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  1. #include <tuple>
  2.  
  3. struct Cache;
  4.  
  5. template<int, typename> struct cache_getter;
  6. template<int, typename...> struct tuple_walker;
  7.  
  8. template<int I, typename... Ts> struct cache_getter<I, std::tuple<Ts...> > {
  9.     static std::tuple<Ts...> get(Cache & c);
  10. };
  11.  
  12. struct Cache {
  13. protected:
  14. 	template<int, typename...> friend struct tuple_walker;
  15. private:
  16. 	/* here T is a type from within a std::tuple<...> */
  17. 	template<int I, typename T> std::tuple<T> get_ex() {
  18. 		return std::tuple<T>();
  19. 	}
  20. public:
  21. 	/* here T is actually a std::tuple<...> */
  22. 	template<typename T> T get() {
  23. 		return cache_getter<0, T>::get(*this);
  24. 	}
  25. };
  26.  
  27. template<typename...> struct my_tuple_cat;
  28. template<typename H, typename... T> struct my_tuple_cat<H, T...> {
  29. 	static auto cat(H h, T... t) -> decltype(std::tuple_cat(h, my_tuple_cat<T...>::cat(t...)))
  30. 	{ return std::tuple_cat(h, my_tuple_cat<T...>::cat(t...)); }
  31. };
  32. template<typename T> struct my_tuple_cat<T> {
  33. 	static T cat(T t) { return t; }
  34. };
  35.  
  36.  
  37. template<int I, typename H, typename... T> struct tuple_walker<I, H, T...> {
  38. 	static std::tuple<H, T...> get(Cache & c) {
  39. 		return my_tuple_cat<std::tuple<H>, std::tuple<T...>>::cat(c.get_ex<I, H>(), tuple_walker<I + 1, T...>::get(c));
  40. 	}
  41. };
  42. template<int I, typename H> struct tuple_walker<I, H> {
  43. 	static std::tuple<H> get(Cache & c) {
  44. 		return c.get_ex<I, H>();
  45. 	}
  46. };
  47.  
  48. template<int I, typename... Ts> std::tuple<Ts...> cache_getter<I, std::tuple<Ts...> >::get(Cache & c) {
  49. 	return tuple_walker<I, Ts...>::get(c);
  50. }
  51.  
  52. int main(int argc, char ** argv) {
  53. 	Cache cache;
  54. 	typedef std::tuple<int, double, bool> InstrumentTuple;
  55. 	InstrumentTuple tuple = cache.get<InstrumentTuple>();
  56. 	return 0;
  57. }
Success #stdin #stdout 0s 2892KB
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https://ideone.com/0gXNYT
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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