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  1. #include <cstddef>
  2. #include <iostream>
  3. #include <type_traits>
  4.  
  5. namespace mpl {
  6. 	template<typename... Ts>
  7. 	struct TypeList
  8. 	{
  9. 	    static constexpr std::size_t size{ sizeof... (Ts) };
  10. 	};
  11.  
  12. 	// -----
  13.  
  14. 	template<typename, typename>
  15. 	struct IndexOf {};
  16.  
  17. 	// IndexOf base case: found the type we're looking for.
  18. 	template <typename T, typename... Ts>
  19. 	struct IndexOf<T, TypeList<T, Ts...>>
  20. 	    : std::integral_constant<std::size_t, 0>
  21. 	{
  22. 	};
  23.  
  24. 	// IndexOf recursive case: 1 + IndexOf the rest of the types.
  25. 	template <typename T, typename TOther, typename... Ts>
  26. 	struct IndexOf<T, TypeList<TOther, Ts...>>
  27. 	    : std::integral_constant<std::size_t,
  28. 	    1 + IndexOf<T, TypeList<Ts...>>::value>
  29. 	{
  30. 	};
  31. } // namespace mpl
  32.  
  33. struct T1
  34.     {
  35.     };
  36.  
  37. struct T2
  38.         {
  39.         };
  40.  
  41. struct T3
  42.         {
  43.         };
  44.  
  45. using Types = mpl::TypeList<T1, T2, T3>;
  46.  
  47. int main() {
  48.     using namespace mpl;
  49.  
  50.     std::cout << "Given TypeList<T1, T2, T3>..."
  51.               << "\nT1 is type #"  << IndexOf<T1, Types>::value
  52.               << "\nT2 is type #"  << IndexOf<T2, Types>::value
  53.               << "\nT3 is type #"  << IndexOf<T3, Types>::value
  54.  
  55. 			  // Breaks if uncommented.
  56.               // << "\nint is type #" << IndexOf<int, Types>::value
  57.  
  58.               << std::endl;
  59. }
Success #stdin #stdout 0s 3468KB
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Standard input is empty
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Given TypeList<T1, T2, T3>...
T1 is type #0
T2 is type #1
T3 is type #2
https://ideone.com/34KEN8
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
secret

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