Can I implement += in my class to increase a contained value?

Question

Consider

class Container

  def initialize(value = 0)
    @value = value
  end

  def + (other)
    return @value + other
  end

  def - (other)
    return @value - other
  end

  def * (other)
    return @value * other
  end

  def / (other)
    return @value / other
  end

  def get
    return @value
  end

end

I would like to use += to increase the value in the container, like this:

c = Container.new(100)
c += 100
print c.get   # Expecting  200

The above won't work, as 100 will overwrite c.

I know I could use something like an attr_accessor to generate a getter and setter for the value, but I'm curious if I could do this in a prettier way such as using +=.

Wouldn't you just use `c.value += 100` ? Is there any reason not to use attr_accessor for this? — tigeravatar, Sep 06 '13 at 18:46
@tigeravatar: That's an option, but I just want to know if I could just do c += 100. If not, then sure, I'll use attr_accessor. — Saturn, Sep 06 '13 at 18:47

Sergio Tulentsev · Accepted Answer · 2013-09-06T19:01:13.000

10

Since c += 100 is just a sugar for c = c + 100, you can't escape overwriting c. BUT you can overwrite it with a similar object (and not with fixnum, as in your question).

class Container
  def initialize(value = 0)
    @value = value
  end

  def + (other)
    Container.new(@value + other)
  end

  def get
    @value
  end
end

c = Container.new(100)
c += 100
c.get # => 200

edited Sep 06 '13 at 19:01

answered Sep 06 '13 at 18:48

Sergio Tulentsev

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It's an incredibly bad idea to make `+` mutate the object. `+` should return a new object with the increased value. The way you implemented it, writing `d = c + 100` would change `c` as well as `d`. – sepp2k Sep 06 '13 at 18:58
1

right, my bad. Amending – Sergio Tulentsev Sep 06 '13 at 18:58

score 3 · Answer 2 · answered Sep 06 '13 at 18:48

3

x += y is just syntactic sugar for x = x + y. So you only have to implement + in your class and you get += for free.

answered Sep 06 '13 at 18:48

Mori

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Correct, however in this case the `+` method returns the result of the added value, and not the instance that implements the method. Which means `c += 100` would change `c` to a totally different type of object, from a `Container` to a number. I'm guessing this isn't what @Omega wants. – Alex Wayne Sep 06 '13 at 18:50

score 1 · Answer 3 · edited May 23 '17 at 12:28

1

No, you can't overload +=. See list of ruby operators that can be overridden/implemented for the full list of overloadable operators.

In Ruby x += y always means x = x + y. The only way to change the meaning of += for a given x is overriding + in x.class. However, + has a different semantics, and the user most probably expects that + returns a new object. If you make + return the original x, that may confuse some of your users. If you make + return a different object, then x would point to that other object in your example, and as far as I understand your question you don't want that.

edited May 23 '17 at 12:28

Community

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1

answered Sep 06 '13 at 18:48

pts

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It wouldn't hurt to say *why not*. – Borodin Sep 06 '13 at 18:56
@Borodin: I don't know any deeper explanation, maybe the authors know. The Ruby language was designed and implemented this way. – pts Sep 06 '13 at 18:58
1

As others have noted, it is simply that `a += b` is simply `a = a + b`, so `+=` isn't a "real" operator at all. It may not be in the list of *operators that can be overloaded*, but neither is it in the list of *operators*. `=` cannot be overloaded, because then the dark arts would reign, but `+` *can*. – Borodin Sep 06 '13 at 19:09

Can I implement += in my class to increase a contained value?

3 Answers3