ruby-on-rails - 为什么我不能访问 ruby​​ 中的实例变量？

Question

好的，我有这个简单的类

class Test
  @var = 99

  def initialize(var2)
    @var2 = var2
  end 

  attr_reader :var, :var2
end

> t = Test.new(100)
 => #<Test:0x007f9b8118ac30 @var2=100> 
> t.var2
 => 100 
> t.var
 => nil 

为什么最后一个 t.var 语句返回 nil 而我希望它返回 99 因为类顶部的 @var = 99。也许我对范围的想法不是 100 正确......关于这个的任何想法

Answer 1

查看代码中的注释。

class Test
  @var = 99 # self is Test class at this point. You're defining 
            # an instance variable on a class instance.

  def initialize(var2)
    @var2 = var2 # This is normal instance variable.
  end 

  attr_reader :var, :var2
end

t = Test.new(100)
t.var # => nil
t.var2 # => 100

# don't bother with creating an accessor and get the ivar from Test directly.
Test.instance_variable_get(:@var) # => 99