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### 4.7.2 Attaching User Objects to CS Objects

This section explains how you can assign your own game specific data to Crystal Space objects (i.e. like meshes and sectors). In this section we explain how to do it from within code. In the next howto (see section Attaching User Objects to CS Objects in a map file) we explain how you can do this from a map file.

#### The Crystal Space Object System

Almost all objects in the Crystal Space engine implement the `iObject` interface. Because of this you can attach your own objects to them. This can be very useful to attach game specific data to Crystal Space objects. For example, you may want to attach the state of an actor to the mesh object representing that actor (like current amount of ammunition, health, ...).

#### Defining Game Data

To define data that you can attach to any other Crystal Space object (or any object implementing `iObject`) you also have to create an object that implements `iObject`. The easiest way to do that is to inherit from `csObject` which implements `iObject` for you. Here is an example on how you can do this easily:

    class MyOwnData : public scfImplementationExt1<MyOwnData, csObject,
    	scfFakeInterface<MyOwnData> >
    {
    private:
      int my_data_int;

    public:
      SCF_INTERFACE (MyOwnData, 1, 0, 0);
      ...
      MyOwnData () : scfImplementationType (this)
      {
        ...
      }
      virtual ~MyOwnData ()
      {
        ...
      }
    };

In this example you must use some SCF templates so that your own class becomes a valid SCF object that inherits from `csObject` properly.

Basically you create a class called ``MyOwnData'` which inherits from `csObject`. In that class you can do whatever you want with respect to your own game data. In the example above we only have a variable ``my_data_int'` but of course you can do whatever you want. The SCF templates make sure that you can later query for that object (using `scfQueryInterface` for example).

#### Attaching Game Data

To attach your own data to some Crystal Space object you can use `iObject::ObjAdd()`. The example below shows how you can attach an instance of `MyOwnData` to a mesh:

    MyOwnData* mydata = new MyOwnData ();
    mydata->my_data_int = ...;
    iMeshWrapper* mesh = ...;
    mesh->QueryObject ()->ObjAdd (mydata);
    mydata->DecRef ();

`mesh->QueryObject()` is interesting. Many Crystal Space objects that implement `iObject` have a convenience function called `QueryObject()` which will return a reference to the `iObject` implementation. This code is equivalent to:

    csRef<iObject> obj = scfQueryInterface<iObject> (mesh);

with the important difference that `QueryObject()` does not increment the reference count. If an object doesn't have this convenience function you must use `scfQueryInterface` instead.

#### Finding Game Data

To find if some object has your data attached to it you can use the following code:

    csRef<MyOwnData> mydata =
      CS_GET_CHILD_OBJECT(mesh->QueryObject(), MyOwnData);
    if (mydata)
    {
      ... (use mydata)
    }

#### Removing Game Data

To remove your data you can use:

    csRef<MyOwnData> mydata =
      CS_GET_CHILD_OBJECT(mesh->QueryObject(), MyOwnData);
    if (mydata)
    {
      mesh->QueryObject ()->ObjRemove (mydata);
    }

#### Include Files

The include files useful for this section are:

    #include <iutil/object.h>
    #include <csutil/csobject.h>

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