ruby - `String#===` 文档示例

Question

字符串类文档 (2.1.1) 将 === 运算符描述为:

If obj is not an instance of String but responds to to_str, then the two strings are compared using case equality Object#===.

我在一个例子中试过这个，但它不起作用。这是我的尝试方式:

class Foo
  def to_str
    'some_txt'
  end
end

foo = Foo.new
'some_txt' === foo #=> false

我预计这应该返回 true。但事实并非如此。我问是否有人可以在示例中显示此描述。我在这里做错了什么？

Answer 1

If obj is not an instance of String but responds to to_str, then the two strings are compared using case equality Object#===.

在你的情况下 foo回应#to_str , 但不是 String 的实例.所以 'some_txt' === foo实际上正在使用 Object#===

Case Equality – For class Object, effectively the same as calling #==, but typically overridden by descendants to provide meaningful semantics in case statements.

Foo是 Object 的后代 . Ruby 中的任何类默认继承自 Object .现在Foo没有覆盖方法 #=== ，因此根据文档，它将使用 Object#== 方法。 Object#== 的文档说:

Equality — At the Object level, == returns true only if obj and other are the same object. Typically, this method is overridden in descendant classes to provide class-specific meaning.

根据上述原则 - As 'some_txt' String 的实例和 foo是 Foo 的实例, 'some_txt' === foo (实际上是 foo == 'some_txt' ，因为 Object#=== 将调用委托(delegate)给 Object#== )给出了 false .

Object#== 的文档和 Object#===都说 -

Typically, this method is overridden in descendant classes to provide class-specific meaning.

这意味着您可以覆盖 Object#===或 Object#==实现你的比较逻辑。只是为了模拟我刚才所说的，我放了一个虚拟示例代码:-

class Foo
  def to_str
    'some_txt'
  end

  def ==(other)
    true
  end
end

foo = Foo.new
'some_txt' === foo # => true
foo.class.superclass # => Object