From: Xavier Noria <fxn@...>
Date: 2008-08-28T20:27:29+09:00
Subject: Re: how to avoid passing by reference and how to copy objects

On Thu, Aug 28, 2008 at 1:03 PM, David A. Black <dblack@rubypal.com> wrote:

> C is actually a poor point of reference (ha ha) for understanding Ruby
> variables and references, because the two are so different. I'd
> encourage you not to worry about how Ruby semantics align with C
> semantics

Nevertheless C is only pass by value. That could help.

There's no way in C to accomplish this

   a = 7;
   some_function(a);
   // a is 7 for certain here

You can pass the address of a:

   a = 7;
   p = &a;
   some_other_function(p);
   // a may be 5 now

but note that the argument of the function is p, it is not a. So you
are clearly just passing p by value.

Again, when some_other_function returns there's no way p is pointing
to anything that is not a, because of pass by value semantics. In some
vague sense this second example has some resemblances to Ruby: you are
always passing ps in Ruby so to speak (that's an analogy, it is clear
from the rest of the thread there are no actual pointers involved).

Through those ps, if the object is mutable you may change the state of
the object, but cannot change the very reference, the object pointed
at by p, the same way you cannot in C.

Java is pass by value as well.