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  1. #include <tuple>
  2. #include <iostream>
  3.  
  4. struct A {
  5.   static void tip();
  6. };
  7.  
  8. struct B {
  9.   static void tip();
  10. };
  11.  
  12. struct Z {
  13. };
  14.  
  15. // Indicates whether the template parameter contains a static member named tip.
  16. template<class T>
  17. struct has_tip {
  18.     template<class U>
  19.     struct clean_type {
  20.         typedef typename std::remove_const<typename std::remove_reference<U>::type>::type type;
  21.     };
  22.  
  23.     template<class U>
  24.     static char test(decltype(&U::tip));
  25.  
  26.     template<class U>
  27.     static float test(...);
  28.  
  29.     static const bool value = sizeof(test<typename clean_type<T>::type>(0)) == sizeof(char);
  30. };
  31.  
  32. // Indicates whether the n-th type contains a tip static member
  33. template<size_t n, typename... Ts>
  34. struct nth_type_has_tip {
  35.     static const bool value = has_tip<typename std::tuple_element<n, std::tuple<Ts...>>::type>::value;
  36. };
  37.  
  38. // Generic iteration
  39. template<size_t index, size_t n>
  40. struct TupleIter
  41. {
  42.   template< typename... Ts >
  43.   typename std::enable_if< nth_type_has_tip<index, Ts...>::value , void >::type
  44.   static Iter(const std::tuple<Ts...>& tpl) 
  45.   {
  46.     std::cout << "tip\n";
  47.     TupleIter<index + 1, n>::Iter(tpl );
  48.   }
  49.  
  50.   template< typename... Ts >
  51.   typename std::enable_if< !nth_type_has_tip<index, Ts...>::value , void >::type
  52.   static Iter(const std::tuple<Ts...>& tpl) {
  53.     std::cout << "no tip\n";
  54.     TupleIter<index + 1, n>::Iter(tpl );
  55.   }
  56. };
  57.  
  58. // Base class, we've reached the tuple end
  59. template<size_t n>
  60. struct TupleIter<n, n>
  61. {
  62.   template<typename... Ts >
  63.   static void Iter( const std::tuple<Ts...>& tpl ) {
  64.     std::cout << "end\n";
  65.   }
  66. };
  67.  
  68. // Helper function that forwards the first call to TupleIter<>::Iter
  69. template<typename... Ts>
  70. void iterate(const std::tuple<Ts...> &tup) {
  71.     TupleIter<0, sizeof...(Ts)>::Iter(tup);
  72. }
  73.  
  74. int main() {
  75.     A a;
  76.     B b;
  77.     Z z;
  78.     std::tuple<const A&,const B&,const Z&> tup(a,b,z);
  79.     iterate(tup);
  80. }
  81.  
Success #stdin #stdout 0s 2928KB
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stdin
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Standard input is empty
stdout
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tip
tip
no tip
end
https://ideone.com/6Puku
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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