Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!tut.cis.ohio-state.edu!YAHI.STANFORD.EDU!tiemann From: tiemann@YAHI.STANFORD.EDU (Michael Tiemann) Newsgroups: gnu.g++.bug Subject: size of empty classes Message-ID: <8908141630.AA14115@yahi.Stanford.EDU> Date: 14 Aug 89 16:30:37 GMT References: <8908141613.AA23693@dsys.ncsl.nist.gov> Sender: daemon@tut.cis.ohio-state.edu Reply-To: tiemann@lurch.stanford.edu Distribution: gnu Organization: GNUs Not Usenet Lines: 83 Date: Mon, 14 Aug 89 12:13:26 EDT From: Root Boy JimOrganization: National Institute of Standards and Technology formerly National Bureau of Standards Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. ? From: chatty%FRLRI61.BITNET@cunyvm.cuny.edu ? I just discovered that an empty class has a non-null size. Is that ? intentional? If it is, what are those 2 bytes for? Until now, I ? thought having an empty base class was for free. I am disappointed. ? I use g++ 1.35.1- on a Sun3 running OS 3.5. Just a bit of speculation here. I hope you don't think all that whiz-bang object-oriented stuff comes for free. Most likely those two bytes are a pointer into some table telling you what class the thing really is. Remember that when deriving one class from another, some means of dynamically determining to which class an object belongs must be provided. The higher level the language, the farther you get from what the thing really is. Live with it. ? Stephane Chatty chatty@frlri61.bitnet, chatty@lri.lri.fr ? LRI, Universite d'Orsay ? Orsay, FRANCE Root Boy Jim Have GNU, Will Travel. 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. >From above: > Just a bit of speculation here. I hope you don't think all that > whiz-bang object-oriented stuff comes for free. > ... > The higher level the language, the farther you > get from what the thing really is. Live with it. Jim: Don't assume the compiler is gratuitously stupid. One of the big features of C++ is the absense of such baggage, in spite of its higher level. When virtual functions are used (which was not the case above), when used carefully, the overhead is small, and competes very favorably with the C alternative of using switch statements. Michael