为什么instance_eval()在类上调用时定义了一个类方法？

Question

Foo = Class.new
Foo.instance_eval do
  def instance_bar
    "instance_bar"
  end
end
puts Foo.instance_bar       #=> "instance_bar"
puts Foo.new.instance_bar   #=> undefined method ‘instance_bar’

我的理解是,在对象上调用instance_eval应该允许您为该对象定义实例变量或方法.

但是在上面的示例中,当您在类Foo上调用它来定义instance_bar方法时,instance_bar将成为可以使用"Foo.instance_bar"调用的类方法.很明显,这段代码没有创建实例方法,因为Foo.new.instance_bar导致"未定义的方法'instance_bar'".

为什么instance_eval在此上下文中定义类方法而不是实例方法？

Answer 1

x.instance_eval更改您的上下文,以便self评估为x.

这允许您执行许多操作,包括定义实例变量和实例方法,但仅限于x.

 x = Object.new
 y = Object.new

 # define instance variables for x and y
 x.instance_eval { @var = 1 }
 y.instance_eval { @var = 2 }

 # define an instance method for all Objects
 class Object
   def var
     @var
   end
 end

 x.var #=> 1
 y.var #=> 2

Ruby允许您为几个地方的对象定义实例方法.通常,在类中定义它们,并且那些实例方法在该类的所有实例之间共享(def var如上所述).

但是,我们也可以只为一个对象定义一个实例方法:

# here's one way to do it
def x.foo
  "foo!"
end
# here's another
x.instance_eval do
  # remember, in here self is x, so bar is attached to x.
  def bar
    "bar!"
  end
end

尽管x并且y具有相同的类,但它们不共享这些方法,因为它们仅被定义为x.

x.foo #=> "foo!"
x.bar #=> "bar!"
y.foo #=> raises NoMethodError
y.bar #=> raises NoMethodError

现在在红宝石中,一切都是对象,甚至是类.类方法只是该类对象的实例方法.

# we have two ways of creating a class:
class A 
end
# the former is just syntatic sugar for the latter
B = Class.new

# we have do ways of defining class methods:

# the first two are the same as for any other object
def A.baz
  "baz!"
end
A.instance_eval do
   def frog
     "frog!"
   end
end

# the others are in the class context, which is slightly different
class A
  def self.marco
    "polo!"
  end
  # since A == self in here, this is the same as the last one.
  def A.red_light
    "green light!"
  end

  # unlike instance_eval, class context is special in that methods that
  # aren't attached to a specific object are taken as instance methods for instances
  # of the class
  def example
     "I'm an instance of A, not A itself"
  end
end
# class_eval opens up the class context in the same way
A.class_eval do
  def self.telegram
    "not a land shark"
  end
end

再次注意,所有这些方法都是A特定的,B不能访问其中任何一个:

A.baz #=> "baz!"
B.telegram #=> raises NoMethodError

从这里开始,重要的是类方法只是类对象的实例方法 Class