I was talking to a coworker today about the Ruby, and we were
discussing how in Ruby everything is an object. He wondered if
everything was call-by-reference, and my knee jerk reaction was yes.
Then I ran the following irb session…
------------------- START IRB -------------------------
def foo(x)
x = ‘k’ + x[1…x.size]
x
end
def bar(x)
x[0] = ‘k’
x
end
k = ‘crazy’
=> “crazy”
foo(k)
=> “krazy”
k
=> “crazy” # !?!?!?!?!
bar(k)
=> “krazy”
k
=> “krazy” # As expected.
------------------- END IRB -------------------------
Can someone explain to me why the method foo() doesn’t modify k while
bar() does? I suppose it has something to do with the fact that I’m
using an asignment statement in foo() but modifiying the parameter
“in-place” for bar(). Still, I’m a bit confused.
[author’s note: Yes, I know my example isn’t very “Rubyish”. I just
want to get a better handle on the language.]
Thanks!
-Nathan
On Fri, Feb 24, 2006 at 12:58:38PM +0900, Nathan Morse wrote:
def foo(x)
x = ‘k’ + x[1…x.size]
x
end
def bar(x)
x[0] = ‘k’
x
end
String#+ creates a new object. String#[] edits the object in place.
def foo(x)
x = ‘k’ + x[1…x.size]
[x, x.object_id]
end
def bar(x)
x[0] = ‘k’
[x, x.object_id]
end
k = ‘crazy’
=> “crazy”
k.object_id
=> 2584064
foo k
=> [“krazy”, 2580574]
bar k
=> [“krazy”, 2584064]
foo k
=> [“krazy”, 2577004]
bar k
=> [“krazy”, 2584064]
marcel
What about when I don’t use String#+ ?
def foo(x)
x = “krazy”
x
end
=> nil
def bar(x)
x[0] = ‘k’
x
end
=> nil
k = ‘crazy’
=> “crazy”
foo(k)
=> “krazy”
k
=> “crazy”
bar(k)
=> “krazy”
k
=> “krazy”
Nathan Morse wrote:
What about when I don’t use String#+ ?
Look at it this way. You’re not “passing by reference.” You’re
passing references by value. 
Learn to distinguish between variables and objects. The former
are merely “labels” if you will.
a = b = c = "hello" # three variables, one object
Then remember that assignment always “wipes out” the old reference.
Assignment doesn’t change the old object at all.
a = b = "hi" # a and b both refer to "hi"
a = "bye" # but now a refers to "bye"
So assigning a variable is different from changing an object.
Look at this:
a = b = "hi"
b[0] = "f"
p a # "fi"
We’re changing the object referred to by b (which is the same
one referred to by a). We’re not making b refer to a different
object.
Or check this out. Both result in the value “foobar” – but there
is a difference.
a = "foo"
b = "foo"
p a.object_id # -542440758
p b.object_id # -542444578
a << "bar"
b += "bar"
p a.object_id # -542440758
p b.object_id # -542466128 (a new object!)
Does this help any?
Hal
<< … remember that assignment always “wipes out” the old reference.
Assignment doesn’t change the old object at all. >>
Ah ha! I get it now.
Thanks!
To the best of my knowledge everything (apart from integers) is passed
by
reference. However, 99% of the time this doesn’t matter. In ‘pure OOP’
programming, ‘messages’ are sent to ‘methods’ and they respond with new
data. If you only ever use return values you won’t see inconsistent
behaviour and you won’t break encapsulation (which, in effect, using
ByRef
arguments to obtain new values does).
To clarify. There are a few string methods which alter the original
string
object. This is also the case if you index into a string to change a
char.
However, when you evaluate an expression, this yields a new object so
that
in the following:
x = x + 1
The expression on the right yields a value which is assigned to a new
object, x, on the left. In other words, the x on the right is a
different
object from the x on the left. Since methods which alter data usually do
so
in the process of evaluation and assignment, most of the time any object
(such as x) which “goes in” is assigned to a new object (with the same
name)
by the time the method ends. This explains why many people believe that
arguments are sent ‘by value’ - because, in most cases, the value of any
argument that goes into a method is not changed. Rather, a new object is
created during the process of evaluation and assignment.
If this is confusing, just stick to the ‘pure OOP’ way of doing things
and
always use return values; never try to use the values of ingoing
arguments
as ‘ByRef’ parameters. Life is much simpler that way 
best wishes
Huw C.
Hi –
On Fri, 24 Feb 2006, Huw C. wrote:
To the best of my knowledge everything (apart from integers) is passed by
reference.
Have a look at Hal’s post: it’s more a question of passing by value,
where the values happen to be references. For example:
a = “hi”
meth(a)
you’re actually passing the value of a – which is a reference to the
string that was assigned to a.
If you use a literal construct as the argument:
meth(“hi”)
then it will be wrapped in a reference for the occasion.
in the following:
argument that goes into a method is not changed. Rather, a new object is
created during the process of evaluation and assignment.
If this is confusing, just stick to the ‘pure OOP’ way of doing things and
always use return values; never try to use the values of ingoing arguments
as ‘ByRef’ parameters. Life is much simpler that way
Except that it takes you right back to the original problem – namely,
not understanding why the “values” you’re manipulating are actually
behaving like references 
I’d discourage splitting the matter into the simple part and the
confusing part. It’s all reasonably simple, and certainly internally
consistent – and it’s good to know what’s actually happening, partly
to avoid (or at least explain) unexpected errors, and partly to have
as large a technique kit as possible should this or that need arise.
David
–
David A. Black ([email protected])
Ruby Power and Light (http://www.rubypowerandlight.com)
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