Found the answer on the coding standard page :
nullptr should be used instead of the
C-style NULL macro in all cases. One
exception to this is that nullptr in
C++/CX builds (such as for Xbox One)
is actually the managed null reference
type. It is mostly compatible with
nullptr from native C++ except in its
type and some template instantiation
contexts, and so you should use the
TYPE_OF_NULLPTR macro instead of the
more usual decltype(nullptr) for
compatibility.