Advanced Rails - Building Industrial-Strength Web Apps in Record Time

6 | Chapter 1: Foundational Techniques

1. Ruby follows the receiver’sklasspointer and searches them_tblof that class
   object for a matching method. (The target of aklasspointer will always be a
   class object.)


2. If no method is found, Ruby follows that class object’ssuperpointer and contin-
   ues the search in the superclass’sm_tbl.


3. Ruby progresses in this manner until the method is found or the top of thesuper
   chain is reached.


4. If the method is not found in any object on the chain, Ruby invokesmethod_
   missingon the receiver of the original method. This starts the process over again,
   this time looking formethod_missing rather than the original method.

These rules apply universally. All of the interesting things that method lookup
involves (mixins, class methods, and singleton classes) are consequences of the struc-
ture of theklass andsuper pointers. We will now examine this process in detail.

Class inheritance

The method lookup process can be confusing, so we’ll start simple. Here is the sim-
plest possible class definition in Ruby:

class A

end

This code generates the following data structures in memory (see Figure 1-1).

The double-bordered boxes represent class objects—objects whoseklasspointer
points to theClassobject.A’ssuperpointer refers to theObjectclass object, indicat-
ing thatAinherits fromObject. For clarity, from now on we will omit defaultklass
pointers toClass,Module, andObject where there is no ambiguity.

Figure 1-1. Data structures for a single class

Object

A

super Class

klass

klass

klass