5 Questions about instance variables

What are instance variables? Why are instance variables confusing?

Transcript

1. 5 questions about Ruby instance v@riables Enrique Mogollan @mogox Software Engineer - Desk.com
2. 1. What are instance variables? class User def initialize(name, email) @name = name @email = email end end puts User.new("Peter", "peter@spider.com").instance_variables @name @email
3. 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
4. 2. Why so many @?
5. 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
6. Why so many @? class User ... def name @name end def email @email end def send_notification notification.push(name, email) end end
7. Why so many @? class User ... attr_reader :name, :email def send_notification notification.push(name, email) end end
8. class User attr_accessor :name, :email ... End > user = User.new("Peter Barker", "peter@spider.com") > puts user.name "Peter Barker" > user.name = "Peter Parker" > puts user "Peter Parker" Why so many @?
9. 3. Why instance v@rs confusing?
10. 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
11. 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
12. 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
13. 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
14. 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
15. 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
16. 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?
17. 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
18. That is how Ruby works… however 4. How does YARV works for v@riable assignations?
19. 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
20. # age = == disasm: <RubyVM::InstructionSequence:calculate_age@<compiled>>======= 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 <callinfo!mid:calculate_age, argc:0, FCALL|VCALL|ARGS_SKIP> 0007 dup 0008 setlocal_OP__WC__0 2 0010 trace 16 ( 15) 0012 leave ( 14) # self.age = == disasm: <RubyVM::InstructionSequence:calculate_age@<compiled>>======= 0000 trace 8 ( 13) 0002 trace 1 ( 14) 0004 putnil 0005 putself 0006 putself 0007 opt_send_simple <callinfo!mid:calculate_age, argc:0, FCALL|VCALL|ARGS_SKIP> 0009 setn 2 0011 opt_send_simple <callinfo!mid:age=, argc:1, FCALL|ARGS_SKIP> 0013 pop 0014 trace 16 ( 15) 0016 leave ( 14) # @age = == disasm: <RubyVM::InstructionSequence:calculate_age@<compiled>>======= 0000 trace 8 ( 13) 0002 trace 1 ( 14) 0004 putself 0005 opt_send_simple <callinfo!mid:calculate_age, argc:0, FCALL|VCALL|ARGS_SKIP> 0007 dup 0008 setinstancevariable :@age, <is:0> 0011 trace 16 ( 15) 0013 leave
21. age = calculate_age local table (size: 2, argc: ...) 0008 setlocal_OP__WC__0 2 @age = calculate_age 0008 setinstancevariable :@age, <is:0> self.age = calculate_age 0011 opt_send_simple <callinfo!mid:age=, ...> How ruby parses v@riable setting?
22. 5. How do I usually interact with instance v@rs?
23. I use accessors (YARV optimized methods) class User attr_accessor :name def to_s "Name #{name}" end end
24. 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
25. Ruby is an OO language We like sending messages https://twitter.com/mogox