C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  passing a template function to qsort          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Andrey Guest Posted: Fri Dec 12, 2003 9:34 am    Post subject: passing a template function to qsort Hi all, aCC returns "Unsatisfied symbols: compare(const void *,const void *)", when I compile code that uses this SortObjects function. template <class T> void SortObjects(int numObjects, T* objects) { qsort(objects, numObjects, sizeof(T), (int(*)(const void *, const void *)) compare<T>); } compare is declared and defined like this just above the SortObjects definition. template <class T> int compare(const void* one, const void* another) { if (*((T*)one) > *((T*)another)) { return 1; } else if (*((T*)one) < *((T*)another)) { return -1; } else { return 0; } } The same code compiles fine on g++. Am I missing some compiler flags or this cannot be done on aCC at all? Thanks in advance, Andrey [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dhruv Guest Posted: Fri Dec 12, 2003 6:21 pm    Post subject: Re: passing a template function to qsort On Fri, 12 Dec 2003 04:34:59 -0500, Andrey wrote: Quote: Hi all, aCC returns "Unsatisfied symbols: compare(const void *,const void *)", when I compile code that uses this SortObjects function. template <class T void SortObjects(int numObjects, T* objects) { qsort(objects, numObjects, sizeof(T), (int(*)(const void *, const void *)) compare } compare is declared and defined like this just above the SortObjects definition. template int compare(const void* one, const void* another) { I *think* this is where your problem lies... You can only pass functions with c linkage to qsort, and template functions cannot have c linkage, so, you will have to use some other strategy here... Regards, -Dhruv. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Roger Orr Guest Posted: Sat Dec 13, 2003 2:27 am    Post subject: Re: passing a template function to qsort "Andrey" <ae5880 (AT) yahoo (DOT) com> wrote Quote: Hi all, aCC returns "Unsatisfied symbols: compare(const void *,const void *)", when I compile code that uses this SortObjects function. template <class T void SortObjects(int numObjects, T* objects) { qsort(objects, numObjects, sizeof(T), (int(*)(const void *, const void *)) compare I'd recommend you take a look at using the standard library 'sort' function rather than trying to fit your C++ code into using C runtime functions. Roger Orr -- MVP in C++ at www.brainbench.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Tiomkin Guest Posted: Sat Dec 13, 2003 2:35 am    Post subject: Re: passing a template function to qsort [email]ae5880 (AT) yahoo (DOT) com[/email] (Andrey) wrote in message news:<395332d2.0312111330.7c9449a4 (AT) posting (DOT) google.com>... Quote: Hi all, aCC returns "Unsatisfied symbols: compare(const void *,const void *)", when I compile code that uses this SortObjects function. template <class T void SortObjects(int numObjects, T* objects) { qsort(objects, numObjects, sizeof(T), (int(*)(const void *, const void *)) compare } compare is declared and defined like this just above the SortObjects definition. This compare should be a C function for qsort. You can add the 'extern "C"' attribute to the function, but in this case you wouldn't be able to use this template more than once in the same compilation unit: the C objects cannot use name mangling, and only one object with the same name will be allowed. You'd better use std::sort instead. Quote: template <class T int compare(const void* one, const void* another) { if (*((T*)one) > *((T*)another)) { return 1; } else if (*((T*)one) < *((T*)another)) { return -1; } else { return 0; } } The same code compiles fine on g++. Am I missing some compiler flags or this cannot be done on aCC at all? This cannot be done safely on most C++/C compilers. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Sat Dec 13, 2003 2:37 am    Post subject: Re: passing a template function to qsort Dhruv wrote: Quote: I *think* this is where your problem lies... You can only pass functions with c linkage to qsort, and template functions cannot have c linkage, so, you will have to use some other strategy here... The C++ standard library provides two versions of qsort. One takes a pointer to a function with C linkage and the other takes a pointer to a function with C++ linkage. Same thing for bsearch. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dietmar Kuehl Guest Posted: Sat Dec 13, 2003 11:13 am    Post subject: Re: passing a template function to qsort [email]ae5880 (AT) yahoo (DOT) com[/email] (Andrey) wrote: Quote: template <class T void SortObjects(int numObjects, T* objects) { qsort(objects, numObjects, sizeof(T), (int(*)(const void *, const void *)) compare } You probably wanted to write std::sort(objects, objects + numObjects); It is not only easier to use but normally also faster than 'qsort()'. Also, it operates on any assignable type, not just on PODs... Quote: template <class T int compare(const void* one, const void* another) { if (*((T*)one) > *((T*)another)) { return 1; } else if (*((T*)one) < *((T*)another)) { return -1; } else { return 0; } } For a function to be a viable argument to the standard C function taking function pointer arguments, it has to be declared as 'extern "C"'. However, I think you cannot declare templates as 'extern "C"'. BTW, you should not cast away constness unless you have a very good reason to do so. The above code should probably look more like this: if (*static_cast ... Quote: The same code compiles fine on g++. Am I missing some compiler flags or this cannot be done on aCC at all? This forum is concerned with standard C++, not with the details of any attempt to implement this standard. In standard C++ the above code is not required to work. The simple work-around is the use 'std::sort()' instead of 'std::qsort()'. -- <mailto:dietmar_kuehl (AT) yahoo (DOT) com> <http://www.dietmar-kuehl.de/> Phaidros eaSE - Easy Software Engineering: <http://www.phaidros.com/> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Sat Dec 13, 2003 3:43 pm    Post subject: Re: passing a template function to qsort Dietmar Kuehl wrote: Quote: For a function to be a viable argument to the standard C function taking function pointer arguments, it has to be declared as 'extern "C"'. It can also be extern "C++". 25.4/4. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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