c++ - How to make policy dictate a member variable type? -

while trying apply policy-based design, got stuck on 1 (simplified):

template <class tprintpolicy, typename t> struct : private tprintpolicy {     using tprintpolicy::print;     t t;     void foo() {         print(t);     } };  struct intpolicy {     void print(int n) {         std::cout << n << std::endl;     } };  int main(int argc, char* argv[]) {     a<intpolicy, int> a;     a.foo();     return 0; } 

and here's question: how should redefine class possible provide policy parameter template, let infer t on own, this:

a<intpolicy> a; 

preferrably, policy definition should not more complicated now. ideas?

edit:
forgot mention not want policy export typedef. of course easy solution, cannot infer type of t on own?

make typedef within each policy gives policy's data type.

template <class tprintpolicy> struct : private tprintpolicy {     using tprintpolicy::print;     typename tprintpolicy::data_type t;     void foo() {         print(t);     } };  struct intpolicy {     typedef int data_type;     void print(int n) {         std::cout << n << std::endl;     } };  int main(int argc, char* argv[]) {     a<intpolicy> a;     a.foo();     return 0; } 

edit: personally, use typedef, though adds duplication, because believe makes code clearer. however, if want avoid it, try similar this.

#include <boost/typeof/typeof.hpp>  template<typename t, typename class> t deduceargumenttype(void (class::*ptr)(t)) {}  template <class tprintpolicy> struct : private tprintpolicy {     using tprintpolicy::print;     typedef boost_typeof(deduceargumenttype(&tprintpolicy::print)) data_type;     data_type t;     void foo() {         print(t);     } };  struct intpolicy {     void print(int n) {         std::cout << n << std::endl;     } };  int main(int argc, char* argv[]) {     a<intpolicy> a;     a.foo();     return 0; } 

it might possible boost.typetraits this, i'm not sure how.