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  1. #include <iostream> // std::cout, std::boolalpha, std::endl
  2. #include <string> // std::basic_string
  3. #include <type_traits> // std::enable_if_t, std::is_same
  4. #include <utility> // std::swap, std::move
  5.  
  6. namespace danger_zone // enter at your own peril
  7. {
  8. 	template<typename CharT>
  9. 	struct nullable_basic_string : public std::basic_string<CharT>
  10. 	{
  11. 	protected:
  12. 		using basic_string = std::basic_string<CharT>;
  13.  
  14. 		bool _isNull = false;
  15.  
  16. 	public:
  17. 		using basic_string::basic_string; // inherit constructors
  18.  
  19. 		nullable_basic_string(basic_string const &str) : basic_string{ str }
  20. 		{
  21. 		}
  22.  
  23. 		nullable_basic_string(decltype(nullptr)) : basic_string{}, _isNull{ true }
  24. 		{
  25. 		}
  26.  
  27. 		nullable_basic_string() : nullable_basic_string{ nullptr }
  28. 		{
  29. 		}
  30.  
  31. 		nullable_basic_string& operator=(nullable_basic_string rhs)
  32. 		{
  33. 			using std::swap;
  34. 			basic_string::swap(rhs);
  35. 			swap(_isNull, rhs._isNull);
  36. 			return *this;
  37. 		}
  38.  
  39. 		friend bool operator==(nullable_basic_string const &lhs, decltype(nullptr) const &rhs)
  40. 		{
  41. 			return lhs._isNull;
  42. 		}
  43.  
  44. 		friend bool operator==(decltype(nullptr) const &lhs, nullable_basic_string const &rhs)
  45. 		{
  46. 			return rhs == lhs;
  47. 		}
  48.  
  49. 		friend bool operator!=(nullable_basic_string const &lhs, decltype(nullptr) const &rhs)
  50. 		{
  51. 			return !(lhs == rhs);
  52. 		}
  53.  
  54. 		friend bool operator!=(decltype(nullptr) const &lhs, nullable_basic_string const &rhs)
  55. 		{
  56. 			return rhs != lhs;
  57. 		}
  58. 	};
  59.  
  60. 	using nullable_string = nullable_basic_string<char>;
  61.  
  62. 	////////////////////////////////////////////////////
  63. 	// TODO: throw exception on null access           //
  64. 	////////////////////////////////////////////////////
  65.  
  66. } // namespace danger_zone
  67.  
  68. int main(int, char**) noexcept
  69. {
  70. 	using danger_zone::nullable_string;
  71. 	nullable_string ns{};
  72. 	nullable_string ns_copy{ ns };
  73. 	nullable_string tmp_copy_n{ std::move(nullable_string{}) };
  74. 	nullable_string tmp_copy_h{ std::move(nullable_string{ "Hello World!" }) };
  75. 	nullable_string hello{ "Hello World!" };
  76. 	nullable_string ns_assigned{};
  77. 	ns_assigned = "Goodbye, Galaxy!";
  78. 	nullable_string hello_assigned{ hello };
  79. 	hello_assigned = nullptr;
  80. 	std::cout << std::boolalpha << std::endl
  81. 		<< "ns == nullptr? " << (ns == nullptr) << std::endl
  82. 		<< "nullptr == ns? " << (nullptr == ns) << std::endl
  83. 		<< "ns_copy == nullptr? " << (ns_copy == nullptr) << std::endl
  84. 		<< "tmp_copy_n == nullptr? " << (tmp_copy_n == nullptr) << std::endl
  85. 		<< "tmp_copy_h == nullptr? " << (tmp_copy_h == nullptr) << std::endl
  86. 		<< "hello == nullptr? " << (hello == nullptr) << std::endl
  87. 		<< "ns_assigned == nullptr? " << (ns_assigned == nullptr) << std::endl
  88. 		<< "hello_assigned == nullptr? " << (hello_assigned == nullptr) << std::endl
  89. 		<< ns_assigned << std::endl;
  90. 	return 0;
  91. }
Success #stdin #stdout 0s 3472KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
ns == nullptr? true
nullptr == ns? true
ns_copy == nullptr? true
tmp_copy_n == nullptr? true
tmp_copy_h == nullptr? false
hello == nullptr? false
ns_assigned == nullptr? false
hello_assigned == nullptr? true
Goodbye, Galaxy!
https://ideone.com/nYJR5M
nullable_string
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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