在 Ruby 中初始化类实例变量
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Initializing class instance variables in Ruby
提问by HalloDu
I am working on a small rails app and have a problem with ruby's OOP model. I have the following simplified class structure.
我正在开发一个小型 Rails 应用程序，但 ruby 的 OOP 模型有问题。我有以下简化的类结构。
class Foo
protected
@bar = []
def self.add_bar(val)
@bar += val
end
def self.get_bar
@bar
end
end
class Baz < Foo
add_bar ["a", "b", "c"]
end
My problem is now, that when I call add_bar in the class definition of Baz, @baris apparently not initialized and I get an error that the +Operator is not available for nil. Calling add_baron Foodirectly does not yield this problem. Why is that and how can I initialize @barcorrectly?
我现在的问题是，当我在 Baz 的类定义中调用 add_bar 时，@bar显然没有初始化，并且我收到一个错误，即+Operator 不可用于nil. 调用add_bar上Foo直接不会产生这个问题。为什么会这样，我该如何@bar正确初始化？
To make clear what I want, I will point out the behavior I would expect from these classes.
为了明确我想要什么，我将指出我期望从这些类中得到的行为。
Foo.add_bar ["a", "b"]
Baz.add_bar ["1", "2"]
Foo.get_bar # => ["a", "b"]
Baz.get_bar # => ["a", "b", "1", "2"]
How could I achieve this?
我怎么能做到这一点？
回答by r00k
Short answer:instance variables don't get inherited by subclasses
简短回答：实例变量不会被子类继承
Longer answer:the problem is that you wrote @bar = []in the body of the class (outside any method). When you set an instance variable, it is stored on whatever is currently self. When you're in a class body, selfis the class object Foo. So, in your example, @foogets defined on the class object Foo.
更长的答案：问题是你写@bar = []在类的主体中（在任何方法之外）。当您设置一个实例变量时，它会存储在当前的任何内容上self。当你在一个类体中时，self是类对象 Foo。因此，在您的示例中，@foo在类对象 Foo 上定义。
Later, when you try to look up an instance variable, Ruby looks in whatever is currently self. When you call add_bar from Baz, selfis Baz. Also selfis STILL Baz in the body of add_bar (even though that method is in Foo). So, Ruby looks for @barin Baz and can't find it (because you defined it in Foo).
稍后，当您尝试查找实例变量时，Ruby 会查找当前self. 当你从 Baz 调用 add_bar 时，self就是 Baz。也self仍然是在巴兹add_bar的主体（即使该方法是在富）。所以，Ruby@bar在 Baz 中寻找却找不到（因为你在 Foo 中定义了它）。
Here's an example that might make this clearer
这是一个可能使这一点更清楚的例子
class Foo
@bar = "I'm defined on the class object Foo. self is #{self}"
def self.get_bar
puts "In the class method. self is #{self}"
@bar
end
def get_bar
puts "In the instance method. self is #{self} (can't see @bar!)"
@bar
end
end
>> Foo.get_bar
In the class method. self is Foo
=> "I'm defined on the class object Foo. self is Foo"
>> Foo.new.get_bar
In the instance method. self is #<Foo:0x1056eaea0> (can't see @bar!)
=> nil
This is admittedly a bit confusing, and a common stumbling point for people new to Ruby, so don't feel bad. This concept finally clicked for me when I read the 'Metaprogramming' chapter in Programming Ruby(aka "The Pickaxe").
诚然，这有点令人困惑，并且是 Ruby 新手的常见绊脚石，所以不要感到难过。当我阅读Ruby 编程（又名“镐”）中的“元编程”一章时，这个概念终于对我产生了影响。
How I'd solve your problem:Look at Rails' class_attributemethod. It allows for the sort of thing you're trying to do (defining an attribute on a parent class that can get inherited (and overidden) in its subclasses).
我将如何解决您的问题：查看 Rails 的class_attribute方法。它允许您尝试做的事情（在父类上定义可以在其子类中继承（和覆盖）的属性）。
回答by khelll
Well, since @bar is defined as class instance variable then it's limited to the class Foo. Check this:
好吧，因为@bar 被定义为类实例变量，所以它仅限于类Foo。检查这个：
class Foo
@bar = []
end
class Baz < Foo
end
Foo.instance_variables #=> [:@bar]
Baz.instance_variables #=> []
Anyway, for this simple example you can do this:
无论如何，对于这个简单的例子，你可以这样做：
class Foo
protected
def self.add_bar(val)
@bar ||=[]
@bar += val
end
def self.get_bar
@bar
end
end
class Baz < Foo
add_bar ["a", "b", "c"]
end
Read more about this here.
在此处阅读更多相关信息。
回答by Nicolas Goy
I do it like so:
我这样做：
class Base
class << self
attr_accessor :some_var
def set_some_var(value)
self.some_var = value
end
end
end
class SubClass1 < Base
set_some_var :foo
end
class SubClass2 < Base
set_some_var :bar
end
Then it should do what you want.
然后它应该做你想做的。
[8] pry(main)> puts SubClass1.some_var
foo
[9] pry(main)> puts SubClass2.some_var
bar
Note that the set_some_var method is optional, you can do SubClass1.some_var = ...if you prefer.
请注意， set_some_var 方法是可选的，您可以根据需要进行操作SubClass1.some_var = ...。
If you want some default value, add something like that under class << self
如果您想要一些默认值，请在下面添加类似的内容 class << self
def some_var
@some_var || 'default value'
end
回答by Guilherme Bernal
This appears to work well:
这似乎运作良好：
class Foo
protected
@@bar = {}
def self.add_bar(val)
@@bar[self] ||= []
@@bar[self] += val
end
def self.get_bar
(self == Foo ? [] : @@bar[Foo] || []) + (@@bar[self] || [])
end
end
class Baz < Foo
end
Foo.add_bar ["a", "b"]
Baz.add_bar ["1", "2"]
Foo.get_bar # => ["a", "b"]
Baz.get_bar # => ["a", "b", "1", "2"]
