+/// TrackingVH - This is a value handle that tracks a Value (or Value subclass),
+/// even across RAUW operations.
+///
+/// TrackingVH is designed for situations where a client needs to hold a handle
+/// to a Value (or subclass) across some operations which may move that value,
+/// but should never destroy it or replace it with some unacceptable type.
+///
+/// It is an error to do anything with a TrackingVH whose value has been
+/// destroyed, except to destruct it.
+///
+/// It is an error to attempt to replace a value with one of a type which is
+/// incompatible with any of its outstanding TrackingVHs.
+template<typename ValueTy>
+class TrackingVH : public ValueHandleBase {
+ void CheckValidity() const {
+ Value *VP = ValueHandleBase::getValPtr();
+
+ // Null is always ok.
+ if (!VP)
+ return;
+
+ // Check that this value is valid (i.e., it hasn't been deleted). We
+ // explicitly delay this check until access to avoid requiring clients to be
+ // unnecessarily careful w.r.t. destruction.
+ assert(ValueHandleBase::isValid(VP) && "Tracked Value was deleted!");
+
+ // Check that the value is a member of the correct subclass. We would like
+ // to check this property on assignment for better debugging, but we don't
+ // want to require a virtual interface on this VH. Instead we allow RAUW to
+ // replace this value with a value of an invalid type, and check it here.
+ assert(isa<ValueTy>(VP) &&
+ "Tracked Value was replaced by one with an invalid type!");
+ }
+
+ ValueTy *getValPtr() const {
+ CheckValidity();
+ return static_cast<ValueTy*>(ValueHandleBase::getValPtr());
+ }
+ void setValPtr(ValueTy *P) {
+ CheckValidity();
+ ValueHandleBase::operator=(GetAsValue(P));
+ }
+
+ // Convert a ValueTy*, which may be const, to the type the base
+ // class expects.
+ static Value *GetAsValue(Value *V) { return V; }
+ static Value *GetAsValue(const Value *V) { return const_cast<Value*>(V); }
+
+public:
+ TrackingVH() : ValueHandleBase(Tracking) {}
+ TrackingVH(ValueTy *P) : ValueHandleBase(Tracking, P) {}
+ TrackingVH(const TrackingVH &RHS) : ValueHandleBase(Tracking, RHS) {}
+
+ operator ValueTy*() const {
+ return getValPtr();
+ }
+
+ ValueTy *operator=(ValueTy *RHS) {
+ setValPtr(RHS);
+ return getValPtr();
+ }
+ ValueTy *operator=(const TrackingVH<ValueTy> &RHS) {
+ setValPtr(RHS.getValPtr());
+ return getValPtr();
+ }
+
+ ValueTy *operator->() const { return getValPtr(); }
+ ValueTy &operator*() const { return *getValPtr(); }
+};
+
+// Specialize simplify_type to allow TrackingVH to participate in
+// dyn_cast, isa, etc.
+template<typename From> struct simplify_type;
+template<> struct simplify_type<const TrackingVH<Value> > {
+ typedef Value* SimpleType;
+ static SimpleType getSimplifiedValue(const TrackingVH<Value> &AVH) {
+ return static_cast<Value *>(AVH);
+ }
+};
+template<> struct simplify_type<TrackingVH<Value> >
+ : public simplify_type<const TrackingVH<Value> > {};
+