Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!orion.cf.uci.edu!uci-ics!schmidt@glacier.ics.uci.edu From: schmidt@glacier.ics.uci.edu (Doug Schmidt) Newsgroups: gnu.g++.bug Subject: Re: size of empty classes Message-ID: <21260@paris.ics.uci.edu> Date: 18 Aug 89 20:44:04 GMT References: <8908141613.AA23693@dsys.ncsl.nist.gov> <2271@csun.edu> Sender: news@paris.ics.uci.edu Reply-To: schmidt@glacier.ics.uci.edu (Doug Schmidt) Distribution: gnu Organization: University of California, Irvine - Dept of ICS Lines: 49 In-reply-to: mx!cbcscmrs@csun.edu In article <2271@csun.edu>, mx!cbcscmrs@csun writes: >So g++ does not need or use it, the question to the writers (Michael???) >why force it up to one? In case you missed this before, here is Michael's answer to this exact question from about 3 days ago: ---------------------------------------- Why speculate when you can have the answers? Having GNU, we can travel to the source code and find out: from cplus-class.c (finish_struct): layout_type (t); /* C++: do not let empty structures exist. */ if (integer_zerop (TYPE_SIZE (t))) TYPE_SIZE (t) = TYPE_SIZE (char_type_node); There is one byte reserved by this action on sparc and VAX, and two bytes on m68k and i386. Why not let empty structures exist? Well, for one thing, different implementations of malloc do different things when called with zero for an argument. Some systems will allocate a chunk which gives the user zero size (but a four-byte overhead for malloc). Other systems will just return zero. This means that empty *p = new p; ... if (p == 0) /* Ambiguous: maybe it is NULL, maybe it is `allocated'. */ is highly non-portable. I asked chatty why zero-sized structures were desirable. I have yet to receive an answer. I do permit zero-sized arrays, because they are useful when they are the last element in a struct: struct vector { int len; int elts[0]; /* If len == 0, no need to subtract 1 from size. */ }; Again, if a good case can be made for empty structures, I am willing to be convinced. But first you must convince me. -- Master Swordsman speak of humility; | schmidt@ics.uci.edu (ARPA) Philosophers speak of truth; | office: (714) 856-4034 Saints and wisemen speak of the Tao of no doubt; The moon, sun, and sea speaks for itself. -- Hiroshi Hamada