NULL in/out parameter in COM

Question

My COM object has a method, in IDL defined as:

HRESULT _stdcall my_method( [in] long value, [in, out] IAnotherObject **result );

Is the caller allowed to call this method like so:

ptr->my_method(1234, NULL);

or would the caller be violating the COM specification in doing so?

In other words, should my code which implements this function check result != NULL before proceeding; and if so, does the COM spec require that I return E_INVALIDARG or E_POINTER or something; or would it be acceptable for my function to continue on and return 0 without allocating an AnotherObject ?

My object is intended to be Automation-compatible; and it uses standard marshaling.

Note: Question edited since my original text. After posting this question I discovered that optional should only be used for VARIANT, and an [in, out] parameter where result != NULL but *result == NULL should be treated like an out parameter, and I must allocate an object.

Answer 1

The Rules of the Component Object Model say:

The in-out parameters are initially allocated by the caller, then freed and re-allocated by the callee if necessary. As with out parameters, the caller is responsible for freeing the final returned value. The standard COM memory allocator must be used.

So, passing NULL is a violation. You can see several violations of COM rules even in Microsoft's own interfaces, such as IDispatch, where a few [out] parameters accept NULL, but that's because they have remote interface methods (see [local] and [call_as]) that most probably allocate the needed memory when crossing apartments, or otherwise perform custom marshaling.

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EDIT: To further answer your questions.

I recommend you check for NULL [out] (or [in, out]) arguments and return E_POINTER when you find one. This will allow you to catch/detect most common errors early instead of raising an access violation.