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Re: Struct and Union

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David Kleinecke <dkleinecke@gmail.com> writes:
> On Friday, July 10, 2015 at 9:36:07 AM UTC-7, Ben Bacarisse wrote:
>> "Rick C. Hodgin" <rick.c.hodgin@gmail.com> writes:
>> > On Friday, July 10, 2015 at 3:49:56 AM UTC-4, janus wrote:
>> >> I have 2 options but which one is idiomatic C?
>> >> 
>> >> 1)
>> >> typedef struct CStatement
>> >> {
>> >>     int stype;
>> >> 
>> >>     union Statement
>> >>     {
>> >>         CPrintStatement* printstatement;
>> >>         CInputStatement* inputstatement;
>> >>         CAssignStatement* assignstatement;
>> >>         CGotoStatement* gotostatement;
>> >>         CIfThenStatement* ifthenstatement;
>> >>         void* sstatement;
>> >> 
>> >>     } statement;
>> >> }CStatement;
>
> The real reason for using a union is if the members are of different
> sized types and you want to insure that all of them fit.

That's not the only reason.  A union exposes the fact that only one of
the members is valid at any time.  (You need other information, in this
case `stype`, to keep track of which one is currently valid.)  Using a
union rather than a structure saves some space (not a lot in this case),
but that space saving is not the only reason to use a union.

>                                                          All pointers
> are the same size so a union is inappropriate here.

All pointers are *not* necessarily the same size.  And even if they are
for a given implementation (which is common), that wouldn't be a good
argument against using a union; the pointers are of different *types*,
even if they have exactly the same representation.

Given that they're pointers, using a single void* is also a reasonable
approach.  Again we don't necessarily know (and needn't care) whether
void* and CPrintStatement* are the same size.

-- 
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"

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Struct and Union janus <emekamicro@gmail.com> - 2015-07-10 00:49 -0700
  Re: Struct and Union Richard Heathfield <rjh@cpax.org.uk> - 2015-07-10 09:04 +0100
    Re: Struct and Union janus <emekamicro@gmail.com> - 2015-07-10 01:40 -0700
      Re: Struct and Union Richard Heathfield <rjh@cpax.org.uk> - 2015-07-10 09:50 +0100
        Re: Struct and Union Richard Heathfield <rjh@cpax.org.uk> - 2015-07-10 13:44 +0100
    Re: Struct and Union "Bill Davy" <Bill@SynectixLtd.com> - 2015-07-10 12:37 +0100
      Re: Struct and Union Keith Thompson <kst-u@mib.org> - 2015-07-10 08:24 -0700
        Re: Struct and Union Keith Thompson <kst-u@mib.org> - 2015-07-12 12:53 -0700
  Re: Struct and Union Ian Collins <ian-news@hotmail.com> - 2015-07-10 21:32 +1200
  Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-10 05:12 -0700
    Re: Struct and Union Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-10 17:35 +0100
      Re: Struct and Union David Kleinecke <dkleinecke@gmail.com> - 2015-07-10 10:18 -0700
        Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-10 10:23 -0700
        Re: Struct and Union James Kuyper <jameskuyper@verizon.net> - 2015-07-10 13:47 -0400
        Re: Struct and Union Keith Thompson <kst-u@mib.org> - 2015-07-10 12:38 -0700
      Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-10 10:27 -0700
  Re: Struct and Union Bartc <bc@freeuk.com> - 2015-07-10 13:55 +0100
  Re: Struct and Union Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-10 21:48 +0100
    Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-10 13:51 -0700
      Re: Struct and Union Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-10 23:45 +0100
        Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-10 17:43 -0700
          Re: Struct and Union Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-11 13:04 +0100
            Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-11 05:15 -0700
              Re: Struct and Union Bartc <bc@freeuk.com> - 2015-07-11 14:01 +0100
                Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-11 06:05 -0700
                Re: Struct and Union "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-07-11 06:13 -0700
            Re: Struct and Union Keith Thompson <kst-u@mib.org> - 2015-07-11 13:51 -0700
              Re: Struct and Union Martin Shobe <martin.shobe@yahoo.com> - 2015-07-11 16:51 -0500
              Re: Struct and Union Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-11 23:13 +0100
                Re: Struct and Union Keith Thompson <kst-u@mib.org> - 2015-07-11 17:39 -0700
              Re: Struct and Union Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-11 16:56 -0700
  Re: Struct and Union Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-10 18:52 -0700
  Re: Struct and Union Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-10 19:12 -0700

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