Question

I m宣读了Programing Duncan 1.9的Metaprogramming section of 和I m,在stat_eval /_exec vsinstance_eval//_instance_exec 之间对的内分>。

首先,我的理解是<代码>f。在<代码>本身的方法表中添加一个方法。 (类别物体):

class A puts self # => A def foo; 42; end # added to the method table of self, so becomes an instance method end A.new.foo # => 42

如果我们使用<条码>等值_eval,我们就采取同样的行动:

A.class_eval do puts self # => A def bar; 42; end # same as above end A.new.bar # => 42

但是,<代码>instance_eval中的一些内容。情况不同:

A.instance_eval do puts self # => A def baz; 42; end # added to the method table of an anonymous # singleton class of self, so becomes a class method end puts A.baz # => 42 s = string s.instance_eval do ... end # same behavior, so now def creates an instance method

因此,我理解<代码>_eval和instance_eval之间的功能差异。

但<代码>_eval和_instance_eval各栏目中的情形见exactly 具体来说,<代码>本身指明了同一物体, local_variables也一样。因此,在制作<条码>f的区块内发生的情况如何不同?

我是否可以阅读一些文件? The RDoc for instance_eval 和无帮助。看看来文方,instance_eval 似乎设立了单一州级物体,而 _eval 。否——但这种区别在《刑法》之外在鲁比拉一级是否明显?

Answer 1

我认为,你的混淆来自这样一个事实,即失败并不取决于目前的自我,你可能认为,它是一个“现时的阶级”,它有自己的规则。

下面举例说明:

class A
  # defs here go to A
  puts self  # => A
  class << self
     #defs here go to A s eigenclass
  end
end

A.class_eval do
  #defs here go to A
end

A.instance_eval do
  #defs here go to A s eigenclass     
end

s = "Hello World"

class << s
  #defs here go to s s eigenclass
end

本章中涉及这个问题的部分内容,它非常明确地描述了行为。

class_eval and instance_eval both set self for the duration of the block. However, they differ in the way they set up the environment for method definition. class_eval sets things up as if you were in the body of a class definition, so method definitions will define instance methods In contrast, calling instance_eval on a class acts as if you were working inside the singleton class of self. Therefore, any methods you define will become class methods.

我认为,唯一值得补充的是,你可以指针——在任何物体中,而不仅仅是课堂上,行为不会改变,但会产生不同的后果。

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