Ruby中的单例类到底是什么? [英] What exactly is the singleton class in ruby?

问题描述

Ruby中的单例类本身就是类吗?这是所有对象都属于类"的原因吗?这个概念是 fuzzy ，但是我认为它与为什么我完全可以定义类方法(class foo; def foo.bar ...)有关.

Is the singleton class in Ruby a class in and of itself? Is it the reason why all objects belong to "class?" The concept is fuzzy, but I believe it has something to do with why I can define a class method at all (class foo; def foo.bar ...).

Ruby中的单例类是什么?

What is the singleton class in Ruby?

推荐答案

首先，一个简单的定义:单个方法是仅针对单个对象定义的方法.示例:

First, a little definition: a singleton method is a method that is defined only for a single object. Example:

irb(main):001:0> class Foo; def method1; puts 1; end; end
=> nil
irb(main):002:0> foo = Foo.new
=> #<Foo:0xb79fa724>
irb(main):003:0> def foo.method2; puts 2; end
=> nil
irb(main):004:0> foo.method1
1
=> nil
irb(main):005:0> foo.method2
2
=> nil
irb(main):006:0> other_foo = Foo.new
=> #<Foo:0xb79f0ef4>
irb(main):007:0> other_foo.method1
1
=> nil
irb(main):008:0> other_foo.method2
NoMethodError: undefined method `method2' for #<Foo:0xb79f0ef4>
        from (irb):8

实例方法是类的方法(即在类的定义中定义).类方法是类的Class实例上的单例方法-它们未在类的定义中定义.而是在对象的 singleton类上定义它们.

Instance methods are methods of a class (i.e. defined in the class's definition). Class methods are singleton methods on the Class instance of a class -- they are not defined in the class's definition. Instead, they are defined on the singleton class of the object.

irb(main):009:0> Foo.method_defined? :method1
=> true
irb(main):010:0> Foo.method_defined? :method2
=> false

您可以使用语法class << obj打开对象的单例类.在这里，我们看到此单例类是定义单例方法的地方:

You open the singleton class of an object with the syntax class << obj. Here, we see that this singleton class is where the singleton methods are defined:

irb(main):012:0> singleton_class = ( class << foo; self; end )
=> #<Class:#<Foo:0xb79fa724>>
irb(main):013:0> singleton_class.method_defined? :method1
=> true
irb(main):014:0> singleton_class.method_defined? :method2
=> true
irb(main):015:0> other_singleton_class = ( class << other_foo; self; end )
=> #<Class:#<Foo:0xb79f0ef4>>
irb(main):016:0> other_singleton_class.method_defined? :method1
=> true
irb(main):017:0> other_singleton_class.method_defined? :method2
=> false

因此向对象添加单例方法的另一种方法是在对象的单例类打开的情况下对其进行定义:

So an alternative means of adding singleton methods to an object would be to define them with the object's singleton class open:

irb(main):018:0> class << foo; def method3; puts 3; end; end
=> nil
irb(main):019:0> foo.method3
3
=> nil
irb(main):022:0> Foo.method_defined? :method3
=> false

总结:

• 方法必须始终属于一个类(或:是某个类的实例方法)
• 常规方法属于它们定义的类(即该类的实例方法)
• 类方法只是Class
的单例方法 对象的
• 单个方法不是该对象类的实例方法；相反，它们是对象的单例类的实例方法.

• methods must always belong to a class (or: be instance methods of some class)
• normal methods belong to the class they're defined in (i.e. are instance methods of the class)
• class methods are just singleton methods of a Class
• singleton methods of an object are not instance methods of the class of the object; rather, they are instance methods of the singleton class of the object.

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