C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How to specialize function template when          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Guest Posted: Wed Feb 01, 2006 4:00 pm    Post subject: How to specialize function template when Hi all, does anyone have an idea how to specialice a function template when the type does not occur in the parameter list ? Example: template <class T> class TheClass { private: std::string a_; std::string b_; public: void dosomething() { foo<T>( a_, b_ ); } }; template<class T> void foo( std::string& a, const std::string& b ) { // default do nothing } template<std::string> void foo( std::string& a, const std::string& b ) { // do what has to be done for strings } Now when i use this code: TheClass<std::string> theclass; theclass.dosomething(); // calls default foo not specialized foo Does anyone know what i am doing wrong ? Thanks in advance, Joe [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Wed Feb 01, 2006 6:00 pm    Post subject: Re: How to specialize function template when fondwey (AT) yahoo (DOT) com wrote: Quote: does anyone have an idea how to specialice a function template when the type does not occur in the parameter list ? Example: template <class T> class TheClass { private: std::string a_; std::string b_; public: void dosomething() { foo<T>( a_, b_ ); } }; template<class T> void foo( std::string& a, const std::string& b ) { // default do nothing } template<std::string> void foo( std::string& a, const std::string& b ) { // do what has to be done for strings } Now when i use this code: TheClass<std::string> theclass; theclass.dosomething(); // calls default foo not specialized foo Does anyone know what i am doing wrong ? You're using the wrong syntax to declare and define the the std::string specialization of foo(). Try this: #include <string> #include <iostream> #include <ostream> template<class T> void foo(std::string& a, const std::string& b) { std::cout << "Default foo()"; } template<> void foo<std::string>(std::string& a, const std::string& b ) { std::cout << "foo<std::string>()"; } template <class T> class TheClass { std::string a_, b_; public: void dosomething() { foo<T>( a_, b_ ); } }; int main() { TheClass<std::string> theclass; theclass.dosomething(); } Best regards, Tom [ 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: Wed Feb 01, 2006 6:00 pm    Post subject: Re: How to specialize function template when fondwey (AT) yahoo (DOT) com wrote: Quote: does anyone have an idea how to specialice a function template when the type does not occur in the parameter list ? Example: template <class T> class TheClass { private: std::string a_; std::string b_; public: void dosomething() { foo<T>( a_, b_ ); } }; template<class T> void foo( std::string& a, const std::string& b ) { // default do nothing } template<std::string> void foo( std::string& a, const std::string& b ) You mean: template<> void foo<std::string>( ... don't you? Quote: { // do what has to be done for strings } Now when i use this code: TheClass<std::string> theclass; theclass.dosomething(); // calls default foo not specialized foo Does anyone know what i am doing wrong ? You're using wrong syntax to define a specialisation. Doesn't your compiler tell you that? --------------- This code #include <string> template<class T> class TheClass { std::string a_; std::string b_; public: void dosomething() { foo<T>( a_, b_ ); } }; #include <iostream> template<class T> void foo( std::string& a, const std::string& b ) { // default do nothing std::cout << "default\n"; } template<> void foo<std::string>( std::string& a, const std::string& b ) { // do what has to be done for strings std::cout << "special\n"; } int main() { TheClass<int> tci; TheClass<std::string> tcs; tci.dosomething(); tcs.dosomething(); } --------------- outputs default special ------------------------ Apparently, as you intend it to. I didn't test it on all available to me compilers, but if it doesn't work for you, move the 'foo' definitions before the 'TheClass' template definition. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ale¹ Pergl Guest Posted: Thu Feb 02, 2006 1:00 am    Post subject: Re: How to specialize function template when Quote: Hi all, does anyone have an idea how to specialice a function template when the type does not occur in the parameter list ? Example: template <class T> class TheClass { private: std::string a_; std::string b_; public: void dosomething() { foo<T>( a_, b_ ); } }; template<class T> void foo( std::string& a, const std::string& b ) { // default do nothing } template<std::string> void foo( std::string& a, const std::string& b ) { // do what has to be done for strings } Now when i use this code: TheClass<std::string> theclass; theclass.dosomething(); // calls default foo not specialized foo Does anyone know what i am doing wrong ? Thanks in advance, Joe You thought you wrote a function template specialization but actually you wrote another function template whose template argument is a non-type parameter std::string. The code is legal but means something else than you intended. The correct syntax for template specialization is this: template<> void foo<std::string>( std::string& a, const std::string& b ) {} Regards, A. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Eugene Alterman Guest Posted: Thu Feb 02, 2006 12:01 pm    Post subject: Re: How to specialize function template when "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message news:Xf4Ef.535$a97.432 (AT) newsread1 (DOT) mlpsca01.us.to.verio.net... Quote: fondwey (AT) yahoo (DOT) com wrote: [snip] Quote: template<class T> void foo( std::string& a, const std::string& b ) { // default do nothing } template<std::string> void foo( std::string& a, const std::string& b ) You mean: template<> void foo<std::string>( ... don't you? { // do what has to be done for strings } Now when i use this code: TheClass<std::string> theclass; theclass.dosomething(); // calls default foo not specialized foo Does anyone know what i am doing wrong ? You're using wrong syntax to define a specialisation. Doesn't your compiler tell you that? The "specialization" actually declares an unrelated template with a non-type parameter of std::string type. Since non-type template parameters of class type are not allowed this template declaration is ill-formed. Microsoft VC7.1 compiles it without any diagnostics, but gives a correct error message if a parameter name is provided: template<std::string str> void foo( std::string& a, const std::string& b). It is obviously a compiler bug. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ale¹ Pergl Guest Posted: Thu Feb 02, 2006 6:00 pm    Post subject: Re: How to specialize function template when Oops, correction... class type 'non-type template parameters' are _not_ legal. A. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Guest Posted: Thu Feb 02, 2006 6:00 pm    Post subject: Re: How to specialize function template when Thank you all. You are of course correct, my specialization was completely wrong in the first place in that it was no specialization at all. Should have seen that myself, but got the compiler and me sufficiently confused. What was funny though was, that i had it correct before, but only for one specialization and not the others. It did somehow not work and called the default version consistently. What i also found out is, that when you use the template-inclusion-model and have the code in a header-file, the compiler ( at least VS2003 ) complains about multiply-defined symbols. You need to put the code of the specialized versions into the cpp-file. I can see more clearly now. Thanks. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Eugene Alterman Guest Posted: Thu Feb 02, 2006 9:00 pm    Post subject: Re: How to specialize function template when <fondwey (AT) yahoo (DOT) com> wrote in message news:1138897102.432941.124060 (AT) o13g2000cwo (DOT) googlegroups.com... Quote: What i also found out is, that when you use the template-inclusion-model and have the code in a header-file, the compiler ( at least VS2003 ) complains about multiply-defined symbols. If you define a function template in a header file you have to declare it 'inline'. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Sat Feb 04, 2006 2:00 pm    Post subject: Re: How to specialize function template when Eugene Alterman <eugalt (AT) verizon (DOT) net> wrote: Quote: fondwey (AT) yahoo (DOT) com> wrote in message news:1138897102.432941.124060 (AT) o13g2000cwo (DOT) googlegroups.com... What i also found out is, that when you use the template-inclusion-model and have the code in a header-file, the compiler ( at least VS2003 ) complains about multiply-defined symbols. If you define a function template in a header file you have to declare it 'inline'. I don't think so. This is a valid template declaration in a header template <typename T> void foo(T ) {} it is up to the compiler/linker to remove duplicate instances if the code is not inlined. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Martin Eisenberg Guest Posted: Sat Feb 04, 2006 6:00 pm    Post subject: Re: How to specialize function template when Eugene Alterman wrote: Quote: fondwey (AT) yahoo (DOT) com> wrote in message news:1138897102.432941.124060 (AT) o13g2000cwo (DOT) googlegroups.com... What i also found out is, that when you use the template-inclusion-model and have the code in a header-file, the compiler ( at least VS2003 ) complains about multiply-defined symbols. If you define a function template in a header file you have to declare it 'inline'. No, the reason is that a function template specialization is not a template anymore, it is an ordinary function. Martin -- Quidquid latine scriptum sit, altum viditur. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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