new instance variable created by eval do NOT cease when eval exit

Question

Note that eval evaluates its code in a temporary scope. eval can alter the value of instance variables that already exist. But any new instance variables it defines are local to the invocation of eval and cease to exist when it returns. (It is as if the evaluated code is run in the body of a block—variables local to a block do not exist outside the block.)

-- from "the ruby programming" book , chapter 8.3.1

irb(main):001:0> class Point
irb(main):002:1> def initialize(x,y)
irb(main):003:2> @x,@y=x,y
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> p=Point.new(1,2)
=> #
irb(main):007:0> 
irb(main):008:0* def get_bind
irb(main):009:1> binding
irb(main):010:1> end
=> nil
irb(main):011:0> b=p.get_bind
=> #Binding:0x81915b0
irb(main):012:0> 
irb(main):013:0* eval("@x",b)
=> 1
irb(main):014:0> 
irb(main):015:0* eval("@x=10",b)
=> 10
irb(main):016:0> eval("@x",b)

=> 10
irb(main):017:0> 
irb(main):018:0* eval("@z=2",b)
=> 2
irb(main):019:0> eval("@z",b)

=> 2
irb(main):020:0> p.instance_variables
=> [:@x, :@y, :@z]
irb(main):021:0> 
irb(main):022:0* 
irb(main):023:0*

the instance variable "@z" does NOT cease , why ?

score 1 · Accepted Answer · 2013-04-05 18:28:56Z

1

I don't have the book at hand, but that must be a typo. What the author describes is true for local variables in Ruby >=1.9.

1.9.3:

irb(main):001:0> eval("a=1")
=> 1
irb(main):002:0> a
NameError: undefined local variable or method `a' for main:Object
    from (irb):2
    from /Users/apple/.rbenv/versions/1.9.3-p385/bin/irb:12:in `<main>'

1.8.7:

>> eval("a=1")
=> 1
>> a
=> 1

Edit: Here's a more recent version of the book and it says "local variables".

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