instance_eval vs class_eval

Posted on Sep 17, 2013

Ruby中类其实也是Class的实例。而且instance_eval必须由实例来调用，class_eval必须由类来调用。具体参考下面的实例：

	class A
	end

	a = new A
	a.instance_eval do
		self #=>a
		
		def test
			puts "this is a singleton method of instance a"
		end
	end

	a.test   #=>this is a singleton method of instance a

	b = A.new
	b.test   #=>NoMethodError

test的receiver为实例a.同时类A本身也是Class类的实例，所以也可以作为instance_eval的receiver，作为该类的singleton method，及常说的类方法，只能通过类名调用。

	class A
	end

	A.instance_eval do
		def test
			puts "this is a singleton method of class A"
		end
	end

	A.test  #=>this is ...

而class_eval则receiver则必须为类

	class A
	end

	A.class_eval do
		self  #=>A

		def test
			puts "this is a instance method of class A"
		end
	end

	a = new A
	a.test   #=>this is a instance method of class A

class_eval定义的method为类的instance method。

这就是它们二者之间的差别