ruby_meta08.rb

class A
  def initialize()
    @a = 1
  end  
end

a = A.new

a.instance_variable_get(:@a) # 1
a.instance_variable_set(:@a, 2)
a.instance_variable_get(:@a) # 2
a.instance_variable_set(:@b, 3) # 3
a.instance_variable_defined? :@b # true
a.instance_variable_get(:@b) # 3

ruby_meta09.rb

eval('p 33') # 33

class Demo
  def initialize(n)
    @secret = n
  end
  def binding
    binding
  end
end

k1 = Demo.new(99)
b1 = k1.binding
k2 = Demo.new(-3)
b2 = k2.binding

eval("@secret", b1)   #=> 99
eval("@secret", b2)   #=> -3
eval("@secret")       #=> nil

ruby_meta10.rb

class A
  def initialize
     @a = 1
  end
end

a = A.new

a.instance_eval { p self } # #<A:0x007fd1c194c150 @a=1>
a.instance_eval { p @a } # 1

b = 99
a.instance_eval { @a = b }
a.instance_eval { p @a } # 99

a.instance_exec(10) { |b| @a += b }

a.instance_exec { p @a } # 109

ruby_meta11.rb

class B
end

b = B.new

B.class_eval do
  def b
    'b'
  end
end

b.b # b

B.class_exec('hello') do |method_name|
  define_method method_name do
    p method_name
  end
end

b.hello # hello

ruby_meta12.rb

class A
  def initialize(a)
    @a = a
  end
  
  def plus9
    p @a + 9
  end
end

a1 = A.new(1)
a2 = A.new(2)

plus9 = a1.method :plus9 # <Method: A#plus9>
plus9.call # 10

uplus9 = plus9.unbind # <UnboundMethod: A#plus9>
plus9 = uplus9.bind a2 #<Method: A#plus9>
plus9.call # 11

class B
  def initialize
    @a = 91
  end
end

b = B.new
uplus9.bind b # TypeError: bind argument must be an instance of A

B.send(:define_method, :b_plus9, &plus9)
b.b_plus9 # 11