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  1. #include <iostream>
  2. #include <iterator>
  3. #include <type_traits>
  4.  
  5.  
  6.  
  7. template<typename Fn>
  8. struct CallRepeatedlyIterator
  9. {
  10. 	using difference_type = unsigned;
  11. 	using value_type = typename std::result_of<Fn()>::type;
  12. 	using pointer = value_type *;
  13. 	using reference = value_type &;
  14. 	using iterator_category = std::input_iterator_tag;
  15.  
  16. 	bool is_end;
  17. 	union {
  18. 		Fn fn;
  19. 	};
  20. 	union {
  21. 		value_type buffer;
  22. 	};
  23.  
  24. 	value_type operator*() const
  25. 	{
  26. 		return buffer;
  27. 	}
  28.  
  29. 	CallRepeatedlyIterator & operator++()
  30. 	{
  31. 		buffer = fn();
  32. 		return *this;
  33. 	}
  34.  
  35. 	CallRepeatedlyIterator()
  36. 	: is_end(true)
  37. 	{}
  38.  
  39. 	explicit CallRepeatedlyIterator(Fn f)
  40. 	: is_end(false)
  41. 	{
  42. 		new (&fn) Fn(f);
  43. 		new (&buffer) value_type(fn());
  44. 	}
  45.  
  46. 	bool operator==(CallRepeatedlyIterator const & other) const
  47. 	{
  48. 		return is_end && other.is_end;
  49. 	}
  50.  
  51. 	bool operator!=(CallRepeatedlyIterator const & other) const
  52. 	{
  53. 		return !(*this == other);
  54. 	}
  55. };
  56.  
  57.  
  58. int foo()
  59. {
  60. 	static int i = 42;
  61. 	std::cerr << "foo() called" << std::endl;
  62. 	return ++i;
  63. }
  64.  
  65.  
  66. int main() {
  67. 	using fn_type = decltype(foo);
  68. 	CallRepeatedlyIterator<fn_type *> it(foo);
  69. 	CallRepeatedlyIterator<fn_type *> the_end;
  70. 	int calls = 4;
  71. 	for (; it != the_end; ++it)
  72. 	{
  73. 		std::cout << *it << std::endl;
  74. 		if (! (--calls)) break;
  75. 	}
  76. }
Success #stdin #stdout #stderr 0s 4504KB
comments (?)
stdin 
Standard input is empty
stdout 
43
44
45
46
stderr 
foo() called
foo() called
foo() called
foo() called
https://ideone.com/GZZliQ
language:
C++14 (gcc 8.3)
created:
5 years ago
visibility:
public

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