c++ - How to subclass a superclass chosen by traits? -

i have template of class (let's call a) should subclass 1 of other classes (b or c), depending on compile time conditional in custom trait struct. i'm attaching snippet reproduces behaviour.

#include <type_traits>  template<typename t> class cls_template { public:   using method_arg_type = t;   virtual void method(method_arg_type) = 0; };  using = cls_template<float>; using b = cls_template<int>;  template<typename t> struct traits {   using cls = std::conditional<std::is_floating_point<t>::value, a, b>; };  //class c : public traits<float>::cls { class c : public { public:   virtual void method(method_arg_type arg) {}; };  int main() {   *a = new c(); } 

if leave (hardcoding superclass) works fine. however, once replace class definition commented out line i'm getting following error messages:

test.cpp:21:27: error: unknown type name 'method_arg_type'   virtual void method(method_arg_type arg) {};                       ^ test.cpp:25:10: error: cannot initialize variable of type 'a *'   (aka 'cls_template<float> *') rvalue of type 'c *'   *a = new c();      ^   ~~~~~~~ 

why method_arg_type no longer defined? why c no longer recognised subclass of a? i've found out if traits aren't template (if hardcode type struct), works fine.

you're trying derive traits<float>::cls of type std::conditional<std::is_floating_point<t>::value, a, b>. it's neither a nor b, it's specialization of conditional template. either derive ::type of it, work expected, or use conditional_t (c++14).

class c : public traits<float>::cls::type { // ok                                    ^^^^^^ public:   virtual void method(method_arg_type arg) {}; }; 

demo

template<typename t> struct traits {   using cls = std::conditional_t<std::is_floating_point<t>::value, a, b>;                               ^^   // above c++14, in c++11 you'd have write   // using cls = typename std::conditional<std::is_floating_point<t>::value, a, b>::type; };  class c : public traits<float>::cls { // ok public:   virtual void method(method_arg_type arg) {}; }; 

demo