c++ - template specialization for derived classes -


i have template class (that cannot modify), let's call someclass, i'd specialize classes derive particular class only. following this answer able in gcc 6.3.1, unfortunately need in gcc 4.9.2, , there fails @ compile time saying "partial specialization someclass<t> not specialize template arguments".

is there way change below make work gcc 4.9.2?

#include <iostream> #include <string>  using namespace std;  struct {     string name() { return "a"; } };  struct b : {     string name() { return "b"; } };  struct c {     string name() { return "c"; } };  template<typename t, typename = std::enable_if_t<std::is_base_of<a, t>::value>> using enable_if_a = t;  template<typename t> struct someclass {     using type = t; };  template<typename t> struct someclass<enable_if_a<t>> {     using type = a; };  int main(int, char**) {     someclass<a>::type el1;     someclass<b>::type el2;     someclass<c>::type el3;      cout << el1.name() << "," << el2.name() << "," << el3.name() << endl; }

output:

a,a,c

a bit contrived, here machinery @ least works.
basic idea hide a , not inherit directly it. instead, can heavily rely on mixins , combine few classes in detector can specialize someclass.
drawback classes b become more abstruse , i'm not sure it's worth @ end of day. direct specializations better.

that being said, here working example:

#include <iostream> #include <string> #include <utility>  using namespace std;  class aderivedfactory {     struct {         string name() { return "a"; }     };      template<typename t>     struct detector: t { using type = a; };  public:     template<template<typename> class c>     using type = detector<c<a>>; };  template<typename t> struct @ : t {};  template<typename t> struct bt : t {     string name() { return "b"; } };  using = aderivedfactory::type<at>; using b = aderivedfactory::type<bt>;  struct c {     string name() { return "c"; } };  template<typename t> struct someclass {     using type = t; };  template<template<typename> class c> struct someclass<aderivedfactory::type<c>> {     using type = typename aderivedfactory::type<c>::type; };  int main(int, char**) {     someclass<a>::type el1;     someclass<b>::type el2;     someclass<c>::type el3;      cout << el1.name() << "," << el2.name() << "," << el3.name() << endl; }

see , running on wandbox.


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