class_eval和instance_eval的区别

#### instance_eval从名字上看，它的调用者必须是一个实例：

class A
end
a = A.new
a.instance_eval do
self #===>a
def method1
puts "it's a's instance method, only for a"
end
end

a.method1 #=it's a's instance method, only for a

同样，因为类class本身也是Class类的一个实例，instance_eval也可以用在类上，这个时候就可以在其中定义该类的singleton_method，即为该类的类函数.

class A
end
A.instance_eval do
self # A
def method1
puts "it's A's class method"
end
end

A.method1 # it's A's class method

#### class_eval 的调用者receiver必须是一个类，而在class_eval block的内部，self即为receiver类本身.

class A
end

A.class_eval do
self  # => A
# current class => A
end

　　根据这个定义，如果在一个类上调用了class_eval，就可以在其中定义该类的实例函数 instance_method

class A
end

a = A.new
a.method1
#=> NoMethodError: undefined method `method1' for #<A:0x10043ff70>

A.class_eval do
self  # => A
# current class => A
def method1
puts 'this is a instance method of class A'
end
end

a.method1
#=> this is a instance method of class A

　　详细讲解：https://ruby-china.org/topics/2442