From: Charles Calvert <cbciv@...>
Date: 2008-11-12T06:47:27+09:00
Subject: Re: Array variable.

On Fri, 07 Nov 2008 23:33:12 +0100, Andreu <root@sys1.org> wrote in
<gf2flo$te8$1@aioe.org>:

>PROBLEM SOLVED
>At the beginning of my program I have written this line:
>
>idate = mdate = Array.new
>
>but it seems Ruby doesn't support this syntax (like C) and assume
>that the two arrays are the same one.

It's not a difference in syntax, but a difference in how variables are
handled.  When you write something like this in C:

int a, b;

a = b = 0;

You are creating two variables on the stack and assigning an integer
to them.  The actual value 0 is stored in borth variables because they
are intrinsic types that have been allocated on the stack.

When you write in Ruby:

idate = mdate = Array.new

something different happens.  Array.new returns a reference to an
instance of array that has been allocated on the heap.  The variable
mdate holds the value of the reference, not the instance itself.  This
same reference is then assigned to idate.  Now both variables have a
reference to the same instace on the heap.

A reference is really just a pointer with nicer semantics.  If you're
familiar with a language that has pointers, like C, then think of it
like this:

int * GetInt()
{
	int * i = (int *) malloc(sizeof(int));
	return i;
}

int * i, * j;

i = j = GetInt();

Now if you modify j:

*j = 5;
printf("%d", *i); // prints "5"
-- 
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