shlwapi: Use an iface instead of an vtbl pointer in threadref.

dlls/shlwapi/thread.c

@@ -122,13 +122,18 @@ typedef struct tagSHLWAPI_THREAD_INFO
 
 typedef struct
 {
-  const IUnknownVtbl* lpVtbl;
+  IUnknown IUnknown_iface;
   LONG  *ref;
 } threadref;
 
+static inline threadref *impl_from_IUnknown(IUnknown *iface)
+{
+  return CONTAINING_RECORD(iface, threadref, IUnknown_iface);
+}
+
 static HRESULT WINAPI threadref_QueryInterface(IUnknown *iface, REFIID riid, LPVOID *ppvObj)
 {
-  threadref * This = (threadref *)iface;
+  threadref * This = impl_from_IUnknown(iface);
 
   TRACE("(%p, %s, %p)\n", This, debugstr_guid(riid), ppvObj);
 
@@ -149,7 +154,7 @@ static HRESULT WINAPI threadref_QueryInterface(IUnknown *iface, REFIID riid, LPV
 
 static ULONG WINAPI threadref_AddRef(IUnknown *iface)
 {
-  threadref * This = (threadref *)iface;
+  threadref * This = impl_from_IUnknown(iface);
 
   TRACE("(%p)\n", This);
   return InterlockedIncrement(This->ref);
@@ -158,7 +163,7 @@ static ULONG WINAPI threadref_AddRef(IUnknown *iface)
 static ULONG WINAPI threadref_Release(IUnknown *iface)
 {
   LONG refcount;
-  threadref * This = (threadref *)iface;
+  threadref * This = impl_from_IUnknown(iface);
 
   TRACE("(%p)\n", This);
 
@@ -199,11 +204,11 @@ HRESULT WINAPI SHCreateThreadRef(LONG *lprefcount, IUnknown **lppUnknown)
     return E_INVALIDARG;
 
   This = HeapAlloc(GetProcessHeap(), 0, sizeof(threadref));
-  This->lpVtbl = &threadref_vt;
+  This->IUnknown_iface.lpVtbl = &threadref_vt;
   This->ref = lprefcount;
 
   *lprefcount = 1;
-  *lppUnknown = (IUnknown *) This;
+  *lppUnknown = &This->IUnknown_iface;
   TRACE("=> returning S_OK with %p\n", This);
   return S_OK;
 }