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  1. #include <cstddef>
  2. #include <type_traits>
  3. #include <iostream>
  4.  
  5. namespace detail {
  6.     template<typename>
  7.     constexpr std::size_t locate(std::size_t ind) {
  8.         return static_cast<std::size_t>(-1);
  9.     }
  10.  
  11.     template<typename IndexedType, typename T, typename... Ts>
  12.     constexpr std::size_t locate(std::size_t ind = 0) {
  13.         if (std::is_same<IndexedType, T>::value) {
  14.             return ind;
  15.         } else {
  16.             return locate<IndexedType, Ts...>(ind + 1);
  17.         }
  18.     }
  19.  
  20.     // This could also be useful.
  21.     // From a previous version of IndexOf, not used in the current version.
  22. /*
  23.     template<typename IndexedType, typename...>
  24.     struct is_in : std::false_type {};
  25.     
  26.     template<typename IndexedType, typename Item, typename... Items>
  27.     struct is_in<IndexedType, Item, Items...>
  28.       : std::integral_constant<bool,
  29.                                std::is_same<IndexedType, Item>::value ||
  30.                                  is_in<IndexedType, Items...>::value> {};
  31.  */
  32. } // namespace detail
  33.  
  34.  
  35. namespace mpl {
  36.     template<typename... Ts>
  37.     struct TypeList
  38.     {
  39.         static constexpr std::size_t size{ sizeof... (Ts) };
  40.     };
  41.  
  42.     // -----
  43.  
  44.     template<typename T, typename>
  45.     struct IndexOf;
  46.  
  47.     template<typename T, typename... ListedTypes>
  48.     struct IndexOf<T, mpl::TypeList<ListedTypes...>>
  49.       : std::integral_constant<std::size_t, detail::locate<T, ListedTypes...>()>
  50.     {
  51.     };
  52. } // namespace mpl
  53.  
  54. struct T1
  55.     {
  56.     };
  57.  
  58. struct T2
  59.         {
  60.         };
  61.  
  62. struct T3
  63.         {
  64.         };
  65.  
  66. using Types = mpl::TypeList<T1, T2, T3>;
  67.  
  68. int main() {
  69.     using namespace mpl;
  70.  
  71.     std::cout << "Given TypeList<T1, T2, T3>..."
  72.               << "\nT1 is type #"  << IndexOf<T1, Types>::value
  73.               << "\nT2 is type #"  << IndexOf<T2, Types>::value
  74.               << "\nT3 is type #"  << IndexOf<T3, Types>::value
  75.               << "\nint is type #" << IndexOf<int, Types>::value
  76.               << std::endl;
  77.  
  78.     // Double-checking with a different TypeList.
  79.     if (IndexOf<int, TypeList<char, bool, int, double>>::value == 2) {
  80.         std::cout << "Huzzah!\n";
  81.     }
  82. }
Success #stdin #stdout 0s 3468KB
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stdin
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Standard input is empty
stdout
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Given TypeList<T1, T2, T3>...
T1 is type #0
T2 is type #1
T3 is type #2
int is type #4294967295
Huzzah!
https://ideone.com/thAkQG
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
secret

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