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

Paavo Helde <>
Fri, 08 Jul 2011 00:39:59 -0500
Christopher <> wrote in news:8f3b364d-798b-4a27-ba68-

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;
3) IUnknown * result = NULL;

This is my attempt:

if( ppRS && *ppRS )
   (*ppRS) = NULL;


// Do stuff

// If failed

// 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.


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