Correct implementation and use of == operator in ruby

Question

In continuation of another question I also have the following question.

I have a class which has a very central instance variable called var. It is so central to the class that when I print an object of the class, it should just print the instance variable.

When I compare that same object with a string which matches the instance variable, I would like it to return true, but that doesn't always work in the implementation below:

class MyClass
    attr_accessor :var
    def initialize(v)
        @var = v
    end
    def to_s
        @var.to_s
    end
    def inspect
        @var.inspect
    end
    def ==(other)
        self.var == other
    end
    # ... methods to manipulate @var
end

str = "test"
obj = MyClass.new("test")

puts obj # prints the var string - great this is what i want
p obj # prints the var string - great this is what i want
puts obj==str # returns true - great this is what i want 
puts str==obj # returns false - oops! Fails since i didn't overload the == operator in String class. What to do? I don't think i want to "monkey patch" the String class.

I understand that it's not my overwritten == operator which is used. It's the one in the String class. What do you suggest I do to get around this? Am I going down the wrong path?

I read through the question What's the difference between equal?, eql?, ===, and ==? without getting an answer to my question.

score 5 · Accepted Answer · answered Jan 10 '23 at 14:32

I understand that it's not my overwritten == operator which is used. It's the one in the String class. What do you suggest I do to get around this? Am I going down the wrong path?

Before trying anything else, have a look at the docs for String#==:

If object is not an instance of String but responds to to_str, then the two strings are compared using object.==.

In other words: you have to implement to_str and return the object's string value:

class MyClass
  # ...

  def to_str
    @var.to_s
  end
end

You can also return to_s or use alias to_str to_s.

With any of the above, you'd get:

"test" == MyClass.new("test")
#=> true

Note that to_str should only be implemented by objects that are string-like.

Correct implementation and use of == operator in ruby

1 Answers1