C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Default copy operator on derivated classes          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Alexander Tumarov Guest Posted: Thu Jul 22, 2004 10:59 pm    Post subject: Default copy operator on derivated classes I tried to run next code compiles with g++ //-------------------------------- #include <stdio.h> class B { public: B& operator=(const B &b) { printf("BBBBBBBB n"); return *this; }; }; class C { public: C& operator=(const C &c) { printf("CCCCCCC n"); return *this; }; }; class D : public B,public C { public: D& operator=(const C &c) { printf("DDDDDD n"); return *this; }; }; int main(int argc, char* argv[]) { C c1; D d1,d2; d1=d2; d1=c1; return 0; } // -------------------------- And got next output BBBBBBBB CCCCCCC DDDDDD The question is why compiler builds call to both B& operator=(const B &b) C& operator=(const C &c) on the line d1=d2; ??? A much as I remember on the case of missing copy operator the compiler should build default one that performs bit-to-bit copy and not call base copy operators. Am I wrong? Can anybody point me to the specific paragraph in the ANSI standard? Thank you. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Fri Jul 23, 2004 2:23 am    Post subject: Re: Default copy operator on derivated classes "Alexander Tumarov" <pure-magic (AT) narod (DOT) ru> wrote... Quote: I tried to run next code compiles with g++ //-------------------------------- #include class B { public: B& operator=(const B &b) { printf("BBBBBBBB n"); return *this; }; }; class C { public: C& operator=(const C &c) { printf("CCCCCCC n"); return *this; }; }; class D : public B,public C { public: D& operator=(const C &c) { printf("DDDDDD n"); return *this; }; }; int main(int argc, char* argv[]) { C c1; D d1,d2; d1=d2; d1=c1; return 0; } // -------------------------- And got next output BBBBBBBB CCCCCCC These two are for the d1=d2 expression. Quote: DDDDDD This is your overloaded assignment. Quote: The question is why compiler builds call to both B& operator=(const B &b) C& operator=(const C &c) on the line d1=d2; ??? A much as I remember on the case of missing copy operator the compiler Where do you remember that from? Quote: should build default one that performs bit-to-bit copy and not call base copy operators. Am I wrong? Yes, you are. Not bit-to-bit copy. Member-wise _assignment_. IOW, the compiler creates code that is semantically equivalent to calling all base classes' and members' assignment operators. That's what you get, the default operator= called the B::operator= and C::operator= for the respective base classes of D. Quote: Can anybody point me to the specific paragraph in the ANSI standard? 12.8, paragraph 13. Victor [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrey Tarasevich Guest Posted: Fri Jul 23, 2004 2:24 am    Post subject: Re: Default copy operator on derivated classes Alexander Tumarov wrote: Quote: A much as I remember on the case of missing copy operator the compiler should build default one that performs bit-to-bit copy and not call base copy operators. Am I wrong? You are wrong. There's no such thing at bit-to-bit copying in C++. When compiler defines implicit copy assignment operator for some class type, it is required to perform memberwise copying of this class' subobjects in accordance with their own copy semantics. For subobjects of class type their copy assignment operators are called, for example. See 12.8/13. -- Best regards, Andrey Tarasevich [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Fri Jul 23, 2004 9:51 am    Post subject: Re: Default copy operator on derivated classes Andrey Tarasevich wrote: Quote: Alexander Tumarov wrote: A much as I remember on the case of missing copy operator the compiler should build default one that performs bit-to-bit copy and not call base copy operators. Am I wrong? You are wrong. There's no such thing at bit-to-bit copying in C++. When compiler defines implicit copy assignment operator for some class type, it is required to perform memberwise copying of this class' subobjects in accordance with their own copy semantics. For subobjects of class type their copy assignment operators are called, for example. See 12.8/13. Member-wise copy was not always the way of C++. If I recall correctly, many things were different in the very old days. C++ has been around for at least 15 years, and was not always as the standard describes it. -- Antoun Kanawati [email]antounk.at (AT) comcast (DOT) dot.net[/email] [remove .dot and .at before use] [ 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: Fri Jul 23, 2004 2:17 pm    Post subject: Re: Default copy operator on derivated classes In article <c6vsm5$166a$1 (AT) news (DOT) f.de.plusline.net>, Alexander Tumarov <pure-magic (AT) narod (DOT) ru> writes Quote: The question is why compiler builds call to both B& operator=(const B &b) C& operator=(const C &c) on the line d1=d2; ??? A much as I remember on the case of missing copy operator the compiler should build default one that performs bit-to-bit copy and not call base copy operators. Am I wrong? Can anybody point me to the specific paragraph in the ANSI standard? Yes, you are wrong. Bitwise copying was removed from the language many years ago. We now use memberwise copying (see 12.8, and in particular para 8) -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Fri Jul 23, 2004 3:40 pm    Post subject: Re: Default copy operator on derivated classes In article <CJ%Lc.147233$JR4.30264@attbi_s54>, Antoun Kanawati <antounk (AT) comcast (DOT) net> writes Quote: C++ has been around for at least 15 years, and was not always as the standard describes it. Well the first edition of 'The C++ Programming Language' has a prefaced dated 1985 which suggests that C++ is almost 21. -- 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. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Sat Jul 24, 2004 12:05 am    Post subject: Re: Default copy operator on derivated classes Francis Glassborow wrote: Quote: In article <CJ%Lc.147233$JR4.30264@attbi_s54>, Antoun Kanawati [email]antounk (AT) comcast (DOT) net[/email]> writes C++ has been around for at least 15 years, and was not always as the standard describes it. Well the first edition of 'The C++ Programming Language' has a prefaced dated 1985 which suggests that C++ is almost 21. 2004 - 1985 = 19, which is somewhat different from 21. My first contact with C++ was either in 1986 or 1987, when I bought the purple book on impulse. It wasn't clear to me back then whether the language was used outside AT&T. It wasn't until late 1988 that I ran into a real C++ compiler (good old CFront) after I switched colleges. At that time, the language version was still 1.x (I forget what x was), strange things happened to "this" in constructors, the word "member-wise" was not known, etc... I don't think we had multiple inheritance back then, but I could be wrong. So, I rounded "late 1988" to 1989, and figure "at least" 15 years; a number that does not exclude 19 or 21. I admit that I do not the exact date of birth of C++. Luckily, there are still some living people who know these things, and we don't have to go digging for archeological evidence -- Antoun Kanawati [email]antounk.at (AT) comcast (DOT) dot.net[/email] [remove .dot and .at before use] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sun Jul 25, 2004 12:24 am    Post subject: Re: Default copy operator on derivated classes In article <NOeMc.153337$JR4.139837@attbi_s54>, Antoun Kanawati <antounk (AT) comcast (DOT) net> writes Quote: Francis Glassborow wrote: In article <CJ%Lc.147233$JR4.30264@attbi_s54>, Antoun Kanawati [email]antounk (AT) comcast (DOT) net[/email]> writes C++ has been around for at least 15 years, and was not always as the standard describes it. Well the first edition of 'The C++ Programming Language' has a prefaced dated 1985 which suggests that C++ is almost 21. 2004 - 1985 = 19, which is somewhat different from 21. It takes time to write a book and get it published I just checked in 'The Design and Evolution of C++' There it lists August 1983 as the date for the first implementation of C++. So, yes, C++ has been in use for 21 years some time next month. The name is listed as December 1983. -- 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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