Declarators are used within the body of an implementation statement (described above) to specify the instance variables that are introduced by a class, and the staticdata variables pointed to by the class's ClassData structure. These variables are declared using ANSI C syntax for variable declarations, restricted to valid SOM IDL types (see "Type and constant declarations," above). For example, the following implementation statement declares two instance variables, x and y, and a staticdata variable, z, for class "Hello," :
implementation
{
short x;
long y;
double z;
z: staticdata;
};
Instance variables are normally intended to be accessed only by the class's methods and not by client programs or subclasses' methods. For data to be accessed by client programs or subclass methods, attributes should be used instead of instance variables. (Note, however, that declaring an attribute has the effect of also declaring an instance variable of the same name, unless the "nodata" attribute modifier is specified.)
Staticdata variables, by contrast, are publicly available and are associated specifically with their introducing class. They are, however, very different in concept from class variables. Class variables are really instance variables introduced by a metaclass, and are therefore present in any class that is an instance of the introducing metaclass (or of any metaclass derived from this metaclass). As a result, class variables present in any given class will also be present in any class derived from this class (that is, class variables are inherited). In contrast, staticdata variables are introduced by a class (not a metaclass) and are (only) accessed from the class's ClassData structure - they are not inherited.