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## 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);
/**
* 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`](https://v8docs.nodesource.com/io.js-3.3/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/io.js-3.3/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/io.js-3.3/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/io.js-3.3/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/io.js-3.3/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/io.js-3.3/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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