instance_eval和singleton方法之间的区别

Question

Singleton method is a method that is defined only on one instance.

foo = Foo.new
def foo.case
  #singleton method
end

Doesn't instance_eval do the same thing? Defining a method for a particular instance? What is the difference?

Answer 1

Object# instance_eval is a method, using which you indeed can define method for an object.

Singleton class is a "place", where the singleton method defined for the object "lives".

So these are two absolutely different things.

Answer 2

The metaprogramming tool instance_eval does the same thing right? Defining a method for a particular instance?

No. instance_eval has nothing whatsoever to do with defining methods. It evaluates a block in the context of the receiver. Normally, blocks are lexically scoped, including self , a block that is evaluated using instance_eval is evaluated with self bound to the receiver of the message.

one = Object.new
two = Object.new

def one.my_eval; yield end

instance_variables
# => []

one.my_eval { @ivar_one = 1 }

one.instance_variables
# => []

instance_variables
# => [:@ivar_one]

two.instance_eval { @ivar_two = 2 }

two.instance_variables
# => [:@ivar_two]

So what is really the difference?

They are completely unrelated. It doesn't really make sense to ask about the difference between two unrelated things.