C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  References and multi-data lists          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Zaharije Pasalic Guest Posted: Mon Aug 23, 2004 11:57 am    Post subject: References and multi-data lists I tried to implement multi-data list with a reference instead of pointer to data, but code compiled with gcc perform "segmentation fault". Generated code from other compilers: Borland C++ 5.0 and Micros. C++ 6.0 is OK. What is wrong? #include <iostream> using namespace::std; class base { public: virtual void dump() = 0; }; class chr : public base { char ch; public: chr(char s) { ch = s; } ~chr() { } void dump() { cout << ch ; } }; class num : public base { int n; public: num(int i) {n = i; } ~num() {} void dump() { cout << n; } }; struct node { base &data; node *next; node(base &d) : data(d) {} }; class list { node *n; public: list() : n(0) {} void add(base &d); void dump() { for(node *tmp = n; tmp; tmp = tmp->next) /* THIS IS LINE WHERE SEGMENTATION OCCURS */ tmp->data.dump(); } }; void list::add(base &d) { node *tmp = n; if (!n) { n = new node(d); Back to top Rolf Magnus Guest Posted: Mon Aug 23, 2004 12:20 pm    Post subject: Re: References and multi-data lists Zaharije Pasalic wrote: Quote: I tried to implement multi-data list with a reference instead of pointer to data, but code compiled with gcc perform "segmentation fault". Generated code from other compilers: Borland C++ 5.0 and Micros. C++ 6.0 is OK. What is wrong? #include <iostream using namespace::std; class base { public: virtual void dump() = 0; }; class chr : public base { char ch; public: chr(char s) { ch = s; } ~chr() { } void dump() { cout << ch ; } }; class num : public base { int n; public: num(int i) {n = i; } ~num() {} void dump() { cout << n; } }; struct node { base &data; node *next; node(base &d) : data(d) {} }; class list { node *n; public: list() : n(0) {} void add(base &d); void dump() { for(node *tmp = n; tmp; tmp = tmp->next) /* THIS IS LINE WHERE SEGMENTATION OCCURS */ tmp->data.dump(); } }; void list::add(base &d) { node *tmp = n; if (!n) { n = new node(d); You didn't show how you use your list. Are you sure none of the objects you reference went out of scope before you traverse your list? Back to top Karl Heinz Buchegger Guest Posted: Mon Aug 23, 2004 12:21 pm    Post subject: Re: References and multi-data lists Zaharije Pasalic wrote: Quote: I tried to implement multi-data list with a reference instead of pointer to data, but code compiled with gcc perform "segmentation fault". Generated code from other compilers: Borland C++ 5.0 and Micros. C++ 6.0 is OK. What is wrong? #include <iostream using namespace::std; class base { public: virtual void dump() = 0; }; class chr : public base { char ch; public: chr(char s) { ch = s; } ~chr() { } void dump() { cout << ch ; } }; class num : public base { int n; public: num(int i) {n = i; } ~num() {} void dump() { cout << n; } }; struct node { base &data; node *next; node(base &d) : data(d) {} }; How about initializing next to something usefull ? Quote: class list { node *n; public: list() : n(0) {} void add(base &d); void dump() { for(node *tmp = n; tmp; tmp = tmp->next) /* THIS IS LINE WHERE SEGMENTATION OCCURS */ tmp->data.dump(); } }; When working with pointers, always check if they point to something usefull. Part of that check is if they get initialized to something usefull. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Ron Natalie Guest Posted: Mon Aug 23, 2004 12:26 pm    Post subject: Re: References and multi-data lists "Zaharije Pasalic" <zaharije.pasalic (AT) pmf (DOT) unsa.ba> wrote Quote: I tried to implement multi-data list with a reference instead of pointer to data, but code compiled with gcc perform "segmentation fault". Generated code from other compilers: Borland C++ 5.0 and Micros. C++ 6.0 is OK. What is wrong? Your example is incomplete...but one guess, are you sure the objects passed to list::add continue to exist when the dump routine is called? Back to top Ron Natalie Guest Posted: Mon Aug 23, 2004 1:05 pm    Post subject: Re: References and multi-data lists "Karl Heinz Buchegger" <kbuchegg (AT) gascad (DOT) at> wrote Quote: When working with pointers, always check if they point to something usefull. Part of that check is if they get initialized to something usefull. -- ....of course, it's not always possible to check to see if a pointer or reference is still valid. As I suspect in this case, the references refer to objects that have ceased to be, expired and gone to meet it's maker, bereft of life, run down the curtain and joined the choir invisible... Back to top Karl Heinz Buchegger Guest Posted: Mon Aug 23, 2004 1:49 pm    Post subject: Re: References and multi-data lists Ron Natalie wrote: Quote: "Karl Heinz Buchegger" <kbuchegg (AT) gascad (DOT) at> wrote When working with pointers, always check if they point to something usefull. Part of that check is if they get initialized to something usefull. -- ...of course, it's not always possible to check to see if a pointer or reference is still valid. True. But surpisingly often, if I have a pointer, a quick look at the thing it is pointing to with the debugger shows that the pointer is garbage. Anyway, what I wanted to get at, was that the OP didn't show code to set the next pointer in his structure to NULL. So when he adds the first struct to his list, I guess he is expecting a 0 pointer in there, which is not there. And that *can* be detected easily with the debugger Quote: As I suspect in this case, the references refer to objects that have ceased to be, expired and gone to meet it's maker, bereft of life, run down the curtain and joined the choir invisible... Could also be. The OP simply presented not enough code to decide. In fact the OP needs to learn one lesson: When dealing with dynamic data structures (own implemented) and the display code crashes, then most likely it is not the display code that is in error, but the code that builds up the structure. At least that was my experience during my apprentice years :-) -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Jon Bell Guest Posted: Mon Aug 23, 2004 2:28 pm    Post subject: Re: References and multi-data lists In article <4129ea97$0$2419$9a6e19ea (AT) news (DOT) newshosting.com>, Ron Natalie <ron (AT) sensor (DOT) com> wrote: Quote: ...of course, it's not always possible to check to see if a pointer or reference is still valid. As I suspect in this case, the references refer to objects that have ceased to be, expired and gone to meet it's maker, bereft of life, run down the curtain and joined the choir invisible... In other words, they're ex-objects! Monty C++, anyone? -- Jon Bell <jtbellm4h (AT) presby (DOT) edu> Presbyterian College Dept. of Physics and Computer Science Clinton, South Carolina USA Back to top Zaharije Pasalic Guest Posted: Thu Sep 09, 2004 12:55 pm    Post subject: Re: References and multi-data lists "Ron Natalie" <ron (AT) sensor (DOT) com> wrote Quote: "Zaharije Pasalic" <zaharije.pasalic (AT) pmf (DOT) unsa.ba> wrote I tried to implement multi-data list with a reference instead of pointer to data, but code compiled with gcc perform "segmentation fault". Generated code from other compilers: Borland C++ 5.0 and Micros. C++ 6.0 is OK. What is wrong? Your example is incomplete...but one guess, are you sure the objects passed to list::add continue to exist when the dump routine is called? OK! Code is not so good (maybe has errors) becuase I wrote this without compiling Main problem was adding new node when temporaly object is created, so after calling "add" object is gonne forevere, or better: "expired and gone to meet it's maker, bereft of life, run down the curtain and joined the choir invisible..." (Thanks Ron for this description :) Main reaseon for this is my thinking that temporaly object MUST exist if exist reference to him. So, i was wrong! Thanks a lot. 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