`&method(:method_ name)` 在 ruby​​ 中是什么意思? [英] What does `&method(:method_ name)` mean in ruby?

问题描述

我试图创建一个具有私有类方法的类.我希望这个私有类方法可以在实例方法中使用.

以下是我的第一次尝试:

类动物类<<自己def public_class_greeter(名称)private_class_greeter(姓名)结尾私人的def private_class_greeter(名称)puts "#{name} 迎接私有类方法"结尾结尾def public_instance_greeter(名称)self.class.private_class_greeter(姓名)结尾结尾

Animal.public_class_greeter('John') 工作正常，打印 John 问候私有类方法.

然而，Animal.new.public_instance_greeter("John") 抛出错误:NoMethodError: private method 'private_class_greeter' called for Animal:Class.

这是预期的，因为调用 self.class.private_class_greeter 与 Animal.private_class_greeter 相同，这显然会引发错误.

在搜索如何解决此问题后，我想出了以下代码，它可以完成工作:

类动物类<<自己def public_class_greeter(名称)private_class_greeter(姓名)结尾私人的def private_class_greeter(名称)puts "#{name} 迎接私有类方法"结尾结尾定义方法:public_instance_greeter，&method(:private_class_greeter)结尾

我不太明白这里发生了什么:&method(:private_class_greeter).

你能解释一下这是什么意思吗?

如果我要替换:

define_method :public_instance_greeter, &method(:private_class_greeter)

与:

def public_instance_greeterXYZ结尾

那么，代替XYZ的内容应该是什么?

解决方案

Ruby 如何解析&method(:private_class_greeter)?

表达式&method(:private_class_greeter)是

• 方法调用的值method(:private_class_greeter)
• 以 & 运算符为前缀.

<小时>

method 方法有什么作用?

方法 方法在当前上下文中查找指定的方法名称，并返回一个表示它的 Method 对象.irb 中的示例:

def foo酒吧"结尾my_method = 方法(:foo)#=>#<方法:对象#foo>

一旦你有了这个方法，你就可以用它做各种事情:

my_method.call#=>酒吧"my_method.source_location # 为您提供定义方法的文件和行#=>["(irb)", 5]# 等等.

<小时>

& 运算符的用途是什么?

& 运算符用于将 Proc 作为块传递给方法，该方法需要一个块传递给它.它还隐式调用您传入的值的 to_proc 方法，以便将不是 Proc 的值转换为 Proc.>

Method 类实现了 to_proc —它将方法的内容作为 Proc 返回.因此，您可以在 Method 实例前面加上 & 并将其作为块传递给另一个方法:

def call_block屈服结尾call_block &my_method # 与 `call_block &my_method.to_proc` 相同#=>酒吧"

define_method 方法恰好采用了一个块，其中包含正在定义的新方法的内容.在您的示例中，&method(:private_class_greeter) 将现有的 private_class_greeter 方法作为一个块传入.

<小时>

&:symbol 是这样工作的吗?

是的.Symbol 实现了 to_proc 以便您可以像这样简化代码:

["foo", "bar"].map(&:upcase)#=>[FOO"，酒吧"]# 这相当于:["foo", "bar"].map { |item|item.upcase }# 因为:upcase.to_proc# 返回这个过程:过程 { |val|val.send(:upcase) }

<小时>

如何复制&method(:private_class_greeter)?

你可以传入一个调用目标方法的块:

define_method :public_instance_greeter do |name|self.class.send(:private_class_greeter, name)结尾

当然，那你就不需要再使用define_method了，这就产生了与他的回答:

def public_instance_greeter(name)self.class.send(:private_class_greeter, name)结尾

I was trying to create a class that has a private class method. I want this private class method available to be used inside an instance method.

The following was my first attempt:

class Animal
  class << self
    def public_class_greeter(name)
      private_class_greeter(name)
    end

  private
    def private_class_greeter(name)
      puts "#{name} greets private class method"
    end
  end

  def public_instance_greeter(name)
    self.class.private_class_greeter(name)
  end
end

Animal.public_class_greeter('John') works fine, printing John greets private class method.

However, Animal.new.public_instance_greeter("John") throws an error: NoMethodError: private method 'private_class_greeter' called for Animal:Class.

That is expected, as the invocation self.class.private_class_greeter is same as Animal.private_class_greeter, which obviously throws an error.

After searching on how this can be fixed, I came up with the following code, that does the job:

class Animal
  class << self
    def public_class_greeter(name)
      private_class_greeter(name)
    end

  private
    def private_class_greeter(name)
      puts "#{name} greets private class method"
    end
  end

  define_method :public_instance_greeter, &method(:private_class_greeter)
end

I don't exactly understand what is happening here: &method(:private_class_greeter).

Could you please explain what does this mean?

If I were to replace:

define_method :public_instance_greeter, &method(:private_class_greeter)

with:

def public_instance_greeter
  XYZ
end

then, what should be the content in place of XYZ?

解决方案

How does Ruby parse &method(:private_class_greeter)?

The expression &method(:private_class_greeter) is

• the value of the method call method(:private_class_greeter)
• prefixed with the & operator.

---

What does the method method do?

The method method looks up the specified method name in the current context and returns a Method object that represents it. Example in irb:

def foo
  "bar"
end

my_method = method(:foo)
#=> #<Method: Object#foo>

Once you have this method, you can do various things with it:

my_method.call
#=> "bar"

my_method.source_location   # gives you the file and line the method was defined on
#=> ["(irb)", 5]

# etc.

---

What is the & operator for?

The & operator is used to pass a Proc as a block to a method that expects a block to be passed to it. It also implicitly calls the to_proc method on the value you pass in, in order to convert values that are not Proc into a Proc.

The Method class implements to_proc — it returns the contents of the method as a Proc. Therefore, you can prefix a Method instance with & and pass it as a block to another method:

def call_block
  yield
end

call_block &my_method   # same as `call_block &my_method.to_proc`
#=> "bar"

The define_method method just happens to take a block with the contents of the new method that is being defined. In your example, &method(:private_class_greeter) passes in the existing private_class_greeter method as a block.

---

Is this how &:symbol works?

Yes. Symbol implements to_proc so that you can simplify your code like this:

["foo", "bar"].map(&:upcase)
#=> ["FOO", "BAR"]

# this is equivalent to:
["foo", "bar"].map { |item| item.upcase }

# because
:upcase.to_proc

# returns this proc:
Proc { |val| val.send(:upcase) }

---

How can I replicate &method(:private_class_greeter)?

You can pass in a block that calls the target method:

define_method :public_instance_greeter do |name|
  self.class.send(:private_class_greeter, name)
end

Of course, then you don't need to use define_method anymore, which results in the same solution Eric mentioned in his answer:

def public_instance_greeter(name)
  self.class.send(:private_class_greeter, name)
end

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