Question

I’m slightly puzzled by the lack of documentation on the issue, so I may be completely off track here:

When I allocate memory in order to return an object through an unique pointer whose value I have modified, what allocater should I use?

The documentation says that I can provide MIDL_user_allocate() and MIDL_user_free() and the stub will use these — however that does not make sense in CLSCTX_INPROC_SERVER, as the calling object would need to use (and hence resolve) my allocater.

So, how should I allocate memory here, so that the stub code can properly free the list if the DLL is loaded into SVCHOST, and applications can still use the DLL directly if they so desire.

idl:

HRESULT GetItems([out] DWORD *count, [out, size_is(,count)] ITEM **items);

cpp:

HRESULT STDMETHODCALLTYPE impl::GetBuffer(DWORD *count, ITEM **items)
{
    *count = 0;
    *items = reinterpret_cast<ITEM *>(/* ??? */);
    if(!*items)
        return E_OUTOFMEMORY;
    *count = 5;
    /* fill in items */
    return S_OK;
}

Answer 1

From here:

Out-parameters must be allocated by the one called; they are freed by the caller using the standard COM task memory allocator.

where COM task memory allocator is either the set of IMalloc methods or the set of CoTaskMemAlloc()/CoTaskMemRealloc()/CoTaskMemFree() functions that provide the same functionality.

The midl_user-*() functions you mention are used for RPC memory management. You need them in case you deal with RPC interfaces, not COM interfaces.