C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Template specialization with partial types          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message fabioppp Guest Posted: Tue Dec 14, 2004 7:58 pm    Post subject: Template specialization with partial types Consider this struct: template <typename T> struct TypeOf { // ... }; I have many explicit specialization: template<> struct TypeOf<int> { //... }; //... and many partial specialization: template <typename T> struct TypeOf<T*> { //... }; template <typename T> struct TypeOf<std::vector { //... }; //.. Now I'd like to have another special behavior for incomplete types, so that i could write: TypeOf< std::vector >::.... Is there some way to achieve this specialization?! Now I've introduced another class (ContainerTypeOf), but I wanna use just TypeOf class. template <template struct ContainerTypeOf<Container> { //... }; -- Fabio. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Graeme Prentice Guest Posted: Wed Dec 15, 2004 1:06 pm    Post subject: Re: Template specialization with partial types On 14 Dec 2004 14:58:16 -0500, fabioppp wrote: Quote: Consider this struct: template <typename T struct TypeOf { // ... }; I have many explicit specialization: [snip] Quote: Now I'd like to have another special behavior for incomplete types, so that i could write: TypeOf< std::vector >::.... Is there some way to achieve this specialization?! Now I've introduced another class (ContainerTypeOf), but I wanna use just TypeOf class. template <template struct ContainerTypeOf<Container { //... }; I'm not sure I understand. In TypeOf< std::vector >:: std:vector is not an incomplete type - it's the name of a class template. I assume you mean it's "incomplete" because it doesn't specify the container element type e.g. vector<int> - incomplete type is an official term defined in 3.9 para 6 (an incomplete class, array of unknown size etc). Assuming you mean that you want to be able to declare something like TypeOf<int> object1; TypeOf<std::vector> object2; that isn't possible because C++ doesn't allow you to overload template class definitions. You can partially specialize TypeOf like this template < template < typename elem, typename alloc = std::allocator class Container, typename t3,typename t4 > struct TypeOf<Container { //... }; and this specialization will be used whenever the template argument is type generated from a class template that takes two parameters - unless there is a more specialised specialization - such as the one you had for vector<T> template <typename T> struct TypeOf<std::vector{}; Function templates can be overloaded, so if foo was a template function with appropriate overloads, you could write foo<std::vector>(); and foo<std::vector(); So if you don't mind writing TypeOf as a macro, you may be able to achieve the result you want i.e. TypeOf(std::vector)::result or TypeOf(std::vector<int>)::result - or just leave off the ::result bit altogether The macro could use overloaded template functions to distinguish between an argument that is the name of a class template (e.g. std::vector) and an argument that is a type (e.g. vector<int>) as in this code - note the spaces around "y" in the macro. #include <vector> #include <typeinfo> #include <iostream> #include <ostream> template <int x> struct tx { typedef char size[x]; }; template <int x> struct type_decode; template < template < typename elem, typename alloc = std::allocator class Container > tx<1>::size & func(); template <> struct type_decode<sizeof(tx<1>::size)> { typedef int result; }; template < typename T > tx<2>::size & func(); template <> struct type_decode<sizeof(tx<2>::size)> { typedef char result; }; #define TypeOf(y) type_decode< sizeof func< y >() >::result int main() { TypeOf(std::vector) abc; TypeOf(std::vector<int>) abc2; std::cout << typeid( TypeOf(std::vector) ).name(); std::cout << typeid( TypeOf(std::vector } Graeme [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tom Widmer Guest Posted: Wed Dec 15, 2004 1:08 pm    Post subject: Re: Template specialization with partial types On 14 Dec 2004 14:58:16 -0500, fabioppp <fabioppp_it (AT) yahoo (DOT) it> wrote: Quote: Consider this struct: template <typename T struct TypeOf { // ... }; I have many explicit specialization: template struct TypeOf { //... }; //... and many partial specialization: template struct TypeOf { //... }; template struct TypeOf { //... }; //.. Now I'd like to have another special behavior for incomplete types, so that i could write: TypeOf< std::vector >::.... std::vector isn't an incomplete type or even a type of any kind - it's a class template, e.g. a template for creating types. Quote: Is there some way to achieve this specialization?! No, since your TypeOf takes a type, which is a very different thing from a template. Quote: Now I've introduced another class (ContainerTypeOf), but I wanna use just TypeOf class. You should call it "TemplateContainerOf" or similar. Again, a class template isn't a type. Quote: template <template struct ContainerTypeOf { //... }; What is your usage such that you need a specialization for non-types? What does your TypeOf template do? Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top fabioppp Guest Posted: Wed Dec 15, 2004 4:48 pm    Post subject: Re: Template specialization with partial types Tom Widmer wrote: Quote: On 14 Dec 2004 14:58:16 -0500, fabioppp <fabioppp_it (AT) yahoo (DOT) it> wrote: What is your usage such that you need a specialization for non-types? What does your TypeOf template do? To retrieve the descriptor of a type. class metaclass {}; class A { static metaclass _mc; }; template <typename T> struct TypeOf<T> { static metaclass& get() { return T::_mc; } }; I want to describe template class too, not just their instantiation and I want to retrieve the metaclass of a template class through TypeOf. -- Fabio. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top fabioppp Guest Posted: Thu Dec 16, 2004 1:45 pm    Post subject: Re: Template specialization with partial types Graeme Prentice wrote: Quote: On 14 Dec 2004 14:58:16 -0500, fabioppp wrote: ... Thanks a lot. I've done in this way: #include <vector> template <int id, class C, class C1, class C2> struct Select; template <class C, class C1, class C2> struct Select<1,C,C1,C2> { typedef C Result; }; template <class C, class C1, class C2> struct Select<2,C,C1,C2> { typedef C1 Result; }; template <class C, class C1, class C2> struct Select<3,C,C1,C2> { typedef C2 Result; }; template <int id> class SizeOfHelper { char dummy[id]; }; template <class T> SizeOfHelper<1> func(); template <template SizeOfHelper<2> func(); template <template SizeOfHelper<3> func(); template <typename T> struct TypeOf { static void get() {} }; template <template struct TypeOf1 { static void get() {} }; template <template struct TypeOf2 { static void get() {} }; struct TypeOfInfo { template <class T> static void get() { TypeOf<T>::get(); } }; struct TypeOfInfo1 { template <template static void get() { TypeOf1<T>::get(); } }; struct TypeOfInfo2 { template <template static void get() { TypeOf2<T>::get(); } }; #define TYPEOF(Ty) Select< sizeof(func TypeOfInfo, TypeOfInfo1, TypeOfInfo2 Quote: ::Result::get<Ty>() int main() { TYPEOF(std::vector); TYPEOF(int); return 0; } -- Fabio. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Graeme Prentice Guest Posted: Fri Dec 17, 2004 5:33 pm    Post subject: Re: Template specialization with partial types On 16 Dec 2004 08:45:50 -0500, fabioppp wrote: [snip] Quote: template <int id class SizeOfHelper { char dummy[id]; }; template SizeOfHelper<1> func(); template <template SizeOfHelper<2> func(); template <template SizeOfHelper<3> func(); [snip] Quote: #define TYPEOF(Ty) Select< sizeof(func TypeOfInfo, TypeOfInfo1, TypeOfInfo2 ::Result::get<Ty>() This code would fail if the compiler added any packing bytes to the end of SizeOfHelper struct which was why I used the size of the array instead of the size of the struct. Graeme [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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