5 questions about Ruby instance v@riables Enrique Mogollan @mogox Software Engineer - Desk.com 

1. What are instance variables? class User def initialize(name, email) @name = name @email = email end end puts User.new("Peter", "[email protected]").instance_variables @name @email 

class User def initialize(name, email) @name = name @email = email end def to_s "Name: #{@name}" End def send_notification notification.push(@name, @email) end end 

2. Why so many @? 

Why so many @? class User def initialize(name, email) @name = name @email = email end def to_s "Name: #{@name}" End def send_notification notification.push(@name, @email) end end 

Why so many @? class User ... def name @name end def email @email end def send_notification notification.push(name, email) end end 

Why so many @? class User ... attr_reader :name, :email def send_notification notification.push(name, email) end end 

class User attr_accessor :name, :email ... End > user = User.new("Peter Barker", "[email protected]") > puts user.name "Peter Barker" > user.name = "Peter Parker" > puts user "Peter Parker" Why so many @? 

3. Why instance v@rs confusing? 

Why are instance v@rs confusing? class User def initialize(name, dob) @name = name @dob = dob calculate_age end def to_s "Name #{@name}, Age: #{@age}" end private def calculate_age @age = dob_to_age(@dob) end end 

Why are instance v@rs confusing? class User attr_accessor :name, :dob, :age def initialize(name, dob) @name = name @dob = dob calculate_age end def to_s "Name #{name}, Age: #{age}" end private def calculate_age age = dob_to_age(dob) end end 

class User attr_accessor :name, :dob, :age def initialize(name, dob) @name = name @dob = dob calculate_age end def to_s "Name #{name}, Age: #{age}" end private def calculate_age age = dob_to_age(dob) # this is not an instance var end end 

class User def age @age end def age=(value) @age = value end def to_s "Name #{name}, Age: #{age}" end private def calculate_age age = dob_to_age(dob) # age is a local var end end 

class User def age @age end def age=(value) @age = value end def to_s "Name #{name}, Age: #{age}" end private def calculate_age self.age = dob_to_age(dob) # this sends a message to self end end 

class User def age @age end def age=(value) @age = value end def to_s "Name #{name}, Age: #{age}" end private def calculate_age @age = dob_to_age(dob) # ignores the method end end 

class User attr_accessor :age # Get the instance variable value def to_s "Age: #{age}" "Age: #{self.age}" "Age: #{@age}" end private # Set the instance variable value def calculate_age age = dob_to_age(dob) # creates a local var self.age = dob_to_age(dob) # sends a message @age = dob_to_age(dob) # ignores the method end end Why are instance v@rs confusing? 

class User def age @age end def age=(value) @age = value end def to_s "Name #{name}, Age: #{age}" end private def calculate_age age = dob_to_age(dob) # Why age = doesn’t # set the instance variable? end end 

That is how Ruby works… however 4. How does YARV works for v@riable assignations? 

compiled = RubyVM::InstructionSequence.compile(code) puts compiled.disasm A trick learned from “Ruby Under a Microscope” http://patshaughnessy.net/ruby-under-a-microscope Get YARV instructions 

# age = == disasm: >======= local table (size: 2, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, keyword: 0@3] s1) [ 2] age 0000 trace 8 ( 13) 0002 trace 1 ( 14) 0004 putself 0005 opt_send_simple 0007 dup 0008 setlocal_OP__WC__0 2 0010 trace 16 ( 15) 0012 leave ( 14) # self.age = == disasm: >======= 0000 trace 8 ( 13) 0002 trace 1 ( 14) 0004 putnil 0005 putself 0006 putself 0007 opt_send_simple 0009 setn 2 0011 opt_send_simple 0013 pop 0014 trace 16 ( 15) 0016 leave ( 14) # @age = == disasm: >======= 0000 trace 8 ( 13) 0002 trace 1 ( 14) 0004 putself 0005 opt_send_simple 0007 dup 0008 setinstancevariable :@age, 0011 trace 16 ( 15) 0013 leave 

age = calculate_age local table (size: 2, argc: ...) 0008 setlocal_OP__WC__0 2 @age = calculate_age 0008 setinstancevariable :@age, self.age = calculate_age 0011 opt_send_simple How ruby parses v@riable setting? 

5. How do I usually interact with instance v@rs? 

I use accessors (YARV optimized methods) class User attr_accessor :name def to_s "Name #{name}" end end 

I send messages class User attr_accessor :name def to_s "Name #{name}, Age: #{age}" end ... private def age @age ||= dob_to_age(dob) end end 

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