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  1. #include <iostream>
  2.  
  3.  
  4. // void_t: void type alias
  5. template< typename... >
  6. using void_t = void;
  7. //
  8.  
  9.  
  10. namespace internal
  11. {
  12. 	// Fallback case
  13. 	template<	typename D,
  14. 				typename Void,
  15. 				template< typename... > class Check,
  16. 				typename... Args 
  17. 			>
  18. 	struct detect_impl
  19. 	{
  20. 		using value_t = std::false_type;
  21. 		using type = D;
  22. 	};
  23.  
  24.  
  25. 	// Check succeeded
  26. 	template<	typename D,
  27. 				template< typename... > class Check,
  28. 				typename... Args 
  29. 			>
  30. 	struct detect_impl
  31. 		< D, void_t< Check<Args...> >, Check, Args... >
  32. 	{
  33. 		using value_t = std::true_type;
  34. 		using type = Check<Args...>;
  35. 	};
  36. }
  37.  
  38. // Type representing a missing type.
  39. struct nonesuch
  40. {
  41. 	nonesuch() = delete;
  42. 	~nonesuch() = delete;
  43. 	nonesuch(nonesuch const&) = delete;
  44. 	void operator=(nonesuch const&) = delete;
  45. };
  46.  
  47.  
  48. template<	template< typename... > class Check,
  49. 			typename... Args
  50. 		>
  51. using is_detected = typename internal::detect_impl< nonesuch, void, Check, Args... >::value_t;
  52.  
  53.  
  54.  
  55. // Our test
  56. template< typename T >
  57. using is_addable_impl = decltype( std::declval<T>() + std::declval<T>() );
  58.  
  59. template< typename T >
  60. using is_addable = is_detected<is_addable_impl, T>;
  61.  
  62.  
  63. auto main(int argc, const char* arv[])
  64. 	-> int
  65. {
  66. 	std::cout 	<<	std::boolalpha
  67. 				<<	is_addable<int>::value  << ", "
  68. 				<<	is_addable<nonesuch>::value << std::endl;
  69. }
  70.  
  71.  
  72.  
  73.  
  74.  
  75.  
  76.  
Success #stdin #stdout 0s 3456KB
comments (?)
stdin
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Standard input is empty
stdout
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true, false
https://ideone.com/VqYo3O
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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