C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  partial ordering of template functions & parameter specifica          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Marek Vondrak Guest Posted: Sun May 14, 2006 10:21 pm    Post subject: partial ordering of template functions & parameter specifica Quote: Hello. I have written the following program and am curious why it prints "1" "2". What are the exact effects of explicitly providing function template parameters at the call? Is the second assign() function really a specialization of the first assign() or is it an assign() overload? Thank you. -- Marek -- cut -- #include <iostream class O { }; class M { }; template <class Left, class Right class Plus { }; template <class Left, class Right, class Op int assign( Left & left, const Right & right, const Op & op ) { return 1; } template <class Op int assign( M & left, const Plus<M, M> & right, const Op & op ) { return 2; } int main() { M m; std::cout << assign<M, Plus<M, M>, O>( m, Plus<M, M>(), O() ) std::endl; std::cout << assign( m, Plus<M, M>(), O() ) << std::endl; } assign function templates are disparate function templates. There is no partial template specialization for functions. Really? I think this was true in the prestandard era. Since C++98 there is a partial specialization of functions and partional ordering rules. Quote: The first function template has three template arguments and the second has one. Therefore the second function template can not be used with three template arguments specified as you do. Although I see the rationale behind your explanation, please consider the following program. class O { }; class M { }; template <class Left, class Right> class Plus { }; // AS1 template <class Left, class Right, class Op> void assign( Left & left, const Right & right, const Op & op ) { } // Primary template // AS2 template <> void assign( M & left, const Plus<M, M> & right, const O & op ) { } // Explicit specialization of AS1 int main() { M m; assign<M, Plus<M, M>, O>( m, Plus<M, M>(), O() ); // Calls AS2 } Why does it call AS2? It occurs to me, that AS1 is treated as the primary template and AS2 as its explicit specialization. In the same sense I would then believe that in the original program the first assign is the primary template and the second assign its partial specialization: template <class Left, class Right> class Plus { }; // AS1 template <class Left, class Right, class Op> void assign( Left & left, const Right & right, const Op & op ) { } // Primary template again. // AS2 template <class Op> void assign( M & left, const Plus<M, M> & right, const Op & op ) { } // Should be partial specialization of AS1? int main() { M m; assign<M, Plus<M, M>, O>( m, Plus<M, M>(), O() ); // Calls AS1! Why? This is inconsistent with the explicit specialization example! } -- Marek [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Sun May 14, 2006 11:21 pm    Post subject: Re: partial ordering of template functions & parameter speci * Marek Vondrak: Quote: Since C++98 there is a partial specialization of functions and partional ordering rules. Nope. 1) See §14/2 (second para at the start of section 14 [temp]), which currently forbids partial specialization of function templates; it was corrected in C++2003 but still forbidding partial specialization of function templates. 2) See also Peter Dimov's paper at e.g. <url: http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1295.asc>, 3) Perhaps also see Sutter and Alexendrescu's article "Why not specialize function templates" in CUJ 91, <url: http://www.gotw.ca/publications/mill17.htm>, esp. note 2. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marek Vondrak Guest Posted: Mon May 15, 2006 3:21 pm    Post subject: Re: partial ordering of template functions & parameter speci Quote: Hello. I have written the following program and am curious why it prints "1" "2". What are the exact effects of explicitly providing function template parameters at the call? Is the second assign() function really a specialization of the first assign() or is it an assign() overload? Thank you. -- Marek -- cut -- #include <iostream class O { }; class M { }; template <class Left, class Right class Plus { }; template <class Left, class Right, class Op int assign( Left & left, const Right & right, const Op & op ) { return 1; } template <class Op int assign( M & left, const Plus<M, M> & right, const Op & op ) { return 2; } int main() { M m; std::cout << assign<M, Plus<M, M>, O>( m, Plus<M, M>(), O() ) std::endl; std::cout << assign( m, Plus<M, M>(), O() ) << std::endl; } assign function templates are disparate function templates. There is no partial template specialization for functions. Really? I think this was true in the prestandard era. Since C++98 there is a partial specialization of functions and partional ordering rules. Quote: The first function template has three template arguments and the second has one. Therefore the second function template can not be used with three template arguments specified as you do. Although I see the rationale behind your explanation, please consider the following program. class O { }; class M { }; template <class Left, class Right> class Plus { }; // AS1 template <class Left, class Right, class Op> void assign( Left & left, const Right & right, const Op & op ) { } // Primary template // AS2 template <> void assign( M & left, const Plus<M, M> & right, const O & op ) { } // Explicit specialization of AS1 int main() { M m; assign<M, Plus<M, M>, O>( m, Plus<M, M>(), O() ); // Calls AS2 } Why does it call AS2? It occurs to me, that AS1 is treated as the primary template and AS2 as its explicit specialization. In the same sense I would then believe that in the original program the first assign is the primary template and the second assign its partial specialization: template <class Left, class Right> class Plus { }; // AS1 template <class Left, class Right, class Op> void assign( Left & left, const Right & right, const Op & op ) { } // Primary template again. // AS2 template <class Op> void assign( M & left, const Plus<M, M> & right, const Op & op ) { } // Should be partial specialization of AS1? int main() { M m; assign<M, Plus<M, M>, O>( m, Plus<M, M>(), O() ); // Calls AS1! Why? This is inconsistent with the explicit specialization example! } -- Marek [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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