void SetWeak(P *parameter,
typename WeakCallbackInfo::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 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)_
```c++
template class NonCopyablePersistentTraits {
public:
typedef Persistent > NonCopyablePersistent;
static const bool kResetInDestructor = false;
template
static void Copy(const Persistent &source,
NonCopyablePersistent *dest);
template 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.
### 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
class CopyablePersistentTraits {
public:
typedef Persistent > CopyablePersistent;
static const bool kResetInDestructor = true;
template
static void Copy(const Persistent &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.
### 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 >
class Persistent;
template class Persistent : public PersistentBase {
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 Persistent(v8::Local 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
Persistent &operator=(const Persistent &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.
### Nan::Global
A type of `PersistentBase` which has move semantics.
```c++
template class Global : public PersistentBase {
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 Global(v8::Local 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 Global(const PersistentBase &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.
### 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 class WeakCallbackInfo {
public:
typedef void (*Callback)(const WeakCallbackInfo& 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 &data) {
int *parameter = data.GetParameter();
delete parameter;
}
Persistent 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.
### 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` being made weak.
Note that only `v8::Object`s and derivatives can have internal fields.
Definition:
```c++
enum class WeakCallbackType { kParameter, kInternalFields };
```