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
   

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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
   

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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
   

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4. 2. Why so many @?
   

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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
   

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

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

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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 @?
   

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9. 3. Why instance v@rs confusing?
   

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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?
    

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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
    

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

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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
    

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20. # 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
    

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21. 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?
    

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

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

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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
    

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25. Ruby is an OO language
    We like sending messages
    https://twitter.com/mogox
    

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