Edit File: persistent.md
## 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. - <a href="#api_nan_persistent_base"><b><code>Nan::PersistentBase & v8::PersistentBase</code></b></a> - <a href="#api_nan_non_copyable_persistent_traits"><b><code>Nan::NonCopyablePersistentTraits & v8::NonCopyablePersistentTraits</code></b></a> - <a href="#api_nan_copyable_persistent_traits"><b><code>Nan::CopyablePersistentTraits & v8::CopyablePersistentTraits</code></b></a> - <a href="#api_nan_persistent"><b><code>Nan::Persistent</code></b></a> - <a href="#api_nan_global"><b><code>Nan::Global</code></b></a> - <a href="#api_nan_weak_callback_info"><b><code>Nan::WeakCallbackInfo</code></b></a> - <a href="#api_nan_weak_callback_type"><b><code>Nan::WeakCallbackType</code></b></a> Also see the V8 Embedders Guide section on [Handles and Garbage Collection](https://developers.google.com/v8/embed#handles). <a name="api_nan_persistent_base"></a> ### 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)_ ```c++ 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); /** Returns true if the handle is empty. */ bool IsEmpty() const; /** * If non-empty, destroy the underlying storage cell * IsEmpty() will return true after this call. */ 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`](https://v8docs.nodesource.com/node-8.16/d4/dca/classv8_1_1_persistent_base.html) for further information. **Tip:** To get a `v8::Local` reference to the original object back from a `PersistentBase` or `Persistent` object: ```c++ v8::Local<v8::Object> object = Nan::New(persistent); ``` <a name="api_nan_non_copyable_persistent_traits"></a> ### 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)_ ```c++ 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`](https://v8docs.nodesource.com/node-8.16/de/d73/classv8_1_1_non_copyable_persistent_traits.html) for further information. <a name="api_nan_copyable_persistent_traits"></a> ### 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)_ ```c++ 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`](https://v8docs.nodesource.com/node-8.16/da/d5c/structv8_1_1_copyable_persistent_traits.html) for further information. <a name="api_nan_persistent"></a> ### Nan::Persistent A type of `PersistentBase` which allows copy and assignment. Copy, assignment and destructor behavior is controlled by the traits class `M`. Definition: ```c++ 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`](https://v8docs.nodesource.com/node-8.16/d2/d78/classv8_1_1_persistent.html) for further information. <a name="api_nan_global"></a> ### Nan::Global A type of `PersistentBase` which has move semantics. ```c++ 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`](https://v8docs.nodesource.com/node-8.16/d5/d40/classv8_1_1_global.html) for further information. <a name="api_nan_weak_callback_info"></a> ### 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: ```c++ 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: ```c++ 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`](https://v8docs.nodesource.com/node-8.16/d8/d06/classv8_1_1_weak_callback_info.html) for further information. <a name="api_nan_weak_callback_type"></a> ### 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::Object`s and derivatives can have internal fields. Definition: ```c++ enum class WeakCallbackType { kParameter, kInternalFields }; ```
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