Re: pointer to pointer intialize to NULL but still point to NULL

From:

Paavo Helde <myfirstname@osa.pri.ee>

Newsgroups:

comp.lang.c++

Date:

Fri, 08 Jul 2011 00:39:59 -0500

Message-ID:

<Xns9F1C582906704myfirstnameosapriee@216.196.109.131>

Christopher <cpisz@austin.rr.com> wrote in news:8f3b364d-798b-4a27-ba68-
3e2beb3a51ef@gh5g2000vbb.googlegroups.com:

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.

Too bad as in C++ one could make use of smartpointers instead or error-
prone raw pointers (IUnknownPtr in this case).

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?

1) Foo(NULL);
2) IUnknown * result = somePointerToAlreadyInstantiatedObject;
     Foo(&result);
3) IUnknown * result = NULL;
     Foo(&result);

This is my attempt:

if( ppRS && *ppRS )
{
   (*ppRS)->Release();
   (*ppRS) = NULL;
}

// Do stuff

// If failed
return;

// If successfull
*ppRS = pointerToInterface;

Should probably be
if (ppRS) {
*ppRS = pointerToInterface;
// plus some AddRef() or something
}

Is that correct?

Depends on the details of how to handle COM pointers, I am not too
familiar with them.

hth
Paavo