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Re: C struct compatibility when they have the same initial members

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"James Harris" <james.harris.1@gmail.com> writes:

> "Pierre Asselin" <pa@see.signature.invalid> wrote in message 
> news:mj0u18$t0k$1@reader1.panix.com...
>> James Harris <james.harris.1@gmail.com> wrote:
>>> [ ... ]
>>> 2. Define a struct which has just the pointers and embed that at the
>>> start of each of the other types as in
>>
>>>   struct common {
>>>     struct common *next;
>>>     struct common *prev;
>>>   );
>>
>>>   struct specific1 {
>>>     struct common c;
>>>     <rest of this node type>
>>>   };
>>
>>> Again, I don't think this will work without casts.
>
>> I think it does work without a cast, but you have to name the
>> intermediate field, e.g. ptr->c.next, ptr->c.prev instead of just
>> ptr->next, ptr->prev.
>
>
> If I define the head and tail nodes as
>
>   struct common head, tail;
>
> and define a normal node, using the above name, as
>
>   struct specific1 node;
>
> Then it looks as though head.next will have the same type as 
> node.c.prev, for example.

You'd still need a cast to treat the struct common * you get as a
pointer to one of the specif types you might need to use (and you still
have potential alignment and aliasing issues).

If you put all the types into a union you can have the next and prev
pointers be to that union type which make for simple cast-free accesses
(though you do got a lot of member names -- macros can help with that).

<snip>
-- 
Ben.

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C struct compatibility when they have the same initial members "James Harris" <james.harris.1@gmail.com> - 2015-05-13 11:31 +0100
  Re: C struct compatibility when they have the same initial members Johann Klammer <klammerj@NOSPAM.a1.net> - 2015-05-13 17:17 +0200
  Re: C struct compatibility when they have the same initial members James Kuyper <jameskuyper@verizon.net> - 2015-05-13 12:25 -0400
    Re: C struct compatibility when they have the same initial members Ian Collins <ian-news@hotmail.com> - 2015-05-14 14:33 +1200
    Re: C struct compatibility when they have the same initial members "James Harris" <james.harris.1@gmail.com> - 2015-05-14 08:04 +0100
      Re: C struct compatibility when they have the same initial members James Kuyper <jameskuyper@verizon.net> - 2015-05-15 00:19 -0400
        Re: C struct compatibility when they have the same initial members "James Harris" <james.harris.1@gmail.com> - 2015-05-15 21:32 +0100
          Re: C struct compatibility when they have the same initial members James Kuyper <jameskuyper@verizon.net> - 2015-05-15 17:11 -0400
            Re: C struct compatibility when they have the same initial members Keith Thompson <kst-u@mib.org> - 2015-05-15 15:27 -0700
  Re: C struct compatibility when they have the same initial members Barry Schwarz <schwarzb@dqel.com> - 2015-05-13 11:22 -0700
  Re: C struct compatibility when they have the same initial members pa@see.signature.invalid (Pierre Asselin) - 2015-05-14 01:33 +0000
    Re: C struct compatibility when they have the same initial members "James Harris" <james.harris.1@gmail.com> - 2015-05-14 08:15 +0100
      Re: C struct compatibility when they have the same initial members Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-05-14 11:22 +0100
        Re: C struct compatibility when they have the same initial members Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-05-14 11:27 +0100

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