OBJECTS

• selector - a symbol used to select an appropriate method
• message - a selector and a list of actual arguments
• method - the code that implements a message

Officially, there is no way to see inside an object (look at the values of its instance variables). The only way to communicate with an object is by sending it a message.

You can send a message to an object using the 'send' function. This function takes the object as its first argument, the message selector as its second argument (which must be a symbol) and the message arguments as its remaining arguments.

The 'send' function determines the class of the receiving object and attempts to find a method corresponding to the message selector in the set of messages defined for that class. If the message is not found in the object's class and the class has a super-class, the search continues by looking at the messages defined for the super-class. This process continues from one super-class to the next until a method for the message is found. If no method is found, an error occurs.

To perform a method lookup starting with the method's superclass rather than the object's class, use the function 'send-super'. This allows a subclass to invoke a standard method in its parent class even though it overrides that method with its own specialized version.

When a method is found, the evaluator binds the receiving object to the symbol 'self' and evaluates the method using the remaining elements of the original list as arguments to the method. These arguments are always evaluated prior to being bound to their corresponding formal arguments. The result of evaluating the method becomes the result of the expression.

Two objects, both classes, are predefined: Object and Class. Both Object and Class are of class Class. The superclass of Class is Object, while Object has no superclass. Typical use is to create new classes (by sending :new to Class) to represent application objects. Objects of these classes, created by sending :new to the appropriate new class, are subclasses of Object. The Object method :show can be used to view the contents of any object.