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297 lines
11 KiB
Markdown
297 lines
11 KiB
Markdown
## Persistent references
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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.
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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.
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- <a href="#api_nan_persistent_base"><b><code>Nan::PersistentBase & v8::PersistentBase</code></b></a>
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- <a href="#api_nan_non_copyable_persistent_traits"><b><code>Nan::NonCopyablePersistentTraits & v8::NonCopyablePersistentTraits</code></b></a>
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- <a href="#api_nan_copyable_persistent_traits"><b><code>Nan::CopyablePersistentTraits & v8::CopyablePersistentTraits</code></b></a>
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- <a href="#api_nan_persistent"><b><code>Nan::Persistent</code></b></a>
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- <a href="#api_nan_global"><b><code>Nan::Global</code></b></a>
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- <a href="#api_nan_weak_callback_info"><b><code>Nan::WeakCallbackInfo</code></b></a>
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- <a href="#api_nan_weak_callback_type"><b><code>Nan::WeakCallbackType</code></b></a>
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Also see the V8 Embedders Guide section on [Handles and Garbage Collection](https://developers.google.com/v8/embed#handles).
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<a name="api_nan_persistent_base"></a>
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### Nan::PersistentBase & v8::PersistentBase
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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()`.
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Definition:
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_(note: this is implemented as `Nan::PersistentBase` for older versions of V8 and the native `v8::PersistentBase` is used for newer versions of V8)_
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```c++
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template<typename T> class PersistentBase {
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public:
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/**
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* If non-empty, destroy the underlying storage cell
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*/
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void Reset();
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/**
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* If non-empty, destroy the underlying storage cell and create a new one with
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* the contents of another if it is also non-empty
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*/
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template<typename S> void Reset(const v8::Local<S> &other);
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/**
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* If non-empty, destroy the underlying storage cell and create a new one with
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* the contents of another if it is also non-empty
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*/
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template<typename S> void Reset(const PersistentBase<S> &other);
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/** Returns true if the handle is empty. */
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bool IsEmpty() const;
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/**
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* If non-empty, destroy the underlying storage cell
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* IsEmpty() will return true after this call.
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*/
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void Empty();
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template<typename S> bool operator==(const PersistentBase<S> &that);
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template<typename S> bool operator==(const v8::Local<S> &that);
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template<typename S> bool operator!=(const PersistentBase<S> &that);
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template<typename S> bool operator!=(const v8::Local<S> &that);
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/**
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* Install a finalization callback on this object.
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* NOTE: There is no guarantee as to *when* or even *if* the callback is
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* invoked. The invocation is performed solely on a best effort basis.
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* As always, GC-based finalization should *not* be relied upon for any
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* critical form of resource management! At the moment you can either
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* specify a parameter for the callback or the location of two internal
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* fields in the dying object.
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*/
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template<typename P>
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void SetWeak(P *parameter,
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typename WeakCallbackInfo<P>::Callback callback,
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WeakCallbackType type);
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void ClearWeak();
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/**
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* Marks the reference to this object independent. Garbage collector is free
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* to ignore any object groups containing this object. Weak callback for an
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* independent handle should not assume that it will be preceded by a global
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* GC prologue callback or followed by a global GC epilogue callback.
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*/
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void MarkIndependent() const;
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bool IsIndependent() const;
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/** Checks if the handle holds the only reference to an object. */
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bool IsNearDeath() const;
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/** Returns true if the handle's reference is weak. */
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bool IsWeak() const
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};
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```
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See the V8 documentation for [`PersistentBase`](https://v8docs.nodesource.com/node-8.11/d4/dca/classv8_1_1_persistent_base.html) for further information.
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**Tip:** To get a `v8::Local` reference to the original object back from a `PersistentBase` or `Persistent` object:
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```c++
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v8::Local<v8::Object> object = Nan::New(persistent);
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```
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<a name="api_nan_non_copyable_persistent_traits"></a>
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### Nan::NonCopyablePersistentTraits & v8::NonCopyablePersistentTraits
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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.
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Definition:
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_(note: this is implemented as `Nan::NonCopyablePersistentTraits` for older versions of V8 and the native `v8::NonCopyablePersistentTraits` is used for newer versions of V8)_
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```c++
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template<typename T> class NonCopyablePersistentTraits {
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public:
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typedef Persistent<T, NonCopyablePersistentTraits<T> > NonCopyablePersistent;
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static const bool kResetInDestructor = false;
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template<typename S, typename M>
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static void Copy(const Persistent<S, M> &source,
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NonCopyablePersistent *dest);
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template<typename O> static void Uncompilable();
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};
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```
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See the V8 documentation for [`NonCopyablePersistentTraits`](https://v8docs.nodesource.com/node-8.11/de/d73/classv8_1_1_non_copyable_persistent_traits.html) for further information.
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<a name="api_nan_copyable_persistent_traits"></a>
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### Nan::CopyablePersistentTraits & v8::CopyablePersistentTraits
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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..
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Definition:
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_(note: this is implemented as `Nan::CopyablePersistentTraits` for older versions of V8 and the native `v8::NonCopyablePersistentTraits` is used for newer versions of V8)_
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```c++
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template<typename T>
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class CopyablePersistentTraits {
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public:
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typedef Persistent<T, CopyablePersistentTraits<T> > CopyablePersistent;
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static const bool kResetInDestructor = true;
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template<typename S, typename M>
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static void Copy(const Persistent<S, M> &source,
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CopyablePersistent *dest);
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};
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```
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See the V8 documentation for [`CopyablePersistentTraits`](https://v8docs.nodesource.com/node-8.11/da/d5c/structv8_1_1_copyable_persistent_traits.html) for further information.
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<a name="api_nan_persistent"></a>
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### Nan::Persistent
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A type of `PersistentBase` which allows copy and assignment. Copy, assignment and destructor behavior is controlled by the traits class `M`.
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Definition:
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```c++
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template<typename T, typename M = NonCopyablePersistentTraits<T> >
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class Persistent;
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template<typename T, typename M> class Persistent : public PersistentBase<T> {
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public:
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/**
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* A Persistent with no storage cell.
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*/
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Persistent();
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/**
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* Construct a Persistent from a v8::Local. When the v8::Local is non-empty, a
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* new storage cell is created pointing to the same object, and no flags are
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* set.
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*/
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template<typename S> Persistent(v8::Local<S> that);
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/**
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* Construct a Persistent from a Persistent. When the Persistent is non-empty,
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* a new storage cell is created pointing to the same object, and no flags are
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* set.
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*/
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Persistent(const Persistent &that);
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/**
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* The copy constructors and assignment operator create a Persistent exactly
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* as the Persistent constructor, but the Copy function from the traits class
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* is called, allowing the setting of flags based on the copied Persistent.
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*/
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Persistent &operator=(const Persistent &that);
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template <typename S, typename M2>
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Persistent &operator=(const Persistent<S, M2> &that);
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/**
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* The destructor will dispose the Persistent based on the kResetInDestructor
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* flags in the traits class. Since not calling dispose can result in a
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* memory leak, it is recommended to always set this flag.
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*/
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~Persistent();
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};
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```
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See the V8 documentation for [`Persistent`](https://v8docs.nodesource.com/node-8.11/d2/d78/classv8_1_1_persistent.html) for further information.
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<a name="api_nan_global"></a>
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### Nan::Global
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A type of `PersistentBase` which has move semantics.
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```c++
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template<typename T> class Global : public PersistentBase<T> {
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public:
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/**
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* A Global with no storage cell.
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*/
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Global();
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/**
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* Construct a Global from a v8::Local. When the v8::Local is non-empty, a new
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* storage cell is created pointing to the same object, and no flags are set.
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*/
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template<typename S> Global(v8::Local<S> that);
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/**
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* Construct a Global from a PersistentBase. When the Persistent is non-empty,
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* a new storage cell is created pointing to the same object, and no flags are
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* set.
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*/
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template<typename S> Global(const PersistentBase<S> &that);
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/**
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* Pass allows returning globals from functions, etc.
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*/
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Global Pass();
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};
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```
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See the V8 documentation for [`Global`](https://v8docs.nodesource.com/node-8.11/d5/d40/classv8_1_1_global.html) for further information.
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<a name="api_nan_weak_callback_info"></a>
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### Nan::WeakCallbackInfo
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`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.
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Definition:
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```c++
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template<typename T> class WeakCallbackInfo {
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public:
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typedef void (*Callback)(const WeakCallbackInfo<T>& data);
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v8::Isolate *GetIsolate() const;
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/**
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* Get the parameter that was associated with the weak handle.
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*/
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T *GetParameter() const;
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/**
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* Get pointer from internal field, index can be 0 or 1.
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*/
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void *GetInternalField(int index) const;
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};
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```
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Example usage:
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```c++
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void weakCallback(const WeakCallbackInfo<int> &data) {
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int *parameter = data.GetParameter();
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delete parameter;
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}
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Persistent<v8::Object> obj;
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int *data = new int(0);
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obj.SetWeak(data, callback, WeakCallbackType::kParameter);
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```
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See the V8 documentation for [`WeakCallbackInfo`](https://v8docs.nodesource.com/node-8.11/d8/d06/classv8_1_1_weak_callback_info.html) for further information.
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<a name="api_nan_weak_callback_type"></a>
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### Nan::WeakCallbackType
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Represents the type of a weak callback.
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A weak callback of type `kParameter` makes the supplied parameter to `Nan::PersistentBase::SetWeak` available through `WeakCallbackInfo::GetParameter`.
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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.
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Note that only `v8::Object`s and derivatives can have internal fields.
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Definition:
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```c++
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enum class WeakCallbackType { kParameter, kInternalFields };
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```
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