Re: [boost] [interest] underlying type library

Subject: Re: [boost] [interest] underlying type library
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2011-08-20 14:41:53

Marshall Clow wrote:

> On Aug 20, 2011, at 10:45 AM, Olaf van der Spek wrote:
> 
>> On Sat, Aug 20, 2011 at 6:28 PM, Mathias Gaunard
>> <mathias.gaunard_at_[hidden]> wrote:
>>> You cannot use bitwise copy to implement move semantics.
>>> That just doesn't work.
>> 
>> Hmm, why not? What breaks if you do a bitwise swap?
> 
> 
> Consider a type that looks contains a pointer to one of its member vars.
> 
> 
> struct Foo {
> 	char *curPos;
> 	char buffer [ 8 ];
> 	Foo () : curPos (buffer+4) {}
> 	};
> 
> Foo a, b;
> 
> Let's assume that a is at 0x1000, and b is at 0x2000 (and four byte pointers)
> 
> 0x1000:	00001008
> 0x1004:	aaaaaaaaa
> 0x1008:	bbbbbbbb
> 
> 0x2000:	00002008
> 0x2004:	cccccccccc
> 0x2008:	dddddddd
> 
> Now we do a (bitwise) swap of a and b:
> 
> 0x1000:	00002008
> 0x1004:	cccccccccc
> 0x1008:	dddddddd
> 
> 0x2000:	00001008
> 0x2004:	aaaaaaaaa
> 0x2008:	bbbbbbbb
> 
> a's pointer now points into b's buffer.

I see. I'll have to relax my claim that the swap algorithm will work for *any* type. Authors of such a class would have to be warned that they can't use the swap algorithm which applies move_raw.

I can think of one other case in which it doesn't work: when the object contains a pointer to something that points back to the object. Though I don't know why someone would want to do that.