AngelScript: Garbage collected objects

# Garbage collected objects

Reference counting as memory management has a drawback in that it is difficult to detect circular references when determining dead objects. AngelScript allows the application to register types with special behaviours to support the garbage collection for detecting circular references. These behaviours make the class a bit more complex, but you should only have to register them for a few types, e.g. generic container classes.

    // Registering the garbage collected reference type
    r = engine->RegisterObjectType("ref", 0, asOBJ_REF | asOBJ_GC); assert( r >= 0 );

The difference between the garbage collected and non-garbage collected types is in the addref and release behaviours, the class constructor, and the extra support behaviours.

See also:
  The dictionary add-on for an example of a garbage collected object

## GC support behaviours

The GC determines when objects should be destroyed by counting the references it can follow for each object. If the GC can see all references that points to an object, it knows that the object is part of a circular reference. If all the objects involved in that circular reference have no outside references it means that they should be destroyed.

The process of determining the dead objects uses the first for of the behaviours below, while the destruction of the objects is done by forcing the release of the object's references.

    void CGCRef::SetGCFlag()
    {
        // Set the gc flag as the high bit in the reference counter
        refCount |= 0x80000000;
    }

    bool CGCRef::GetGCFlag()
    {
        // Return the gc flag
        return (refCount & 0x80000000) ? true : false;
    }

    int CGCRef::GetRefCount()
    {
        // Return the reference count, without the gc flag
        return (refCount & 0x7FFFFFFF);
    }

    void CGCRef::EnumReferences()
    {
        // Call the engine::GCEnumCallback for all references to other objects held
        engine->GCEnumCallback(myref);
    }

    void CGCRef::ReleaseAllReferences()
    {
        // When we receive this call, we are as good as dead, but
        // the garbage collector will still hold a references to us, so we
        // cannot just delete ourself yet. Just free all references to other
        // objects that we hold
        if( myref )
        {
            myref->Release();
            myref = 0;
        }
    }

    // Register the GC support behaviours
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_SETGCFLAG, "void f()", asMETHOD(CGCRef,SetGCFlag), asCALL_THISCALL); assert( r >= 0 );
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_GETGCFLAG, "bool f()", asMETHOD(CGCRef,GetGCFlag), asCALL_THISCALL); assert( r >= 0 );
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_GETREFCOUNT, "int f()", asMETHOD(CGCRef,GetRefCount), asCALL_THISCALL); assert( r >= 0 );
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_ENUMREFS, "void f(int&in)", asMETHOD(CGCRef,EnumReferences), asCALL_THISCALL); assert( r >= 0 );
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_RELEASEREFS, "void f(int&in)", asMETHOD(CGCRef,ReleaseAllReferences), asCALL_THISCALL); assert( r >= 0 );

## Factory for garbage collection

Whenever a garbage collected class is created, the garbage collector must be notified of it's existence. The easiest way of doing that is to have the factory behaviour, or the class constructor call the `NotifyGarbageCollectorOfNewObject()` method on the engine when initializing the class.

    CGCRef *GCRef_Factory()
    {
        // Create the object and then notify the GC of its existence
        CGCRef *obj = new CGCRef();
        int typeId = engine->GetTypeIdByDecl("gc");
        engine->NotifyGarbageCollectorOfNewObject(obj, typeId);
        return obj;
    }

You may want to consider caching the typeId, so that it doesn't have to be looked up through the relatively expensive call to GetTypeIdByDecl every time an object of this type is created.

Note, if you create objects of this type from the application side, you must also notify the garbage collector of its existence, so it's a good idea to make sure all code use the same way of creating objects of this type.

## Addref and release for garbage collection

For garbage collected objects it is important to make sure the AddRef and Release behaviours clear the GC flag. Otherwise it is possible that the GC incorrectly determine that the object should be destroyed.

    void CGCRef::AddRef()
    {
        // Clear the gc flag and increase the reference counter
        refCount = (refCount&0x7FFFFFFF) + 1;
    }

    void CGCRef::Release()
    {
        // Clear the gc flag, decrease ref count and delete if it reaches 0
        refCount &= 0x7FFFFFFF;
        if( --refCount == 0 )
            delete this;
    }

    // Registering the addref/release behaviours
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_ADDREF, "void f()", asMETHOD(CGCRef,AddRef), asCALL_THISCALL); assert( r >= 0 );
    r = engine->RegisterObjectBehaviour("gc", asBEHAVE_RELEASE, "void f()", asMETHOD(CGCRef,Release), asCALL_THISCALL); assert( r >= 0 );