);
                // ??? will this release the old value of pBar?
            catch (_com_error& e)
                // ??? if I get here, is pBar left alone or is it corrupted?
                // if it's corrupted, how do I test the incoming variant
                // before I assign it to pBar?
                return E_INVALIDARG;
            return S_OK;
            return E_INVALIDARG;

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