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

Metaprogramming Techniques | 29

TheObject#methodsmethod returns an array of instance methods, including single-
ton methods, defined on the receiver. If the first parameter tomethodsisfalse, only
the object’s singleton methods are returned.

class C

def inst_method

end

def self.cls_method

end

end

c = C.new

class << c

def singleton_method

end

end

c.methods - Object.methods # => ["inst_method", "singleton_method"]

c.methods(false) # => ["singleton_method"]

Module#instance_methodsreturns an array of the class or module’s instance methods.
Note thatinstance_methodsis called on the class, whilemethodsis called on an
instance. Passingfalse toinstance_methods skips the superclasses’ methods:

C.instance_methods(false) # => ["inst_method"]

We can also use Metaid’smetaclass method to examineC’s class methods:

C.metaclass.instance_methods(false) # => ["new", "allocate", "cls_method",

"superclass"]

In my experience, most of the value from these methods is in satisfying curiosity.
With the exception of a few well-established idioms, there is rarely a need in produc-
tion code to reflect on an object’s methods. Far more often, these techniques can be
used at a console prompt to find methods available on an object—it’s usually
quicker than reaching for a reference book:

Array.instance_methods.grep /sort/ # => ["sort!", "sort", "sort_by"]

ObjectSpace

ObjectSpaceis a module used to interact with Ruby’s object system. It has a few use-
ful module methods that can make low-level hacking easier:

• Garbage-collection methods:define_finalizer(sets up a callback to be called
  just before an object is destroyed),undefine_finalizer(removes those call-
  backs), andgarbage_collect (starts garbage collection).


• _id2ref converts an object’s ID to a reference to that Ruby object.


• each_objectiterates through all objects (or all objects of a certain class) and
  yields them to a block.