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  1. #include <iostream>
  2. #include <vector>
  3. #include <memory>
  4. #include <list>
  5. using namespace std;
  6.  
  7. template <typename T>
  8. struct ref_or_copy
  9. {
  10. 	ref_or_copy(T&& rval) 
  11. 	    : storage(new T(std::move(rval)))
  12. 	    , val(*storage) { }
  13. 	ref_or_copy(T& lval) : val(lval) { }
  14.  
  15. 	std::unique_ptr<T> storage;
  16. 	T& val;
  17. };
  18.  
  19. template <typename C1, typename C2>
  20. struct chain_obj_struct {
  21.   ref_or_copy<std::remove_reference_t<C1>> c1;
  22.   ref_or_copy<std::remove_reference_t<C2>> c2;
  23.   struct iterator {
  24.     decltype(std::begin(c1.val)) it1;
  25.   	decltype(std::begin(c1.val)) it1_end;
  26.     decltype(std::begin(c2.val)) it2;
  27.     bool in_first;
  28.  
  29.     iterator& operator++() {
  30.       if (in_first) {
  31.         ++it1;
  32.         if (it1 == it1_end) {
  33.           in_first = false;
  34.         }
  35.       } else { 
  36.         ++it2;
  37.       }
  38.       return *this;
  39.     }
  40.  
  41.     typename std::conditional<
  42.         std::is_const<std::remove_reference_t<decltype(*it1)>>::value,
  43.         decltype(*it1),
  44.         decltype(*it2)>::type
  45.     operator*() 
  46.     {
  47.       if (in_first) return *it1;
  48.       return *it2;
  49.     }
  50.  
  51.     bool operator==(const iterator& i2) { 
  52.       if (in_first != i2.in_first) return false;
  53.       if (in_first)
  54.         return it1 == i2.it1;
  55.       else
  56.         return it2 == i2.it2;
  57.     }
  58.     bool operator!=(const iterator& i2) {
  59.     	return !this->operator==(i2);
  60.     }
  61.   };
  62.  
  63.   iterator begin() {
  64.     return iterator{std::begin(c1.val), std::end(c1.val), std::begin(c2.val), true};
  65.   }
  66.  
  67.   iterator end() {
  68.     return iterator{std::end(c1.val), std::end(c1.val), std::end(c2.val), false};
  69.   }
  70. };
  71.  
  72. template <typename C1, typename C2>
  73. chain_obj_struct<C1, C2> chain_obj(C1&& c1, C2&& c2) {
  74. 	return chain_obj_struct<C1, C2>{std::forward<C1>(c1), std::forward<C2>(c2)};
  75. }
  76.  
  77. template <typename C>
  78. auto chain_it(C&& c) { return std::forward<C>(c); }
  79.  
  80. template <typename C1, typename C2, typename... Cs>
  81. auto chain_it(C1&& c1, C2&& c2)
  82. {
  83. 	return chain_obj(std::forward<C1>(c1), std::forward<C2>(c2));
  84. }
  85.  
  86. template <typename C1, typename... Cs>
  87. auto chain_it(C1&& c1, Cs&&... cs)
  88. {
  89. 	return chain_obj(std::forward<C1>(c1), chain_it(std::forward<Cs>(cs)...));
  90. }
  91.  
  92.  
  93. int main() {
  94. 	vector<int> a = {1, 2, 3, 4};
  95. 	vector<int> b = {5, 6, 7, 8};
  96. 	list<int> c = {9, 9, 4};
  97. 	const auto& ca = a;
  98. 	const auto& cb = b;
  99. 	for (auto& i : chain_it(a, cb, ca, b, c)) { 
  100. 		cout << i << " "; 
  101. 	}
  102.  
  103. 	return 0;
  104. }
Success #stdin #stdout 0s 3460KB
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1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9 9 4
https://ideone.com/kFMo9m
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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