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  1. #include <iostream>
  2. #include <typeinfo>
  3.  
  4. template <typename> struct trait;
  5.  
  6. template <> struct trait<char> {
  7. 	using storage = char;
  8. };
  9.  
  10. template <> struct trait<int> {
  11. 	using storage = double;
  12. };
  13.  
  14. template <> struct trait<float> {
  15. 	// No storage
  16. };
  17.  
  18. template <typename> struct void_t { using type = void; };
  19.  
  20. template <typename T, typename = void>
  21. struct example_user
  22. {
  23. 	static void do_something() {
  24. 		std::cout << "invoked on " << typeid(T).name() << " which doesn't have storage" << std::endl;
  25. 	}
  26. };
  27.  
  28. template <typename T>
  29. struct example_user<T, typename void_t<typename trait<T>::storage>::type>
  30. {
  31. 	static void do_something() {
  32. 		std::cout << "invoked on " << typeid(T).name() << " which does have storage of type "
  33. 		          << typeid(typename trait<T>::storage).name()
  34. 		          << std::endl;
  35. 	}
  36. };
  37.  
  38. int main() {
  39. 	example_user<char>::do_something();
  40. 	example_user<int>::do_something();
  41. 	example_user<float>::do_something();
  42. 	return 0;
  43. }
Success #stdin #stdout 0s 3340KB
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stdin
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Standard input is empty
stdout
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invoked on c which does have storage of type c
invoked on i which does have storage of type d
invoked on f which doesn't have storage
https://ideone.com/TnsPJv
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
secret

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