Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #128159

Re: Does simple inheritance break strict-aliasing rule

Show key headers only | View raw

supercat@casperkitty.com writes:
> On Wednesday, March 21, 2018 at 10:54:06 AM UTC-5, Ben Bacarisse wrote:
>> C has no "strict-aliasing rule".  It has what are called effective type
>> rules and these say that you can't access an object of one type though
>> an lvalue expression of another type.  That's a gross simplification of
>> course, but none of the details are in play here.  Provided s->type is
>> never equal RECT when s is not, in fact, pointing at the Shape inside a
>> Rect, then you are not breaking C's rules when you write r->whatever.
>
> A literal reading of the rules would indicate that given:
>
>     struct foo {int x; } *p;
>
> the lvalue "p->x" has type "int".  As such, it may be used to access any
> object whose type is simply "int", but may not be used to access any object
> of type "struct foo".

Of course p->x is of type int.

> The rules would make a lot of sense if "p->x" were regarded for purposes of
> 6.5p7 as an lvalue of type "struct foo"

No, it would make no sense at all to treat p->x as an lvalue of type
struct foo.  Would you want this to be legal?

    struct foo {int x; } *p;
    struct foo obj;
    // assume reasonable initializations
    p->x = obj;

If p->x is regarded as an lvalue of type struct foo, why would that last
assignment not be legal?

*p is of type struct foo because p is defined as an object of type
struct foo*.  p->x is of type int because x is defined as a member of
type int.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Does simple inheritance break strict-aliasing rule pozz <pozzugno@gmail.com> - 2018-03-21 11:19 +0100
  Re: Does simple inheritance break strict-aliasing rule Thiago Adams <thiago.adams@gmail.com> - 2018-03-21 05:37 -0700
  Re: Does simple inheritance break strict-aliasing rule Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-21 07:02 -0700
  Re: Does simple inheritance break strict-aliasing rule James Kuyper <jameskuyper@verizon.net> - 2018-03-21 10:31 -0400
    Re: Does simple inheritance break strict-aliasing rule supercat@casperkitty.com - 2018-03-21 09:38 -0700
  Re: Does simple inheritance break strict-aliasing rule Nick Bowler <nbowler@draconx.ca> - 2018-03-21 14:46 +0000
  Re: Does simple inheritance break strict-aliasing rule supercat@casperkitty.com - 2018-03-21 08:18 -0700
    Re: Does simple inheritance break strict-aliasing rule Steven Petruzzellis <frelwizzen@gmail.com> - 2018-03-21 23:23 -0700
  Re: Does simple inheritance break strict-aliasing rule Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-03-21 15:53 +0000
    Re: Does simple inheritance break strict-aliasing rule supercat@casperkitty.com - 2018-03-21 09:30 -0700
      Re: Does simple inheritance break strict-aliasing rule Keith Thompson <kst-u@mib.org> - 2018-03-21 10:20 -0700
        Re: Does simple inheritance break strict-aliasing rule supercat@casperkitty.com - 2018-03-21 10:54 -0700
          Re: Does simple inheritance break strict-aliasing rule Keith Thompson <kst-u@mib.org> - 2018-03-21 11:48 -0700
            Re: Does simple inheritance break strict-aliasing rule supercat@casperkitty.com - 2018-03-21 14:28 -0700
  Re: Does simple inheritance break strict-aliasing rule Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-03-21 21:39 +0000

csiph-web