Add CallGetInterface. b=92602 r=jag sr=scc

xpcom/base/nsIInterfaceRequestor.idl

@@ -54,6 +54,20 @@ interface nsIInterfaceRequestor : nsISupports
 #include "nsCOMPtr.h"
 #endif
 
+// a type-safe shortcut for calling the |GetInterface()| member function
+// T must inherit from nsIInterfaceRequestor, but the cast may be ambiguous.
+template <class T, class DestinationType>
+inline
+nsresult
+CallGetInterface( T* aSource, DestinationType** aDestination )
+  {
+    NS_PRECONDITION(aSource, "null parameter");
+    NS_PRECONDITION(aDestination, "null parameter");
+
+    return aSource->GetInterface(NS_GET_IID(DestinationType),
+                                 NS_REINTERPRET_CAST(void**, aDestination));
+  }
+
 class NS_EXPORT nsGetInterface : public nsCOMPtr_helper
   {
     public:

xpcom/tests/TestCallTemplates.cpp

@@ -32,6 +32,7 @@
 #include "nsIComponentManager.h"
 #include "nsIServiceManager.h"
 #include "nsWeakReference.h"
+#include "nsIInterfaceRequestor.h"
 
 #define NS_ITESTSERVICE_IID \
   {0x127b5253, 0x37b1, 0x43c7, \
@@ -101,5 +102,10 @@ int main()
     CallGetService(NS_TEST_SERVICE_CONTRACTID, myShutdownListener,
                    &myITestService);
 
+    /* Test CallGetInterface */
+    nsIInterfaceRequestor *myInterfaceRequestor =
+        NS_STATIC_CAST(nsIInterfaceRequestor*, mySupportsPtr);
+    CallGetInterface(myInterfaceRequestor, &myITestService);
+
     return 0;
 }