C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why is my const pointer not behaving?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message DamonChong Guest Posted: Sat Jan 29, 2005 5:56 pm    Post subject: Why is my const pointer not behaving? Hi, I am new and had this piece of code which I created but it is not behaving as expected. I'm using g++, can someone help me out? Thank you. //--------file (Object.cc)----------- class Object { public: Object(const char * c){ id = c; } ~Object() {} const char * get_id() const { return id; } private: const char * id; }; //-------file (Test.cc)------------- #include <iostream> #include <sstream> #include "Object.cc" class Test{ public: Object ** obj; Test(){ obj = new Object * [3]; for( int i=0; i<3; i++ ){ std::ostringstream os; os << i; obj[i] = new Object( os.str().c_str() ); std::cout << "Created id = " << obj[i]->get_id() << std::endl; } } ~Test(){} }; int main() { Test t; for(int i=0; i<3; i++){ std::cout << "Object id - " << t.obj[i]->get_id() << " // expecting " << i << std::endl; } } //----------------output upon running------------ => ./Test Created id = 0 Created id = 1 Created id = 2 Object id - 2 // expecting 0 Object id - 2 // expecting 1 Object id - 2 // expecting 2 My query is if I changed the "const char *" to simply int as the parameter to pass the Object constructor like this and make the appropriate changes, it works as expected. Now, I know i'm missing something here! //---------------file (Object1.cc)----------------- class Object1 { public: Object1(int c){ id = c; } ~Object1() {} const int get_id() const { return id; } private: int id; }; Back to top Victor Bazarov Guest Posted: Sat Jan 29, 2005 6:24 pm    Post subject: Re: Why is my const pointer not behaving? "DamonChong" <so_excited (AT) excite (DOT) com> wrote... Quote: Hi, I am new and had this piece of code which I created but it is not behaving as expected. I'm using g++, can someone help me out? Thank you. Your 'Object' class is holding onto a pointer that quickly becomes invalid. You need to rethink what you store in 'Object' and why. I recommend making 'id' a "std::string". Quote: [...] Back to top Arrivederci Guest Posted: Sat Jan 29, 2005 6:26 pm    Post subject: Re: Why is my const pointer not behaving? DamonChong wrote: Quote: Hi, I am new and had this piece of code which I created but it is not behaving as expected. I'm using g++, can someone help me out? Thank you. const pointer? char const *id; http://www.possibility.com/Cpp/const.html Back to top Peter Koch Larsen Guest Posted: Sat Jan 29, 2005 6:30 pm    Post subject: Re: Why is my const pointer not behaving? I've already advised you not to use pointers. They really should be used very carefully, and not by beginners. Here your class "Object" should have a std::string as a member: thus id should be a std::string, not a pointer to char. "DamonChong" <so_excited (AT) excite (DOT) com> skrev i en meddelelse news:1107021400.925156.180400 (AT) z14g2000cwz (DOT) googlegroups.com... Quote: Hi, I am new and had this piece of code which I created but it is not behaving as expected. I'm using g++, can someone help me out? Thank you. //--------file (Object.cc)----------- class Object { public: Object(const char * c){ id = c; } ~Object() {} const char * get_id() const { return id; } private: const char * id; }; //-------file (Test.cc)------------- #include <iostream #include #include "Object.cc" class Test{ public: Object ** obj; Test(){ obj = new Object * [3]; for( int i=0; i<3; i++ ){ std::ostringstream os; os << i; obj[i] = new Object( os.str().c_str() ); os.str().c_str() is only valid as long as os is not changed. Thus you keep tenporary pointers in your newly created object. Quote: std::cout << "Created id = " << obj[i]->get_id() std::endl; } } ~Test(){} }; int main() { Test t; for(int i=0; i<3; i++){ std::cout << "Object id - " << t.obj[i]->get_id() " // expecting " << i << std::endl; } } //----------------output upon running------------ => ./Test Created id = 0 Created id = 1 Created id = 2 Object id - 2 // expecting 0 Object id - 2 // expecting 1 Object id - 2 // expecting 2 My query is if I changed the "const char *" to simply int as the parameter to pass the Object constructor like this and make the appropriate changes, it works as expected. Now, I know i'm missing something here! You are missing a good book - perhaps "Accelerated C++", of which I've read the first chapter. That chapter is very good and the book as a whole has been praised by all. [snip] /Peter Back to top Shezan Baig Guest Posted: Sat Jan 29, 2005 10:07 pm    Post subject: Re: Why is my const pointer not behaving? Arrivederci wrote: Quote: DamonChong wrote: Hi, I am new and had this piece of code which I created but it is not behaving as expected. I'm using g++, can someone help me out? Thank you. const pointer? char const *id; http://www.possibility.com/Cpp/const.html It's a nice article, but I'm not sure of its relevance to the OP's question. -shez- Back to top Adieu Guest Posted: Sun Jan 30, 2005 12:32 am    Post subject: Re: Why is my const pointer not behaving? Shezan Baig wrote: Quote: Arrivederci wrote: DamonChong wrote: Hi, I am new and had this piece of code which I created but it is not behaving as expected. I'm using g++, can someone help me out? Thank you. const pointer? char const *id; http://www.possibility.com/Cpp/const.html It's a nice article, but I'm not sure of its relevance to the OP's question. He's trying to understand constness. That's why he's playing. -- I tell you, we are here on Earth to fart around, and don't let anybody tell you different. -- Kurt Vonnegut Back to top DamonChong Guest Posted: Sun Jan 30, 2005 2:02 am    Post subject: Re: Why is my const pointer not behaving? Ah, now i see. Kind of silly, I bought the book but lend it to someone instead. Will get it back and go through it, thanks again! Back to top Ron Natalie Guest Posted: Sun Jan 30, 2005 7:01 pm    Post subject: Re: Why is my const pointer not behaving? Adieu wrote: Quote: He's trying to understand constness. That's why he's playing. The problem is that there is little runtime influence to const. const like access control is a compile time issue. If it compiles then the checking has been done. Assuming the compiler actually enforces constness at the runtime can't be determine without undefined behavior. Back to top Old Wolf Guest Posted: Sun Jan 30, 2005 7:32 pm    Post subject: Re: Why is my const pointer not behaving? Peter Koch Larsen wrote: Quote: "DamonChong" <so_excited (AT) excite (DOT) com> skrev: Object ** obj; Test(){ obj = new Object * [3]; for( int i=0; i<3; i++ ){ std::ostringstream os; os << i; obj[i] = new Object( os.str().c_str() ); All of that guff could have been replaced with Object obj[3]. Or if dynamic sizing is needed, use a vector. (Assuming Object is copyable). Quote: os.str().c_str() is only valid as long as os is not changed. It's only valid until the end of the full-expression it occurs in. But even if you had an implementation where c_str() was valid until its string changed, os.str() returns a temporary string anyway, which is destroyed at the end of the full-expression. 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