Instance Variables

Each instance-variable-clause declares a number of instance variables for instances of the class, or expressions to be evaluated when an instance of the class is created. The first part of a clause is the clause specifier, one of sequence, public, private, etc. A clause specifier determines the properties of instance variables declared in its clause:

• sequence does not declare any instance variables. It only specifies expressions to be evaluated (in order) when an instance of the class is initialized.
• public declares fresh instance variables that are visible outside the class. A derived class can override a public declaration, but a public declaration never overrides an existing declaration in the superclass.
• override is like public, but an override declaration always overrides an existing declaration in the superclass. A derived class can override the instance variable again.
• private declares instance variables that can be accessed only within the class expression. They cannot be overridden in a derived class and do not override declarations in the superclass.
• inherit declares instance variables that must be defined in the superclass or one of its ancestors (as public). An inherit declaration can be overridden in derived classes.
• rename is similar to inherit, but a new name is always used to access the value locally and the reference cannot be overridden. The new name can only be used within the class definition. A rename declaration accesses the superclass-defined variable value, even if the variable is overridden.

The collection of instance variable declarations induces two sets of variables:

• The internal instance variables bound within the class* expression: this is the collection of all instance-variables, internal-instance-variables, and inherited-variables. Along with the variables for initialization variables, this-variable, and super-init-variable, all of these variables must be distinct.
• The external instance variables visible outside the class* expression (for derived classes or reference via ivar): this is the collection of all instance-variables and external-instance-variables. All of these variables must be distinct.

For public, override, and private instance variables, the instance-var-initial-value-expr expression provides a value for the variable in an object; when an initial value expression is not given, (void) is used. The process for evaluating initial value expressions is described in section 6.3.3 and section 6.4.

For each public instance variable defined in a class, the superclass (or any of its ancestors) must not contain a declaration for the variable. For each override instance variable, the superclass must already contain a declaration. These properties are verified when the class*/name expression is evaluated; if a public variable is found in the superclass or a override variable is not found, the exn:object exception is raised.

For inherit and rename instance variables, the inherited-variable or external-inherited-variable specifies a (public) instance variable from the superclass. This inheritance is verified when the class*/name expression is evaluated; if an inherited instance variable is not found in the superclass (or one of its ancestors), the exn:object exception is raised. The process that gives values to inherited variables is described in section 6.4.