`@ count`是Ruby中的实例变量还是类变量？

Question

class Counter
   def self.counted_new
      @count = 0 if @count.nil?
      @count += 1
      new
   end

   def self.count
      @count
   end
end

在格式中,@count看起来像一个实例变量,但是当我在"irb"(接口ruby)中加载它并输入四个命令代码时

Counter.counted_new
Counter.count
Counter.counted_new
Counter.count

@count终于成了2!表现得像一个类变量

Answer 1

@count 始终是一个实例变量,但如果在该上下文中声明它,它可以是类上的实例变量.

在这种情况下,@count是一个类实例变量.换句话说,通过@count在类级方法内部编写,您将为该类分配一个变量.

通过@count在实例方法内部编写,您将分配仅在该特定实例中可用的实例变量.

如果你声明一个变量,@@count你得到一个类变量.

类变量和类实例变量之间的主要区别是类变量保留在继承中.

class Foo
  @@klass = "class level variable"
  @klass_instance = "class instance level variable"
end

class Bar < Foo
end


puts Foo.instance_variables.inspect # => [:@klass_instance]
puts Foo.class_variables.inspect  # => [:@@klass]

puts Foo.instance_variable_get(:@klass_instance)
# => "klass instance level variable"
puts Foo.class_variable_get(:@@klass)
# => "class level variable"

# The class variable is inherited, but the class instance variable is not

puts Bar.instance_variables.inspect # => []
puts Bar.class_variables.inspect  # => [:@@klass]

# The @@klass variable is shared between all classes in the downward inheritance chain
# So for example:
Foo.class_variable_set(:@@klass, "foo")
puts Bar.class_variable_get(:@@klass) # => "foo"