From: fxn@...
Date: 2007-01-28T18:15:05+09:00
Subject: Re: pass by reference?

> dblack@wobblini.net writes:
>
>> Hi --
>>
>> On Sat, 27 Jan 2007, Robert Klemme wrote:
>>
>>> On 27.01.2007 01:43, dblack@wobblini.net wrote:
>>>> Hi --
>>>>
>>>> On Sat, 27 Jan 2007, Martin C. Martin wrote:
>>>>
>>>>> Phrogz wrote:
>>>>>> If you pass an immutable type by reference, does it make a sound?
>>>>>> Er, I mean...
>>>>>> If you passed an immutable type by reference, how would you know
>>>>>> that
>>>>>> it wasn't passed by value?
>>>>>
>>>>> Is there any way for the function you're calling to modify the value
>>>>> of
>>>>> the variable in the caller?  Pass by reference can do that.
>>>>
>>>> You can modify the object to which the variable refers:
>>>>
>>>>   def change_me(obj)
>>>>     obj << "hi"
>>>>   end
>>>>
>>>>   arr = [1,2,3]
>>>>   change_me(arr)
>>>>   p arr          # [1, 2, 3, "hi"]
>>>>
>>>> In this example, arr contains a reference to an array.  In change_me,
>>>> obj contains another copy of the same reference, so you can use it to
>>>> manipulate and change the original array.
>>>>
>>>> I still wouldn't call this pass by reference (see my earlier post in
>>>> this thread).
>>>
>>> Absolutely right: Ruby uses pass by value - with references.
>>>
>>> irb(main):004:0> def foo(x) x = 10 end
>>> => nil
>>> irb(main):005:0> def bar; x = 20; foo(x); x end
>>> => nil
>>> irb(main):006:0> bar
>>> => 20
>>> irb(main):007:0>
>>
>> I'm not sure that demonstrates the "values that are references" thing,
>> though.  You're reassigning to x in foo, which creates a new local x;
>> but I think that's just part of the assignment semantics.  Or are you
>> assuming that if pass by reference were involved, then assignment
>> would work differently?
>>
>
> *** NEWBIE WARNING ***
>
> I'm entering this thread as it gives me a chance to test my own
> understanding.
> My apologies if I've missed the mark!
>
> I agree the example doesn't show pass by reference v pass by value. I
> think it
> shows more about scope.
>
> However, I think what he was trying to show was that if what was being
> passed
> was the address of the variable in the callers environment, then changes
> to
> what that variable pointed at would be seen in the callers environment as
> well.
>
> i.e.
>
> a = Array.new could look like this
>
> -------
> |  a  |        ----------
> |  ----------->|Array Obj|
> |     |        ----------
> -------        Addr = 1000
> Addr = 0
>
> Here 'a' holds the value 1000, the address of the array object. This value
> (1000) is stored in location 0. 'a' points to the storage location. In C
> it
> would be called a pointer, in ruby, its just a variable because yo don't
> have
> anything else. some would say 'a' is bound to the address 1000, which is
> the
> start location of an array object.
>
> Some of this confusion about passed by value and passed by reference is
> due to
> C and how it worked. In C, by default, things are passed by value

<snip>

> Ruby takes a different approach to C. In ruby, all variables are really
> references to objects stored somewhwere else. The overheads associated
> with
> arguement passing are the same regardless of the size or type of object
> the
> variable references - its just an address. This means the traditional C
> pass by
> reference

I think those statements were informal, but just for the archives C only
has pass-by-value.

Sometimes people think that, say, modifying an integer through a pointer
argument shows pass-by-reference. But it doesn't, the argument is the
pointer there, and is passed by value.

-- fxn