Re: pointer to pointer intialize to NULL but still point to NULL
On 8 Jul., Christopher wrote:
void Foo(IUnknown ** ppMicrosoftsOut)
{
// If the operation fails param is NULL
// Otherwise param is a valid pointer to object
}
I can't change the function prototype, because there is already a lot
of code that depends on it. How do I initialize the parameter properly
in the function body, such that all of the following calls from the
client yield the expected results?
Note that this is a MS COM specific question that is off-topic here.
You should try microsoft.public.vc.mfc, that is the only group that
shows at least some activity (the dedicated COM groups were never very
active, even back in the time when MS had not declared its groups
dead) or use the social.msdn site (they dropped their newsgroup in
favour of this credit-driven web-based pseudo-newsgroup).
1) Foo(NULL);
That is a case that you should not be bothered with because the rules
of COM actually forbid that an [out]-parameter is initialized with a
NULL pointer. The correct answer to such a case would be
HRESULT Foo (IUnknown ** ppMicrosoftsOut)
{
if (!ppMicrosoftsOut)
return E_POINTER;
// whatever.
return S_OK;
}
Note that you would have to make the method more COM-compliant by
changing the return type into an HRESULT.
2) IUnknown * result = somePointerToAlreadyInstantiatedObject;
Foo(&result);
3) IUnknown * result = NULL;
Foo(&result);
This depends on whether the parameter ppMicrosoftsOut is declared as
[in, out] or just as [out] parameter. Pointer parameters that are
[out] and not also [in] should be initialized with a NULL pointer be
the caller, so you could write:
HRESULT Foo (IUnknown ** ppMicrosoftsOut)
{
if (!ppMicrosoftsOut)
return E_POINTER;
ASSERT (!*ppMicrosoftsOut);
// whatever.
return S_OK;
}
This is my attempt:
if( ppRS && *ppRS )
{
(*ppRS)->Release();
(*ppRS) = NULL;
}
// Do stuff
// If failed
return;
// If successfull
*ppRS = pointerToInterface;
Is that correct?
Yes, that should always work (for both the [out] and the [in, out]
case).
I strongly recommend that you use the automatically generated wrappers
that you get when you #import type libraries (you'll get those .tlh
and .tli files). They use a smart pointer _com_ptr_t that relieve you
from having to manage the refcount of COM pointers.
Regards,
Stuart