Can't figure out whats wrong here...

gistfile1.cpp

//I often want to typedef shared_pts to classes like:
typedef std::shared_ptr<CThing> TThingPtr;

//Figured it'd be easy to wrap this in a macro like:
#define SHARED_PTR(Name) typedef std::shared_ptr<C##Name> T##NamePtr;

//but:
SHARED_PTR(Thing)
TThingPtr p; //ERROR: Unknown type TThingPtr

//and:
SHARED_PTR(Thing1)
SHARED_PTR(Thing2) //ERROR: Typedef redefinition with different types ('shared_ptr<class CThing1>' vs 'shared_ptr<class CThing2>')

//???