xref: /napi_generator/examples/napitutorials/doc/apiguide/object_reference.md

# Object Reference

`Napi::ObjectReference` is a subclass of [`Napi::Reference`](reference.md), and is equivalent to an instance of `Napi::Reference<Object>`. This means that a `Napi::ObjectReference` holds a [`Napi::Object`](object.md), and a count of the number of references to that Object. When the count is greater than 0, an ObjectReference is not eligible for garbage collection. This ensures that the Object being held as a value of the ObjectReference will remain accessible, even if the original Object no longer is. However, ObjectReference is unique from a Reference since properties can be set and get to the Object itself that can be accessed through the ObjectReference.

For more general information on references, please consult [`Napi::Reference`](reference.md).

## Example
```cpp
#include <napi.h>

using namespace Napi;

void Init(Env env) {

    // Create an empty ObjectReference that has an initial reference count of 2.
    ObjectReference obj_ref = Reference<Object>::New(Object::New(env), 2);

    // Set a couple of different properties on the reference.
    obj_ref.Set("hello", String::New(env, "world"));
    obj_ref.Set(42, "The Answer to Life, the Universe, and Everything");

    // Get the properties using the keys.
    Value val1 = obj_ref.Get("hello");
    Value val2 = obj_ref.Get(42);
}
```

## Methods

### Initialization

```cpp
static Napi::ObjectReference Napi::ObjectReference::New(const Napi::Object& value, uint32_t initialRefcount = 0);
```

* `[in] value`: The `Napi::Object` which is to be referenced.

* `[in] initialRefcount`: The initial reference count.

Returns the newly created reference.

```cpp
static Napi::ObjectReference Napi::Weak(const Napi::Object& value);
```

Creates a "weak" reference to the value, in that the initial count of number of references is set to 0.

* `[in] value`: The value which is to be referenced.

Returns the newly created reference.

```cpp
static Napi::ObjectReference Napi::Persistent(const Napi::Object& value);
```

Creates a "persistent" reference to the value, in that the initial count of number of references is set to 1.

* `[in] value`: The value which is to be referenced.

Returns the newly created reference.

### Empty Constructor

```cpp
Napi::ObjectReference::ObjectReference();
```

Returns a new _empty_ `Napi::ObjectReference` instance.

### Constructor

```cpp
Napi::ObjectReference::ObjectReference(napi_env env, napi_value value);
```

* `[in] env`: The `napi_env` environment in which to construct the `Napi::ObjectReference` object.

* `[in] value`: The Node-API primitive value to be held by the `Napi::ObjectReference`.

Returns the newly created reference.

### Set
```cpp
bool Napi::ObjectReference::Set(___ key, ___ value);
```

* `[in] key`: The name for the property being assigned.

* `[in] value`: The value being assigned to the property.

The `key` can be any of the following types:
- `const char*`
- `const std::string`
- `uint32_t`

The `value` can be any of the following types:
- `napi_value`
- `Napi::Value`
- `const char*`
- `bool`
- `double`

### Get

```cpp
Napi::Value Napi::ObjectReference::Get(___ key) const;
```

* `[in] key`: The name of the property to return the value for.

Returns the [`Napi::Value`](value.md) associated with the key property. Returns NULL if no such key exists.

The `key` can be any of the following types:
- `const char*`
- `const std::string`
- `uint32_t`