www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Zombie Object?

reply "Namespace" <rswhite4 googlemail.com> writes:
[code]
import std.stdio;

class Vec2 {
public:
	short x, y;
	
	this(short x, short y) {
		this.x = x;
		this.y = y;
	}
}

void foo(Vec2 v) {
	delete v;
}

void bar(Vec2 v) {
	
}

void main() {
	foo(new Vec2(42, 23));
	Vec2 v1 = new Vec2(4, 2);
	foo(v1);
	assert(v1 !is null);
	writefln("v1.x = %d", v1.x);
	Vec2 v2 = new Vec2(4, 2);
	bar(v2);
	writefln("v2.x = %d", v2.x);
}
[/code]
This code prints:
v1.x = 0
v2.x = 4

But is this correct? I expected
v1.x = 4
v2.x = 4

Same behaviour with destroy.
Feb 07 2013
parent reply "monarch_dodra" <monarchdodra gmail.com> writes:
On Thursday, 7 February 2013 at 10:15:49 UTC, Namespace wrote:

 [code]
 import std.stdio;

 class Vec2 {
 public:
 	short x, y;
 	
 	this(short x, short y) {
 		this.x = x;
 		this.y = y;
 	}
 }

 void foo(Vec2 v) {
 	delete v;
 }

 void bar(Vec2 v) {
 	
 }

 void main() {
 	foo(new Vec2(42, 23));
 	Vec2 v1 = new Vec2(4, 2);
 	foo(v1);
 	assert(v1 !is null);
 	writefln("v1.x = %d", v1.x);
 	Vec2 v2 = new Vec2(4, 2);
 	bar(v2);
 	writefln("v2.x = %d", v2.x);
 }
 [/code]
 This code prints:
 v1.x = 0
 v2.x = 4

 But is this correct? I expected
 v1.x = 4
 v2.x = 4

 Same behaviour with destroy.


Well, given that you are reading a deleted/destroyed object, I'm 
not sure why you'd expect *any* behavior at all. I'd simply 
expect *undefined* behavior...

In any case, I think what you are seeing is a Vec2 instance init 
state. If you change Vec2 to:

//----
class Vec2 {
public:
	short x = 2, y = 2;
         // ...
//----

Then the output becomes:
v1.x = 2
v2.x = 4

AFAIK, the delete/destroy *might* eagerly reset the object to its 
init state, to make future re-use more efficient. I'm not sure, 
but in any case, what you are observing is undefined, so I 
wouldn't look too much into it...
Feb 07 2013
parent "Namespace" <rswhite4 googlemail.com> writes:
Ah thank you.
I thought I found a bug. ;)
Feb 07 2013