AngelScript: Object handles

# Object handles

In AngelScript an object handle is a reference counted pointer to an object. In the scripts they are used to pass the objects around by reference instead of by value. Depending on how an application type is registered the type will support handles.

See also:
  Registering an object type, Object handles in the script language

## Managing the reference counter in functions

Whenever the object handle is passed by value from the application to the script engine, or vice versa its reference should be accounted for. This means that the application must release any object handles it receives as parameters when it no longer needs them, it also means that the application must increase the reference counter for any object handle being returned to the script engine. Note that this is not the same for the generic calling convention where AngelScript automatically takes care of most of work.

A function that creates an object and returns it to the script engine might look like this:

    // Registered as "obj@ CreateObject()"
    obj *CreateObject()
    {
      // The constructor already initializes the ref count to 1
      return new obj();
    }

A function that receives an object handle from the script and stores it in a global variable might look like this:

    // Registered as "void StoreObject(obj@)"
    obj *o = 0;
    void StoreObject(obj *newO)
    {
      // Release the old object handle
      if( o ) o->Release();

      // Store the new object handle
      o = newO;
    }

A function that retrieves a previously stored object handle might look like this:

    // Registered as "obj@ RetrieveObject()"
    obj *RetrieveObject()
    {
      // Increase the reference counter to account for the returned handle
      if( o ) o->AddRef();

      // It is ok to return null if there is no previous handle stored
      return o;
    }

A function that receives an object handle in the parameter, but doesn't store it looks like this:

    // Registered as "void DoSomething(obj@)"
    void DoSomething(obj *o)
    {
      // When finished with the object it must be released
      if( o ) o->Release();
    }

## Auto handles can make it easier

The application can use auto handles (@+) to alleviate some of the work of managing the reference counter. When registering the function or method with AngelScript, add a plus sign to the object handles that AngelScript should automatically manage. For parameters AngelScript will then release the reference after the function returns, and for the return value AngelScript will increase the reference on the returned pointer. The reference for the returned value is increased before the parameters are released, so it is possible to have the function return one of the parameters.

    // Registered as "obj@+ ChooseObj(obj@+, obj@+)"
    obj *ChooseObj(obj *a, obj *b)
    {
      // Because of the auto handles AngelScript will
      // automatically manage the reference counters
      return some_condition ? a : b;
    }

However, it is not recommended to use this feature unless you can't change the functions you want to register to properly handle the reference counters. When using the auto handles, AngelScript needs to process all of the handles which causes an extra overhead when calling application registered functions.

The auto handles does not affect the behaviour of the handles when the generic calling convention is used.