C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Template function          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Gurikar Guest Posted: Fri Apr 29, 2005 11:25 am    Post subject: Template function Hello, Can anyone tell me whats wrong in this? // template function template<class T> inline void test(T* ptr) { cout<<"Pointer is"< ptr = NULL; cout<<"Pointer is"< } // Some class Class A() { A(); } // main function int main() { A* pObj = new A(); test(pObj ); if (pObj == NULL) { cout<<"Mem fail"< } My question, even after making the pointer pObj = NULL in template function test, when it comes to main the value of pObj wont be NULL, Why is this??? Regards } Back to top Tim Love Guest Posted: Fri Apr 29, 2005 11:38 am    Post subject: Re: Template function "Gurikar" <msgurikar (AT) gmail (DOT) com> writes: Quote: Hello, Can anyone tell me whats wrong in this? ... My question, even after making the pointer pObj = NULL in template function test, when it comes to main the value of pObj wont be NULL, Why is this??? For now forget about templates and look at something like #include <iostream> void test(int *ptr) { ptr=0; } int main() { int* pObj = new int(); test(pObj ); if (pObj == 0) std::cout<<"Mem fail"< } Back to top John Bufton Guest Posted: Fri Apr 29, 2005 11:45 am    Post subject: Re: Template function Gurikar wrote: Quote: Hello, Can anyone tell me whats wrong in this? // template function template inline void test(T* ptr) { cout<<"Pointer is"< ptr = NULL; cout<<"Pointer is"< } // Some class Class A() { A(); } // main function int main() { A* pObj = new A(); test(pObj ); if (pObj == NULL) { cout<<"Mem fail"< } My question, even after making the pointer pObj = NULL in template function test, when it comes to main the value of pObj wont be NULL, Why is this??? Regards } The pointer is passed by value. Hence the pointer value in the calling function remains unchanged. Try passing by reference if you want the pointer of the calling function to be affected. John Bufton Back to top Gurikar Guest Posted: Fri Apr 29, 2005 11:48 am    Post subject: Re: Template function why is this?????? Back to top Gurikar Guest Posted: Fri Apr 29, 2005 11:51 am    Post subject: Re: Template function Can i do this: void test(int*& ptr = NULL) { ptr=0; } Back to top Sharad Kala Guest Posted: Fri Apr 29, 2005 12:01 pm    Post subject: Re: Template function "Gurikar" <msgurikar (AT) gmail (DOT) com> wrote in message Quote: Can i do this: void test(int*& ptr = NULL) { ptr=0; } Why not try on a compiler ? The answer is no, most good compilers will also flash the reason. Back to top John Bufton Guest Posted: Fri Apr 29, 2005 12:02 pm    Post subject: Re: Template function Gurikar wrote: Quote: Can i do this: void test(int*& ptr = NULL) { ptr=0; } No. Prototype definition can have default parameter value, but not the function itself. Hence the following is OK. // Function prototype definition with default parameter value void test(int*& ptr = NULL); // Function implementation void test(int *&ptr) { ptr = NULL; } John Bufton Back to top Sharad Kala Guest Posted: Fri Apr 29, 2005 12:02 pm    Post subject: Re: Template function "Gurikar" <msgurikar (AT) gmail (DOT) com> wrote in message Quote: why is this?????? Because your problem has nothing to do with templates. btw, it's a good idea to not snip the context entirely when you reply. Sharad Back to top John Bufton Guest Posted: Fri Apr 29, 2005 12:10 pm    Post subject: Re: Template function John Bufton wrote: Quote: Gurikar wrote: Can i do this: void test(int*& ptr = NULL) { ptr=0; } No. Prototype definition can have default parameter value, but not the function itself. Hence the following is OK. // Function prototype definition with default parameter value void test(int*& ptr = NULL); // Function implementation void test(int *&ptr) { ptr = NULL; } John Bufton Apologies the above is wrong, (sheer carelessness). The prototype needs to reference an exisiting pointer as shown below. int *Tmp = NULL void test(int*& ptr = Tmp); Back to top Sharad Kala Guest Posted: Fri Apr 29, 2005 12:12 pm    Post subject: Re: Template function "John Bufton" <j.bufton (AT) ntlworld (DOT) com> wrote in message Quote: Gurikar wrote: Can i do this: void test(int*& ptr = NULL) { ptr=0; } No. Prototype definition can have default parameter value, but not the function itself. Hence the following is OK. // Function prototype definition with default parameter value void test(int*& ptr = NULL); // Function implementation void test(int *&ptr) { ptr = NULL; } Incorrect. The problem with test function is that you can't bind a non-const reference to an r-value. Sharad Back to top John Bufton Guest Posted: Fri Apr 29, 2005 12:16 pm    Post subject: Re: Template function Sharad Kala wrote: Quote: "John Bufton" <j.bufton (AT) ntlworld (DOT) com> wrote in message Gurikar wrote: Can i do this: void test(int*& ptr = NULL) { ptr=0; } No. Prototype definition can have default parameter value, but not the function itself. Hence the following is OK. // Function prototype definition with default parameter value void test(int*& ptr = NULL); // Function implementation void test(int *&ptr) { ptr = NULL; } Incorrect. The problem with test function is that you can't bind a non-const reference to an r-value. Sharad I couldn't agree more! As already stated, sheer carelessness on my part. Well spotted anyway. Personally I'm not keen on default parameter values for reference parameters. John Bufton Back to top Gurikar Guest Posted: Fri Apr 29, 2005 12:34 pm    Post subject: Re: Template function You are right sharad kala.. Good.. Back to top Axter Guest Posted: Fri Apr 29, 2005 1:53 pm    Post subject: Re: Template function Gurikar wrote: Quote: Hello, Can anyone tell me whats wrong in this? // template function template<class T inline void test(T* ptr) { cout<<"Pointer is"< ptr = NULL; cout<<"Pointer is"< } // Some class Class A() { A(); } // main function int main() { A* pObj = new A(); test(pObj ); if (pObj == NULL) { cout<<"Mem fail"< } My question, even after making the pointer pObj = NULL in template function test, when it comes to main the value of pObj wont be NULL, Why is this??? Regards } You're passing the pointer by value, and you need to pass the pointer by reference or by pointer (pointer to a pointer). Example code: // template function template inline void test(T*& ptr) { cout<<"Pointer is"< ptr = NULL; cout<<"Pointer is"< } // Some class class A { public: A(){} }; int main(int argc, char* argv[]) { A* pObj = new A; test(pObj ); if (pObj == NULL) { cout<<"Mem fail"< } Back to top whisper Guest Posted: Sun May 01, 2005 2:03 pm    Post subject: Re: Template function in c++, the arguments of function call are always by value. eg. void func(void* ptr); we should know that ptr is by-value, we can feel the changing of the value of ptr(that is *ptr) out of the scope of func, however, ptr itself cannot be modified by func as it is by-value. so ptr is by-value, *ptr is by-ref if we wanna change ptr, we should change the declare of func to void func(void** ptr); Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.