Path: csiph.com!usenet.pasdenom.info!aioe.org!eternal-september.org!feeder.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail From: Keith Thompson Newsgroups: comp.lang.c Subject: Re: Struct and Union Date: Fri, 10 Jul 2015 12:38:26 -0700 Organization: None to speak of Lines: 49 Message-ID: References: <405a385f-f728-45c0-adfa-c6e1a1662938@googlegroups.com> <87oajkvukx.fsf@bsb.me.uk> <44898eb2-560d-444f-bea4-1ddf7bb69f36@googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: mx02.eternal-september.org; posting-host="945944de09706c9b4e29b53c9d2efdc2"; logging-data="26613"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19S+j+XfMu6JypbOT4KqKmf" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux) Cancel-Lock: sha1:cTBgqXs8cKfKlBt3ZTaU6TPVyRo= sha1:UhYOAawF8gh+EIODQtoP2niUfzo= Xref: csiph.com comp.lang.c:65182 David Kleinecke writes: > On Friday, July 10, 2015 at 9:36:07 AM UTC-7, Ben Bacarisse wrote: >> "Rick C. Hodgin" 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 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"