C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Sizeof empty class Goto page 1, 2, 3, 4, 5  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Rohit Dhamija Guest Posted: Wed Jul 28, 2004 1:59 am    Post subject: Sizeof empty class Dear All, I have a small doubt, what is the size of an empty class (having no member variable/functions defined) and why? I tried and ran following program in vc++ version 6.0 and got the answer 1 What does this 1 represent. Is 1 means that the size is 1 byte. But what is stored in this memory location ??? A pointer to the the class ? #include <iostream.h> class base { }; void main() { cout< } Please comment!! Regards, Rohit [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Paul D. DeRocco Guest Posted: Wed Jul 28, 2004 8:08 am    Post subject: Re: Sizeof empty class Quote: "Rohit Dhamija" <rohit_dhamija (AT) rediffmail (DOT) com> wrote I have a small doubt, what is the size of an empty class (having no member variable/functions defined) and why? I tried and ran following program in vc++ version 6.0 and got the answer 1 What does this 1 represent. Is 1 means that the size is 1 byte. But what is stored in this memory location ??? A pointer to the the class ? #include class base { }; void main() { cout< } Please comment!! 1 is correct. Everything has to have a sizeof at least 1, so that the address of each element of an array is unique. -- Ciao, Paul D. DeRocco Paul mailto:pderocco (AT) ix (DOT) netcom.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Townsend Guest Posted: Wed Jul 28, 2004 8:12 am    Post subject: Re: Sizeof empty class I believe the size is set to 1 as this is the smallest non-zero size you can have. I reasoned that a non-zero size would be necessary to give correct meaning to things like foo f[10], if foo had zero size, then indexing into the array would always access the same element, thus it would not be possible to distinguish elements contained at f[0] and f[1] If you want to try this experiment, go ahead, create an empty class and then construct an array of 10 elements, then index through the class and see that the "this" pointer is actually different for each of the elements. "Rohit Dhamija" <rohit_dhamija (AT) rediffmail (DOT) com> wrote Quote: Dear All, I have a small doubt, what is the size of an empty class (having no member variable/functions defined) and why? I tried and ran following program in vc++ version 6.0 and got the answer 1 What does this 1 represent. Is 1 means that the size is 1 byte. But what is stored in this memory location ??? A pointer to the the class ? #include class base { }; void main() { cout< } Please comment!! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Wed Jul 28, 2004 8:14 am    Post subject: Re: Sizeof empty class Rohit Dhamija wrote: Quote: I have a small doubt, what is the size of an empty class (having no member variable/functions defined) and why? Every object has at least size one, because that is required for it to have a unique address. Note: functions don't count in that calculation. Quote: I tried and ran following program in vc++ version 6.0 and got the answer 1 What does this 1 represent. Is 1 means that the size is 1 byte. '1' means that it has the same size as a 'char', on your platform that is the size of a byte, but it doesn't have to. Quote: But what is stored in this memory location ??? A pointer to the the class? Nothing. In case you are interested, you might try to see how deriving from one or more empty classes changes the size of the final object. Search the web for 'empty baseclass optimization'. Quote: #include Ewww, what book did you get that from? Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Wed Jul 28, 2004 10:01 pm    Post subject: Re: Sizeof empty class Ulrich Eckhardt wrote: Quote: Rohit Dhamija wrote: I have a small doubt, what is the size of an empty class (having no member variable/functions defined) and why? Every object has at least size one, because that is required for it to have a unique address. Note: functions don't count in that calculation. To be precise, this is true of every *complete* object. As you know, in an instance of a class derived from a base class with no non-static member data, the size of the sub-object of the base class type can be zero. Quote: I tried and ran following program in vc++ version 6.0 and got the answer 1 What does this 1 represent. Is 1 means that the size is 1 byte. '1' means that it has the same size as a 'char', on your platform that is the size of a byte, but it doesn't have to. snip Yes it does (paragraph 5.3.3/1). [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Thu Jul 29, 2004 3:22 pm    Post subject: Re: Sizeof empty class Ben Hutchings wrote: Quote: Ulrich Eckhardt wrote: Every object has at least size one, because that is required for it to have a unique address. To be precise, this is true of every *complete* object. Hmmm, I don't think I understand you. I know what an incomplete type is, but what is an incomplete object? An object whos ctor hasn't finished yet? Quote: As you know, in an instance of a class derived from a base class with no non-static member data, the size of the sub-object of the base class type can be zero. Just for confirmation: isn't that what 'empty baseclass optimization' is about? And is there a case where sizeof yields zero? As I understand it, the size required for an object can be zero (when it is a sub-object and EBO kicks in) but the result of sizeof can never be zero. Quote: I tried and ran following program in vc++ version 6.0 and got the answer 1 What does this 1 represent. Is 1 means that the size is 1 byte. '1' means that it has the same size as a 'char', on your platform that is the size of a byte, but it doesn't have to. snip Yes it does (paragraph 5.3.3/1). Argh. Yes, The Standard even defines what a byte is, so I should have said 'octet' instead of byte. Uli -- FAQ: http://parashift.com/c++-faq-lite/ /* bittersweet C++ */ default: break; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tobias Güntner Guest Posted: Thu Jul 29, 2004 9:11 pm    Post subject: Re: Sizeof empty class Ulrich Eckhardt wrote: Quote: Ben Hutchings wrote: Ulrich Eckhardt wrote: Every object has at least size one, because that is required for it to have a unique address. To be precise, this is true of every *complete* object. Hmmm, I don't think I understand you. I know what an incomplete type is, but what is an incomplete object? An object whos ctor hasn't finished yet? AFAIK a complete object is the enclosing object, e.g. class B {}; class C {}; class A : public B { C c; }; While B and c are perfectly valid objects, they're only sub-objects of A, i.e. A is the complete object. The required memory for B may be optimized away, but the required memory for c may not. Quote: As you know, in an instance of a class derived from a base class with no non-static member data, the size of the sub-object of the base class type can be zero. Just for confirmation: isn't that what 'empty baseclass optimization' is about? yes Quote: And is there a case where sizeof yields zero? no Quote: As I understand it, the size required for an object can be zero (when it is a sub-object and EBO kicks in) but the result of sizeof can never be zero. yes. In my example above, sizeof(B) is always >=1 but (figuratively speaking) sizeof(A::B) can be zero. Note that in this example: class D : public B {}; class E : public B {}; class F : public D, public E {}; both B sub-objects must have a different memory address, therefore sizeof(F) is at least 2, although the class is basically empty and neither B requires memory. -- Regards, Tobias [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Thu Jul 29, 2004 9:14 pm    Post subject: Re: Sizeof empty class Ulrich Eckhardt wrote: Quote: Ben Hutchings wrote: Ulrich Eckhardt wrote: Every object has at least size one, because that is required for it to have a unique address. To be precise, this is true of every *complete* object. Hmmm, I don't think I understand you. I know what an incomplete type is, but what is an incomplete object? An object whos ctor hasn't finished yet? An "incomplete" object is a sub-object. Actually what you said is true of two kinds of sub-object: members of a class-type object and elements of an array. Quote: As you know, in an instance of a class derived from a base class with no non-static member data, the size of the sub-object of the base class type can be zero. Just for confirmation: isn't that what 'empty baseclass optimization' is about? Right. Quote: And is there a case where sizeof yields zero? As I understand it, the size required for an object can be zero (when it is a sub-object and EBO kicks in) but the result of sizeof can never be zero. snip Right - it always yields the size of a complete object of the given type or of the same type as the given expression. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Falk Tannhäuser Guest Posted: Fri Jul 30, 2004 9:27 am    Post subject: Re: Sizeof empty class Ulrich Eckhardt wrote: Quote: Ben Hutchings wrote: Ulrich Eckhardt wrote: Every object has at least size one, because that is required for it to have a unique address. To be precise, this is true of every *complete* object. Hmmm, I don't think I understand you. I know what an incomplete type is, but what is an incomplete object? An object whos ctor hasn't finished yet? A base class subobject within a derived class object, I suppose. BTW, how does one call objects of incomplete type, like extern char const msg[]; or class Foo; extern Foo f; -- Falk [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Chris Uzdavinis Guest Posted: Fri Jul 30, 2004 9:28 am    Post subject: Re: Sizeof empty class Ben Hutchings <do-not-spam-benh (AT) bwsint (DOT) com> wrote Quote: '1' means that it has the same size as a 'char', on your platform that is the size of a byte, but it doesn't have to. snip Yes it does (paragraph 5.3.3/1). The "it" he was referring to was the size of an empty class, not the size of a char. A char must be 1 byte, as you have shown, but the size of an empty class can be >= 1 byte It's usually only 1 byte since the space isn't used except for its unique address, but that's up to the compiler vendor. -- Chris [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Mon Aug 02, 2004 10:51 pm    Post subject: Re: Sizeof empty class Tobias Güntner <fatbull (AT) users (DOT) sourceforge.net> wrote Quote: class B {}; class C {}; class A : public B { C c; }; While B and c are perfectly valid objects, they're only sub-objects of A, i.e. A is the complete object. The required memory for B may be optimized away, but the required memory for c may not. Okay. Quote: In my example above, sizeof(B) is always >=1 but (figuratively speaking) sizeof(A::B) can be zero. I'm not sure what "figuratively speaking" was supposed to imply, but surely sizeof(A::B) is the same as sizeof(B)? Quote: Note that in this example: class D : public B {}; class E : public B {}; class F : public D, public E {}; both B sub-objects must have a different memory address, therefore sizeof(F) is at least 2, although the class is basically empty and neither B requires memory. Can you tell me where in the standard it says that the B sub-objects have to have different addresses? Microsoft Visual C 6.0 gives sizeof(F)==1, and I don't think this is wrong. Do you know of some system where sizeof(D)==sizeof(E)==1, but sizeof(F)==2? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hyman Rosen Guest Posted: Wed Aug 04, 2004 9:59 am    Post subject: Re: Sizeof empty class Allan W wrote: Quote: I'm not sure what "figuratively speaking" was supposed to imply, but surely sizeof(A::B) is the same as sizeof(B)? He meant the actual number of bytes dedicated to A::B within the larger object, as opposed to the number returned by sizeof (which is the size of the type). Quote: Can you tell me where in the standard it says that the B sub-objects have to have different addresses? 10/5 Quote: Microsoft Visual C 6.0 gives sizeof(F)==1 This is perfectly plausible; the other poster was wrong, because the layout can be D::B, D, pad, E::B, E, F [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bill Wade Guest Posted: Wed Aug 04, 2004 10:13 am    Post subject: Re: Sizeof empty class Quote: Can you tell me where in the standard it says that the B sub-objects have to have different addresses? 10p5 "Note: ... two [base class] subobjects that have the same class type and that belong to the same most derived object must not be allocated at the same address." [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tobias Güntner Guest Posted: Wed Aug 04, 2004 12:07 pm    Post subject: Re: Sizeof empty class Allan W wrote: Quote: In my example above, sizeof(B) is always >=1 but (figuratively speaking) sizeof(A::B) can be zero. I'm not sure what "figuratively speaking" was supposed to imply, but surely sizeof(A::B) is the same as sizeof(B)? Yes, sizeof(A::B) is equal to sizeof(B). I wanted to say sizeof(Base class of A which is A::B)==0 (I don't know how to express that in C++) Quote: Note that in this example: class D : public B {}; class E : public B {}; class F : public D, public E {}; both B sub-objects must have a different memory address, therefore sizeof(F) is at least 2, although the class is basically empty and neither B requires memory. Can you tell me where in the standard it says that the B sub-objects have to have different addresses? I don't know exactly where it is, but AFAIU two objects (any object, not only sub-objects) of the same type (class B in our case) must never share the same memory address (otherwise it would be impossible to distinguish between them) Quote: Microsoft Visual C 6.0 gives sizeof(F)==1, and I don't think this is wrong. Do you know of some system where sizeof(D)==sizeof(E)==1, but sizeof(F)==2? I've compiled it in VC6 as well and I also got sizeof(F)==1. Then I've compiled the same test program in g++ and g++ prints sizeof(F)==2. I believe VC6 is wrong. -- Regards, Tobias [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Thu Aug 05, 2004 3:17 am    Post subject: Re: Sizeof empty class In article <ceokm9$84t$07$1 (AT) news (DOT) t-online.com>, Tobias Güntner <fatbull (AT) users (DOT) sourceforge.net> writes Quote: I've compiled it in VC6 as well and I also got sizeof(F)==1. Then I've compiled the same test program in g++ and g++ prints sizeof(F)==2. I believe VC6 is wrong. But to demonstrate that you must show that the two instances of class B share the same address. Note that C++ in fact only requires distinct addresses and does not permit you to write to those addresses. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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