Overriding OM GET()

Grzegorz Kraszewski

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This article in other languages: Polish

The OM_GET() method, used for getting an attribute from an object, receives an opGet structure as its message. The structure is defined in the <intuition/classusr.h> header file:

struct opGet
{
  ULONG  MethodID;           /* always OM_GET (0x104) */
  ULONG  opg_AttrID;
  ULONG *opg_Storage;
};

Unlike OM_SET(), this method handles only one attribute at a time. The attribute is placed in the opg_AttrID field. The field opg_Storage is a pointer to a place where the attribute value should be stored. It is defined as a pointer to ULONG, but it may point to anything (for example to some larger structure). It allows for passing attributes not fitting in a 32-bit variable. Because OM_GET() does not have a taglist iteration loop, its implementation is simple:

IPTR MyClassGet(Class *cl, Object *obj, struct opGet *msg)
{
  switch (msg->opg_AttrID)
  {
    case Some_Integer_Tag:
      *msg->opg_Storage = /* value of the tag */;
    return TRUE;

    case Some_String_Tag:
      *(char**)msg->opg_Storage = "a fixed string value";
    return TRUE;
  }

  return DoSuperMethodA(cl, obj, (Msg)msg);
}

The implementation consists of a switch statement with cases for all recognized attributes. If an attribute is recognized, the method should return TRUE. It is very important, as MUI notifications rely on OM_GET() and will not work on the attribute if TRUE is not returned. Unknown attributes are passed to the superclass. The DoSuperMethodA() call may be alternatively placed as the default clause of the switch statement. It is important that msg->opg_Storage is dereferenced when storing the attribute value. If the value type is not integer, a typecast is needed. For a value type T, the dereferencing combined with typecast is denoted as *(T*).