Overriding Constructors

Grzegorz Kraszewski

Objects without child objects

An object constructor (OM_NEW() method), takes the same message structure opSet as OM_SET() method. The message contains ops_AttrList field, being a pointer to a taglist containing initial object's attributes. Implementation of a constructor for an object not chaving child objects is simple. The superclass constructor is called first, then, if it succeeds, the constuructor initializes object instance data, allocates resources needed and sets initial values of attributes from tags passed via ops_AttrList.

A rule of thumb when overriding constructor is to never leave a half-constructed object. The counstructor should either return a fully constructed object, or fail completely, freeing all succesfully obtained resources. It is important if the object obtains more than one resource and any of resource allocation may fail (for example allocating a big chunk of memory or opening a file). An example implementation below obtains three resources: A, B and C:

IPTR MyClassNew(Class *cl, Object *obj, struct opSet *msg)
{  
  if (obj = DoSuperMethodA(cl, obj, (Msg)msg))
  {
    struct MyClassData *d = (struct MyClassData*)INST_DATA(cl, obj);

    if ((d->ResourceA = ObtainResourceA()
     && (d->ResourceB = ObtainResourceB()
     && (d->ResourceC = ObtainResourceC())
    {
      return (IPTR)obj;    /* success */
    }
    else CoerceMethod(cl, obj, OM_DISPOSE);
  }
  return NULL;
}

If the object destructor frees resources A, B and C (which would be logical considering the constructor allocates them), the cleanup job may be delegated to the destructor. It requires however, that the destructor must be prepared for destruction of not fully constructed object. It can't assume all three resources have been allocated, so it should check every resource pointer against NULL before calling a freeing function. The desctructor also takes care of calling a superclass destructor when resources are freed. See Overloading Destructors for a destructor example code and explanation.

The only question remaining is what CoerceMethod() does and why it is used instead of a plain DoMethod()? The CoerceMethod() call works exactly the same as DoMethod(), but performs method coercion by forced call to the dispatcher of the class specified as the first argument instead of the dispatcher of the object's true class. It makes a difference, when the class in question is later subclassed. The flowchart below explains the problem:

The class B on the diargam is a subclass of the class A and similarly, the class C is a subclass of B. Let's assume an object of the class C is being constructed. As every constructor calls the superclass first, the call goes up to rootclass (the root of all BOOPSI classes) first. Then going down the class tree, every class constructor allocates its resources. Unfortunately the constructor of class A has been unable to allocate one of resources and decided to fail. If it would just call DoMethod(obj, OM_DISPOSE) it will unneccesarily execute destructors in classes B and C, while constructors in these classes have been not yet fully executed. Even if these destructors can cope with this, calling them is superfluous. With CoerceMethod() destructor in the class A is called directly. Then class A constructor returns NULL, which makes constructors in classes B and C to fail immediately wihout resource allocation attempts.

Objects with child objects

While retaining the same principles, a constructor of object having subobjects is designed a bit differently. The most commonly subclassed classes able to have child objects are Application, Window and Group. The constructor should create its child objects first, then insert them into the ops_AttrList taglist and call the superclass constructor. If it succeeds, then resources may be allocated if needed. As any of the three constructor stages may fail, proper handling of errors becomes complicated. Also inserting objects created into the taglist as values of child tags (like MUIA_Group_Child) is cumbersome. Fortunately one can use the DoSuperNew() function, which merges subobjects creation and calling the superclass into one operation. It also provides automatic handling of failed child object construction. An example below is a constructor for a Group subclass putting two Text objects in the group.

IPTR MyClassNew(Class *cl, Object *obj, struct opSet *msg)
{  
  if (obj = DoSuperNew(cl, obj,
    MUIA_Group_Child, MUI_NewObject(MUIC_Text,
      /* attributes for the first subobject */
    TAG_END),
    MUIA_Group_Child, MUI_NewObject(MUIC_Text,
      /* attributes for the second subobject */
    TAG_END),
  TAG_MORE, msg->ops_AttrList)) 
  {
    struct MyClassData *d = (struct MyClassData*)INST_DATA(cl, obj);

    if ((d->ResourceA = ObtainResourceA()
     && (d->ResourceB = ObtainResourceB()
     && (d->ResourceC = ObtainResourceC())
    {
      return (IPTR)obj;    /* success */
    }
    else CoerceMethod(cl, obj, OM_DISPOSE);
  }
  return NULL;
}

An important thing to observe is the fact, that DoSuperNew() merges the taglist passed to the constructor via the message ops_AttrList field and the one specified in the function arguments list. It is done with a special TAG_MORE tag, which direct a taglist iterator (like NextTagItem() function) to jump to another taglist pointed by the value of this tag. Taglist merging allows for modifying the object being constructed with tags passed to NewObject(), for example adding a frame or background to the group in the above example.