Re: [boost] [interest] underlying type library

Subject: Re: [boost] [interest] underlying type library
From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2011-08-20 14:13:40

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.

-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists_at_[hidden]>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
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