[chuck-users] Passing classes to functions

[Stefan Blixt]

I think you can pretty much say that ChucK follows Java's attitude towards
objects, where the declaration of any non-primitive always contains a
pointer to the memory where the object's data is stored, as opposed to
containing the acual values. That's why this doesn't compile:
class A {
        int x;
}

A a1;
1 => a1.x;
A a2;
a1 => a2;
2 => a2.x;
<<< "a1.x=", a1.x, " a2.x=", a2.x>>>;

Hailstone:~ stefanblixt$ chuck test.ck
[test.ck]:line(8): cannot resolve operator '=>' on types 'A' and 'A'...
[test.ck]:line(8): ...(note: use '@=>' for object reference assignment)

After I've put the @ in there as the error message suggests, I get this
output from the program:

a1.x= 2  a2.x= 2

To create copies of an object you need to actively make a method in that
class that takes an instance as input, creates a completely new instance,
copies each member and then returns the new instance. There is no copy
constructor. See Java's
clone<http://java.sun.com/javase/6/docs/api/java/lang/Object.html#clone()>
method
in java.lang.Object.

Comparing with C/C++, there you can declare a struct/class variable that
isn't a pointer but rather contains all member data where it was declared,
allowing you to do crazy stuff like this:

#include <stdio.h>

struct {
int x;
} a1;
struct {
int x;
} a2;

int main(char** argv) {
        a1.x = 1;
        a2.x = 3;
        *(&(a1.x) +4) = 2;
        // this may print "a1.x=1 a2.x=2" if the alignment is correct
        printf("a1.x=%d a2.x=%d\n", a1.x, a2.x);
}

The confusing bit in ChucK is that it allows declarations with or without a
@ before the variable name. My interpretation of this is that

A a;

should be seen as a kind of shorthand for this:

new A @=> A @ a;

These two staments are identical, I think.

/Stefan



On Sun, Oct 4, 2009 at 8:57 AM, Hans Aberg <haberg at math.su.se> wrote:

>
> On 4 Oct 2009, at 02:21, Adam Tindale wrote:
>
>  This is a more direct version of what I meant to say. The problem I
>> encountered was that I was hoping to do some operations in a function
>> and was hoping that I was passing a copy of an object, much like what
>> happens with primitives. Since objects pass by reference then weird
>> side effects and bugs can easily creep into a fellow ChucKist's code.
>>
>
> Yes, that is one side effect of it: side effects. The reference objects
> reference the same object on the heap, also when passed into a function.
>
>  Anyways, I hope this example is more clear.
>>
>> --art
>>
>> class test{
>>   0 => int val;
>> }
>>
>> test obj1;
>> <<< obj1.val , "original object">>>;
>>
>> fun void testclasspassing(test i)
>> {
>>   4 => i.val;
>>   <<< i.val, "in function" >>>;
>>   // side effect!
>> }
>>
>> testclasspassing(obj1);
>> <<< obj1.val , "original object">>>;
>>
>
> So one way to think about it, is that this is the same as C/C++
>  void fun void testclasspassing(test* i) {
>    4 => i->val;
>    <<< i->val, "in function" >>>;
>    // side effect!
>  }
> which in C++ can also be written as
>  fun void testclasspassing(test& i)
>  {
>    4 => i.val;
>    <<< i.val, "in function" >>>;
>    // side effect!
>  }
> only that in chuck, the extra reference symbols are hidden away by the
> implementation of the reference object.
>
> So when mutating the object within the function, as it is a reference to an
> object on the heap, it will be mutated there too.
>
> By contrast, when one has a copy over type, a copy will be made, which will
> placed in the function stack. So when changing that within the function,
> that is a copy, and not the original object.
>
>  Hans
>
>
>
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>



-- 
Release me, insect, or I will destroy the Cosmos!
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