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  1. #include <iostream>
  2. #include <typeinfo>
  3. #include <type_traits>
  4.  
  5. struct Grandma             {};
  6. struct Mom:        Grandma {};
  7. struct Daughter:   Mom     {};
  8. struct Son:        Mom     {};
  9. struct Grandchild: Son     {};
  10.  
  11. struct Stranger            {};
  12.  
  13. namespace detail
  14. {
  15.  
  16.     struct TypeIsNotPartOfTheHierarchy {};
  17.  
  18.     template<typename T>
  19.     struct TypeWrapper
  20.     {
  21.         static_assert(!std::is_same<TypeIsNotPartOfTheHierarchy, T>::value,
  22.             "using types of different type hierarchies.");
  23.  
  24.         using type = T;
  25.     };
  26.  
  27.     template<typename StillCommonAncestor, typename TypeToCheck, typename... Ts>
  28.     struct IsCommonAncestor;
  29.  
  30.     template<typename StillCommonAncestor, typename TypeToCheck>
  31.     struct IsCommonAncestor<StillCommonAncestor, TypeToCheck>
  32.     {
  33.         static constexpr bool value = StillCommonAncestor::value;
  34.     };
  35.  
  36.     template<typename StillCommonAncestor, typename TypeToCheck, typename T1, typename... Ts>
  37.     struct IsCommonAncestor<StillCommonAncestor, TypeToCheck, T1, Ts...>:
  38.         IsCommonAncestor
  39.         <
  40.             std::integral_constant
  41.             <
  42.                 bool,
  43.                 std::conditional
  44.                 <
  45.                     std::is_base_of<TypeToCheck, T1>::value,
  46.                     std::true_type,
  47.                     std::false_type
  48.                 >::type::value && StillCommonAncestor::value
  49.             >,
  50.             TypeToCheck,
  51.             Ts...
  52.         >
  53.     {};
  54.  
  55.     template<typename Pack, typename... Ts>
  56.     struct LCA;
  57.  
  58.     template<typename... PackParams, typename T1>
  59.     struct LCA<std::tuple<PackParams...>, T1>:
  60.         std::conditional
  61.         <
  62.             IsCommonAncestor<std::true_type, T1, PackParams...>::value,
  63.             TypeWrapper<T1>,
  64.             TypeWrapper<TypeIsNotPartOfTheHierarchy>
  65.         >::type
  66.     {};
  67.  
  68.     template<typename... PackParams, typename T1, typename... Ts>
  69.     struct LCA<std::tuple<PackParams...>, T1, Ts...>:
  70.         std::conditional
  71.         <
  72.             IsCommonAncestor<std::true_type, T1, PackParams...>::value,
  73.             TypeWrapper<T1>,
  74.             LCA<std::tuple<PackParams...>, Ts...>
  75.         >::type
  76.     {};
  77.  
  78. }
  79.  
  80. template<typename... Ts>
  81. struct LCA: detail::LCA<std::tuple<Ts...>, Ts...>
  82. {};
  83.  
  84. int main()
  85. {
  86.     std::cout << typeid(LCA<Son, Daughter, Mom, Grandchild, Grandma, Son, Son>::type).name() << std::endl;
  87.     std::cout << typeid(LCA<Son>::type).name() << std::endl;
  88.     std::cout << typeid(LCA<Grandma, Daughter, Son>::type).name() << std::endl;
  89.  
  90.     // error because Daughter and Son are orphans in the list
  91.     // std::cout << typeid(LCA<Daughter, Son>::type).name() << std::endl;
  92.  
  93.     // error because the Son and his Grandma are not related to Stranger.
  94.     // std::cout << typeid(LCA<Grandma, Stranger, Son>::type).name() << std::endl;
  95.  
  96.     return 0;
  97. }
Success #stdin #stdout 0s 3340KB
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7Grandma
3Son
7Grandma
https://ideone.com/pYEPYl
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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