From: Brian Candler <B.Candler@...>
Date: 2002-10-12T20:56:37+09:00
Subject: Re: Things That Newcomers to Ruby Should Know

On Sat, Oct 12, 2002 at 01:05:26AM +0900, Kontra, Gergely wrote:
> >3) 'x+=b', 'x*=b' and other assignment operators is internally translated
> >into 'x=x+b' and so on, so these operators assign to x reference to
> >newly created object
> 
> If this is true, a+=1 creates a new object. Can't it be avoided?

If I understand my maths correctly: numbers always exist, they are neither
created nor destroyed :-)

Actually,

a=1      # a is a reference to the number 1 (object of class Fixnum)
a+=1     # a is now a reference to the number 2

However in practise this is efficient, since there appears to be a very
compact internal representation of references to Fixnums:

irb(main):001:0> 0.id
1
irb(main):002:0> 1.id
3
irb(main):003:0> 2.id
5

So I don't think that an object representing "the number 2" is actually
allocated on the heap, thankfully :-)

Hence "a+=1" does not increment the object pointed to by 'a' (you can't
increment the number 1, it's meaningless); rather, 'a' is changed to
reference a new object which is the result of applying method +(1) to the
original object.

If 'a' references any other type of object, then 'a+=1 creates a new object'
is surely the desired behaviour?

   a += 1       is shorthand for
   a = a + 1    which is shorthand for
   a = a.+(1)

and in the general case you can't say that the method '+' when applied to
the object referred to by 'a' returns an object of the same class - and even
if it does, changing the original object in-situ is probably not what you
want anyway.

For example:

a="fred"
b=a
a+=" flintstone"

leaves 'b' pointing to the original string object ("fred") whilst 'a'
references to a new object.

Regards,

Brian.