C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  template template parameters question          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Goncalo de Carvalho Guest Posted: Thu Jun 16, 2005 8:12 am    Post subject: template template parameters question Hey, Consider the following code: template<typename T> struct D { }; template<typename T, template struct A : public U<T> { }; template<typename T, template = D> struct B : public S<T, U { }; int main( ) { B<unsigned char, A> a; } Why can't A be used above? All the compilers I tried say "error: class template "A" is not compatible with template template parameter "S"". This is with Intel C++ 8.1 and VC++ .NET 2002 and 2005 beta 2. TIA, Goncalo [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Fri Jun 17, 2005 10:11 am    Post subject: Re: template template parameters question "Goncalo de Carvalho" <glslang_NOSPAM (AT) yah_NOSPAMoo (DOT) com> writes: Quote: Hey, Consider the following code: template<typename T struct D { }; template struct A : public U<T { }; template = D struct B : public S<T, U { }; int main( ) { B } Why can't A be used above? The short answer is: because the standard says so. Template arguments must match in kind and in arity. -- Gabriel Dos Reis [email]gdr (AT) integrable-solutions (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Fri Jun 17, 2005 10:21 am    Post subject: Re: template template parameters question "Goncalo de Carvalho" <glslang_NOSPAM (AT) yah_NOSPAMoo (DOT) com> writes: Quote: Hey, Consider the following code: template<typename T struct D { }; template struct A : public U<T { }; template = D struct B : public S<T, U { }; int main( ) { B } Why can't A be used above? Because a template is not a type. If you want S to match A, it needs to be: template< typename T , template class S ^^^^^^^^^^^^^^^^^^^^^ , template<class> class U = D> struct B : public S<T, U { }; HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Fri Jun 17, 2005 10:22 am    Post subject: Re: template template parameters question Hi, Goncalo de Carvalho wrote: Quote: template<typename T struct D { }; template struct A : public U<T { }; template = D Rather: template<typename T, template class S, template<class> class U = D> Quote: struct B : public S<T, U Rather: struct B : public S Quote: { }; int main( ) { B<unsigned char, A> a; } -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Fri Jun 17, 2005 10:24 am    Post subject: Re: template template parameters question "Goncalo de Carvalho" <glslang_NOSPAM (AT) yah_NOSPAMoo (DOT) com> writes: Quote: template<typename T struct D { }; template struct A : public U<T { }; template = D struct B : public S<T, U { }; int main( ) { B } Why can't A be used above? All the compilers I tried say "error: class template "A" is not compatible with template template parameter "S"". This is pretty much it. A is not compatible with the second parameter of B. Why do you think it should be? FWIW, changing the definition of B to template<typename T, template class S, template<typename> class U = D> struct B : public S<T,U> { }; makes the code compile. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Greg Guest Posted: Fri Jun 17, 2005 1:54 pm    Post subject: Re: template template parameters question Goncalo de Carvalho wrote: Quote: Hey, Consider the following code: template<typename T struct D { }; template struct A : public U<T { }; template = D struct B : public S<T, U { }; int main( ) { B } Why can't A be used above? All the compilers I tried say "error: class template "A" is not compatible with template template parameter "S"". This is with Intel C++ 8.1 and VC++ .NET 2002 and 2005 beta 2. The sample doesn't compile because you really intended to write it like this: template<typename T> struct D { }; template<typename T, template struct A : public U<T> { }; // note that the template template parameter (S) // itself takes a template template parameter (V) template< typename T, template class S, template<class> class U = D> struct B : public S<T, U> // second type parameter must be a // template (U) not a type (U<T>) { }; Even with the sample code compiling, it does have the drawback of being, er, complex. Greg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gene Bushuyev Guest Posted: Fri Jun 17, 2005 2:00 pm    Post subject: Re: template template parameters question "Goncalo de Carvalho" <glslang_NOSPAM (AT) yah_NOSPAMoo (DOT) com> wrote Quote: Hey, Consider the following code: template<typename T struct D { }; template struct A : public U<T { }; template U = D struct B : public S<T, U { }; int main( ) { B } Why can't A be used above? All the compilers I tried say Because, as compiler told you, they are incompatible. Modify struct B this way and then you get a match: template<typename T, template class S, template<class> class U= D> struct B : public S<T, U> { }; Gene [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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