Ruby meta programming 4(eval/class_eval/module_eval/instance_eval)

之所以有不同的eval方法，只是因为在不同的上下文中；你也可以使用eval+bind参数完全替换class_eval/module_eval/instance_eval的

# "#{exp}" evaluates it  as a string and not as an expression, whereas eval( exp ) evaluates a string as an expression.exp = "2+4"puts( eval( exp )) puts( "#{exp}" ) # eval method can evaluate strings spanning many lineseval( 'def aMethod( x )    return( x * 2 ) end num = 100  puts( "This is the result of the calculation:" ) puts( aMethod( num ))' ) # special types of evalclass MyClass  def initialize    @aVar = "Hello world"  end end ob = MyClass.new p( ob.instance_eval { @aVar } )   p( ob.instance_eval( "@aVar" ) ) # eval can evaluate expressions of arbitrary complexity# such ass create method to String classString::class_eval{ define_method(:bye){ puts("goodbye" ) } } "Hello".bye    #=> “goodbye”