Check the class of an object from Box2d's user data - objective-c

Question

I am trying to determine the type of object when detecting collision with Box2d. I want to be able to assign the user data to an object and check to see if its of the correct class type
id object = b->GerUserData():
Then
if([object isKindOfClass:[MyClassObject class]])
However i just get the error "Cannot initialize a variable of type'id' with an rvalue of type 'void*'
Can anyone help me out.
Thanks

Answer 1

Your problem is you are trying to assign an object of type 'id' to a void * type
The method call body->GetUserData(); returns a void pointer. Here it is as defined in the header file of b2Body.h
/// Get the user data pointer that was provided in the body definition.
void* GetUserData() const;
Now, if you are using ARC (Automatic Reference Counting), you need to perform an additional step to cast it back.
id object = (__bridge id) b->GetUserData();
If you are not using ARC, then...
id object = (id) b->GetUserData();
As for checking the type, there are many ways to do this. I personally prefer having an enum called GameObjectType. I can then assign the type in the appropriate constructor of the object. Here is an example of how I do it in my games
for (b2Body * b = world->GetBodyList(); b != NULL; b = b->GetNext()) {
Box2DSprite * sprite = (__bridge Box2DSprite*) b->GetUserData();
id obj = (__bridge id) b->GetUserData();
if (sprite.gameObjectType == kGroundTypeStatic
|| sprite.gameObjectType == kWallType
|| sprite.gameObjectType == kGroundTypeDynamic) {
// Insert Logic here
} // end if
sprite.position = ccp(b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
sprite.rotation = CC_RADIANS_TO_DEGREES(b->GetAngle() * -1);
} // end for
Here is how I would go about creating the sprite (Using ARC)
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;
bodyDef.position = b2Vec2(location.x/PTM_RATIO,
location.y/PTM_RATIO);
// Setting the enum value
self.gameObjectType = kTankType;
self->body = self->world->CreateBody(&bodyDef);
self->body->SetUserData((__bridge void*) self); // Obtain sprite object later
GB2ShapeCache * shapeCache = [GB2ShapeCache sharedShapeCache];
[shapeCache addFixturesToBody:self->body forShapeName:#"Earth_Tank"];
self.anchorPoint = [shapeCache anchorPointForShape:#"Earth_Tank"];
Hope this helps

Answer 2

I'm guessing the error is on this line:
id object = b->GetUserData();
That might be because the return type is void pointer. Try to cast it like that:
id object = (id)b->GetUserData();