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  1.  
  2. #include <utility>
  3.  
  4. template <typename Iterator, typename Runnable>
  5. struct ConditionalIterator
  6. {
  7.    private:
  8.       Iterator iterator;
  9.       Iterator end;
  10.       const Runnable& condition;
  11.    public:
  12.       ConditionalIterator(Iterator b, Iterator e, const Runnable& r)
  13.       :
  14.          iterator(b),
  15.          end(e),
  16.          condition(r)
  17.       {
  18.       }
  19.       auto operator*() -> decltype(*iterator)
  20.       {
  21.          return *iterator;
  22.       }
  23.       ConditionalIterator& operator++()
  24.       {
  25.          do
  26.          {
  27.             ++iterator;
  28.          }
  29.          while ( iterator != end && !condition(*iterator) );
  30.          return *this;
  31.       }
  32.       bool operator==(const ConditionalIterator& second)
  33.       {
  34.          return iterator == second.iterator;
  35.       }
  36.       bool operator!=(const ConditionalIterator& second)
  37.       {
  38.          return !(*this == second);
  39.       }
  40. };
  41.  
  42. template <typename Range, typename Runnable>
  43. struct ConditionalRange;
  44.  
  45. template <typename Range, typename Runnable>
  46. ConditionalRange<Range, Runnable> makeConditionalRange(Range& range, Runnable&& condition)
  47. {
  48.    static_assert(std::is_same<decltype(condition(*std::declval<Range>().begin())), bool>::value, "Condition must return a boolean value.");
  49.    return ConditionalRange<Range, Runnable>(range, std::forward<Runnable>(condition));
  50. }
  51.  
  52. template <typename Range, typename Runnable>
  53. struct ConditionalRange
  54. {
  55.    public:
  56.       friend ConditionalRange makeConditionalRange<>(Range&, Runnable&&);
  57.    public:
  58.       using iterator_type = ConditionalIterator<decltype(std::declval<Range>().begin()), Runnable>;
  59.       iterator_type begin() const
  60.       {
  61. 	     auto b = range.begin();
  62. 	     while ( b != range.end() && !condition(*b) )
  63. 	     {
  64. 	        ++b;
  65. 	     }
  66. 	     return ConditionalIterator<decltype(std::declval<Range>().begin()), Runnable>(b, range.end(), condition);
  67. 	  }
  68.       iterator_type end() const
  69.       {
  70.       	 return ConditionalIterator<decltype(std::declval<Range>().begin()), Runnable>(range.end(), range.end(), condition);
  71.       }
  72.    private:
  73.       ConditionalRange(Range& range_, Runnable&& condition_)
  74. 	  :
  75.    		 range(range_),
  76.    		 condition(std::forward<Runnable>(condition_))
  77.       {
  78. 	  }
  79.    private:
  80.       Range& range;
  81.       Runnable condition;
  82. };
  83.  
  84. // user code begins here!
  85.  
  86. #include <iostream>
  87. #include <vector>
  88.  
  89. int main()
  90. {
  91. 	std::vector<int> ns{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
  92.    for ( const auto& n : makeConditionalRange(ns, [](int a) { return a % 2 == 0; }) )
  93.    {
  94.       std::cout << n << " ";
  95.    }
  96.    std::cout << "\n";
  97.    for ( const auto& n : makeConditionalRange(ns, [](int a) { return a % 2 != 0; }) )
  98.    {
  99.       std::cout << n << " ";
  100.    }
  101.    std::cout << "\n";
  102. }
Success #stdin #stdout 0s 3472KB
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0 2 4 6 8 
1 3 5 7 9
https://ideone.com/XCgCny
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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