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 lines
eval( 'def aMethod( x ) 
   return( x * 2 ) 
end 

num = 100  
puts( "This is the result of the calculation:" ) 
puts( aMethod( num ))' ) 

# special types of eval
class 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 class
String::class_eval{ define_method(:bye){ puts("goodbye" ) } } 
"Hello".bye    #=> “goodbye”