# Class property and descriptor
Property descriptor for use with `Napi::ObjectWrap::DefineClass()`.
This is different from the standalone `Napi::PropertyDescriptor` because it is
specific to each `Napi::ObjectWrap<T>` subclass.
This prevents using descriptors from a different class when defining a new class
(preventing the callbacks from having incorrect `this` pointers).
## Example
```cpp
#include <napi.h>
class Example : public Napi::ObjectWrap<Example> {
public:
static Napi::Object Init(Napi::Env env, Napi::Object exports);
Example(const Napi::CallbackInfo &info);
private:
static Napi::FunctionReference constructor;
double _value;
Napi::Value GetValue(const Napi::CallbackInfo &info);
void SetValue(const Napi::CallbackInfo &info, const Napi::Value &value);
};
Napi::Object Example::Init(Napi::Env env, Napi::Object exports) {
Napi::Function func = DefineClass(env, "Example", {
// Register a class instance accessor with getter and setter functions.
InstanceAccessor("value", &Example::GetValue, &Example::SetValue),
// We can also register a readonly accessor by passing nullptr as the setter.
InstanceAccessor("readOnlyProp", &Example::GetValue, nullptr)
});
constructor = Napi::Persistent(func);
constructor.SuppressDestruct();
exports.Set("Example", func);
return exports;
}
Example::Example(const Napi::CallbackInfo &info) : Napi::ObjectWrap<Example>(info) {
Napi::Env env = info.Env();
// ...
Napi::Number value = info[0].As<Napi::Number>();
this->_value = value.DoubleValue();
}
Napi::FunctionReference Example::constructor;
Napi::Value Example::GetValue(const Napi::CallbackInfo &info) {
Napi::Env env = info.Env();
return Napi::Number::New(env, this->_value);
}
void Example::SetValue(const Napi::CallbackInfo &info, const Napi::Value &value) {
Napi::Env env = info.Env();
// ...
Napi::Number arg = value.As<Napi::Number>();
this->_value = arg.DoubleValue();
}
// Initialize native add-on
Napi::Object Init (Napi::Env env, Napi::Object exports) {
Example::Init(env, exports);
return exports;
}
// Register and initialize native add-on
NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)
```
The above code can be used from JavaScript as follows:
```js
'use strict';
const { Example } = require('bindings')('addon');
const example = new Example(11);
console.log(example.value);
// It prints 11
example.value = 19;
console.log(example.value);
// It prints 19
example.readOnlyProp = 500;
console.log(example.readOnlyProp);
// Unchanged. It prints 19
```
## Methods
### Constructor
Creates new instance of `Napi::ClassPropertyDescriptor` descriptor object.
```cpp
Napi::ClassPropertyDescriptor(napi_property_descriptor desc) : _desc(desc) {}
```
- `[in] desc`: The `napi_property_descriptor`
Returns new instance of `Napi::ClassPropertyDescriptor` that is used as property descriptor
inside the `Napi::ObjectWrap<T>` class.
### Operator
```cpp
operator napi_property_descriptor&() { return _desc; }
```
Returns the original N-API `napi_property_descriptor` wrapped inside the `Napi::ClassPropertyDescriptor`
```cpp
operator const napi_property_descriptor&() const { return _desc; }
```
Returns the original N-API `napi_property_descriptor` wrapped inside the `Napi::ClassPropertyDescriptor`