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  1. #include <iostream>
  2. #include <iterator>
  3. #include <vector>
  4.  
  5. template <class Derived, class Category, class Value, class Reference = Value&,
  6.     class Pointer = Value*, class Distance = std::ptrdiff_t>
  7. class iterator_facade;
  8.  
  9. struct iterator_core_access {
  10.     template <class I>
  11.     static typename I::reference dereference(const I& i) { return i.dereference(); }
  12.  
  13.     template <class I>
  14.     static void increment(I& i) { i.increment(); }
  15.  
  16.     template <class I>
  17.     static void decrement(I& i) { i.decrement(); }
  18.  
  19.     template <class I1, class I2>
  20.     static bool equal(const I1& i1, const I2& i2) { return i1.equals(i2); }
  21. };
  22.  
  23. namespace detail {
  24.  
  25. template <class Iterator, class Category, class Value, class Reference,
  26.     class Pointer, class Distance>
  27. class iterator_facade_base;
  28.  
  29. template <class I, class V, class R, class P, class D>
  30. class iterator_facade_base<I, std::output_iterator_tag, V, R, P, D>
  31.     : public std::iterator<std::output_iterator_tag, V, D, P, R> {
  32. public:
  33.     R operator * () const { return iterator_core_access::dereference(derived()); }
  34.  
  35.     I& operator ++ () {
  36.         iterator_core_access::increment(derived());
  37.         return derived();
  38.     }
  39.  
  40.     I operator ++ (int) {
  41.         I copy = derived();
  42.         ++*this;
  43.         return copy;
  44.     }
  45.  
  46. protected:
  47.     I& derived() { return *static_cast<I*>(this); }
  48.  
  49.     const I& derived() const { return *static_cast<const I*>(this); }
  50. };
  51.  
  52. template <class I, class V, class R, class P, class D>
  53. class iterator_facade_base<I, std::input_iterator_tag, V, R, P, D>
  54.     : public iterator_facade_base<I, std::output_iterator_tag, V, R, P, D> {
  55. public:
  56.     using iterator_category = std::input_iterator_tag;
  57.  
  58.     P operator -> () const { return *iterator_core_access::dereference(this->derived()); }
  59. };
  60.  
  61. template <class I, class V, class R, class P, class D>
  62. class iterator_facade_base<I, std::forward_iterator_tag, V, R, P, D>
  63.     : public iterator_facade_base<I, std::input_iterator_tag, V, R, P, D> {
  64. public:
  65.     using iterator_category = std::forward_iterator_tag;
  66. };
  67.  
  68. template <class I, class V, class R, class P, class D>
  69. class iterator_facade_base<I, std::bidirectional_iterator_tag, V, R, P, D>
  70.     : public iterator_facade_base<I, std::forward_iterator_tag, V, R, P, D> {
  71. public:
  72.     using iterator_category = std::bidirectional_iterator_tag;
  73.  
  74.     I& operator -- () {
  75.         iterator_core_access::decrement(this->derived());
  76.         return this->derived();
  77.     }
  78.  
  79.     I operator -- (int) {
  80.         I copy = this->derived();
  81.         --*this;
  82.         return copy;
  83.     }
  84. };
  85.  
  86. }  // namespace detail
  87.  
  88. template <class I, class C, class V, class R, class P, class D>
  89. class iterator_facade : public detail::iterator_facade_base<I, C, V, R, P, D> {};
  90.  
  91. template <typename Integer, bool INCREMENT = true>
  92. class integer_iterator : public iterator_facade<integer_iterator<Integer, INCREMENT>,
  93.     std::input_iterator_tag, Integer, const Integer, Integer*, Integer> {
  94. public:
  95.     explicit integer_iterator() : value_() {}
  96.  
  97.     explicit integer_iterator(Integer value) : value_(value) {}
  98.  
  99. private:
  100.     friend struct iterator_core_access;
  101.  
  102.     const Integer dereference() const { return value_; }
  103.  
  104.     void increment() { INCREMENT ? ++value_ : --value_; }
  105.  
  106.     bool equals(const integer_iterator& other) const { return value_ == other.value_; }
  107.  
  108.     Integer value_;
  109. };
  110.  
  111. int main() {
  112.     std::cout << std::is_same<integer_iterator<int>::iterator_category, std::input_iterator_tag>::value << std::endl;
  113.     return 0;
  114. }
  115.  
Success #stdin #stdout 0s 15240KB
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1
https://ideone.com/74nIGF
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
secret

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