Persistent references
An object reference that is independent of any HandleScope is a persistent reference. Where a Local handle only lives as long as the HandleScope in which it was allocated, a Persistent handle remains valid until it is explicitly disposed.
Due to the evolution of the V8 API, it is necessary for NAN to provide a wrapper implementation of the Persistent classes to supply compatibility across the V8 versions supported.
Nan::PersistentBase & v8::PersistentBaseNan::NonCopyablePersistentTraits & v8::NonCopyablePersistentTraitsNan::CopyablePersistentTraits & v8::CopyablePersistentTraitsNan::PersistentNan::GlobalNan::WeakCallbackInfoNan::WeakCallbackType
Also see the V8 Embedders Guide section on Handles and Garbage Collection.
Nan::PersistentBase & v8::PersistentBase
A persistent handle contains a reference to a storage cell in V8 which holds an object value and which is updated by the garbage collector whenever the object is moved. A new storage cell can be created using the constructor or Nan::PersistentBase::Reset(). Existing handles can be disposed using an argument-less Nan::PersistentBase::Reset().
Definition:
(note: this is implemented as Nan::PersistentBase for older versions of V8 and the native v8::PersistentBase is used for newer versions of V8)
template<typename T> class PersistentBase {
public:
/**
* If non-empty, destroy the underlying storage cell
*/
void Reset();
/**
* If non-empty, destroy the underlying storage cell and create a new one with
* the contents of another if it is also non-empty
*/
template<typename S> void Reset(const v8::Local<S> &other);
/**
* If non-empty, destroy the underlying storage cell and create a new one with
* the contents of another if it is also non-empty
*/
template<typename S> void Reset(const PersistentBase<S> &other);
/**
* If non-empty, destroy the underlying storage cell
* IsEmpty() will return true after this call.
*/
bool IsEmpty();
void Empty();
template<typename S> bool operator==(const PersistentBase<S> &that);
template<typename S> bool operator==(const v8::Local<S> &that);
template<typename S> bool operator!=(const PersistentBase<S> &that);
template<typename S> bool operator!=(const v8::Local<S> &that);
/**
* Install a finalization callback on this object.
* NOTE: There is no guarantee as to *when* or even *if* the callback is
* invoked. The invocation is performed solely on a best effort basis.
* As always, GC-based finalization should *not* be relied upon for any
* critical form of resource management! At the moment you can either
* specify a parameter for the callback or the location of two internal
* fields in the dying object.
*/
template<typename P>
void SetWeak(P *parameter,
typename WeakCallbackInfo<P>::Callback callback,
WeakCallbackType type);
void ClearWeak();
/**
* Marks the reference to this object independent. Garbage collector is free
* to ignore any object groups containing this object. Weak callback for an
* independent handle should not assume that it will be preceded by a global
* GC prologue callback or followed by a global GC epilogue callback.
*/
void MarkIndependent() const;
bool IsIndependent() const;
/** Checks if the handle holds the only reference to an object. */
bool IsNearDeath() const;
/** Returns true if the handle's reference is weak. */
bool IsWeak() const
};
See the V8 documentation for PersistentBase for further information.
Tip: To get a v8::Local reference to the original object back from a PersistentBase or Persistent object:
v8::Local<v8::Object> object = Nan::New(persistent);
Nan::NonCopyablePersistentTraits & v8::NonCopyablePersistentTraits
Default traits for Nan::Persistent. This class does not allow use of the a copy constructor or assignment operator. At present kResetInDestructor is not set, but that will change in a future version.
Definition:
(note: this is implemented as Nan::NonCopyablePersistentTraits for older versions of V8 and the native v8::NonCopyablePersistentTraits is used for newer versions of V8)
template<typename T> class NonCopyablePersistentTraits {
public:
typedef Persistent<T, NonCopyablePersistentTraits<T> > NonCopyablePersistent;
static const bool kResetInDestructor = false;
template<typename S, typename M>
static void Copy(const Persistent<S, M> &source,
NonCopyablePersistent *dest);
template<typename O> static void Uncompilable();
};
See the V8 documentation for NonCopyablePersistentTraits for further information.
Nan::CopyablePersistentTraits & v8::CopyablePersistentTraits
A helper class of traits to allow copying and assignment of Persistent. This will clone the contents of storage cell, but not any of the flags, etc..
Definition:
(note: this is implemented as Nan::CopyablePersistentTraits for older versions of V8 and the native v8::NonCopyablePersistentTraits is used for newer versions of V8)
template<typename T>
class CopyablePersistentTraits {
public:
typedef Persistent<T, CopyablePersistentTraits<T> > CopyablePersistent;
static const bool kResetInDestructor = true;
template<typename S, typename M>
static void Copy(const Persistent<S, M> &source,
CopyablePersistent *dest);
};
See the V8 documentation for CopyablePersistentTraits for further information.
Nan::Persistent
A type of PersistentBase which allows copy and assignment. Copy, assignment and destructor behavior is controlled by the traits class M.
Definition:
template<typename T, typename M = NonCopyablePersistentTraits<T> >
class Persistent;
template<typename T, typename M> class Persistent : public PersistentBase<T> {
public:
/**
* A Persistent with no storage cell.
*/
Persistent();
/**
* Construct a Persistent from a v8::Local. When the v8::Local is non-empty, a
* new storage cell is created pointing to the same object, and no flags are
* set.
*/
template<typename S> Persistent(v8::Local<S> that);
/**
* Construct a Persistent from a Persistent. When the Persistent is non-empty,
* a new storage cell is created pointing to the same object, and no flags are
* set.
*/
Persistent(const Persistent &that);
/**
* The copy constructors and assignment operator create a Persistent exactly
* as the Persistent constructor, but the Copy function from the traits class
* is called, allowing the setting of flags based on the copied Persistent.
*/
Persistent &operator=(const Persistent &that);
template <typename S, typename M2>
Persistent &operator=(const Persistent<S, M2> &that);
/**
* The destructor will dispose the Persistent based on the kResetInDestructor
* flags in the traits class. Since not calling dispose can result in a
* memory leak, it is recommended to always set this flag.
*/
~Persistent();
};
See the V8 documentation for Persistent for further information.
Nan::Global
A type of PersistentBase which has move semantics.
template<typename T> class Global : public PersistentBase<T> {
public:
/**
* A Global with no storage cell.
*/
Global();
/**
* Construct a Global from a v8::Local. When the v8::Local is non-empty, a new
* storage cell is created pointing to the same object, and no flags are set.
*/
template<typename S> Global(v8::Local<S> that);
/**
* Construct a Global from a PersistentBase. When the Persistent is non-empty,
* a new storage cell is created pointing to the same object, and no flags are
* set.
*/
template<typename S> Global(const PersistentBase<S> &that);
/**
* Pass allows returning globals from functions, etc.
*/
Global Pass();
};
See the V8 documentation for Global for further information.
Nan::WeakCallbackInfo
Nan::WeakCallbackInfo is used as an argument when setting a persistent reference as weak. You may need to free any external resources attached to the object. It is a mirror of v8:WeakCallbackInfo as found in newer versions of V8.
Definition:
template<typename T> class WeakCallbackInfo {
public:
typedef void (*Callback)(const WeakCallbackInfo<T>& data);
v8::Isolate *GetIsolate() const;
/**
* Get the parameter that was associated with the weak handle.
*/
T *GetParameter() const;
/**
* Get pointer from internal field, index can be 0 or 1.
*/
void *GetInternalField(int index) const;
};
Example usage:
void weakCallback(const WeakCallbackInfo<int> &data) {
int *parameter = data.GetParameter();
delete parameter;
}
Persistent<v8::Object> obj;
int *data = new int(0);
obj.SetWeak(data, callback, WeakCallbackType::kParameter);
See the V8 documentation for WeakCallbackInfo for further information.
Nan::WeakCallbackType
Represents the type of a weak callback.
A weak callback of type kParameter makes the supplied parameter to Nan::PersistentBase::SetWeak available through WeakCallbackInfo::GetParameter.
A weak callback of type kInternalFields uses up to two internal fields at indices 0 and 1 on the Nan::PersistentBase<v8::Object> being made weak.
Note that only v8::Objects and derivatives can have internal fields.
Definition:
enum class WeakCallbackType { kParameter, kInternalFields };