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  1. #include <map>
  2. #include <string>
  3. #include <iostream>
  4.  
  5. template<typename T>
  6. struct disable_default
  7. {    
  8. 	T& get_default()
  9. 	{
  10. 		throw 1;
  11. 	}
  12.  
  13.     const T& get_default() const
  14.     {
  15.         throw 1;
  16.     }
  17. };
  18.  
  19. template<typename T>
  20. struct enable_default
  21. {
  22. 	template<typename Value>
  23. 	enable_default(Value&& def_value) : default_value(std::forward<Value>(def_value))
  24. 	{
  25. 	}
  26.  
  27. 	const T& get_default() const
  28. 	{
  29. 		return default_value;
  30. 	}
  31.  
  32. 	T& get_default()
  33. 	{
  34. 		return default_value;
  35. 	}
  36.  
  37. private:
  38. 	T default_value;		
  39. };
  40.  
  41. template<typename K, typename T, template <typename> class default_policy = enable_default, typename key_comp = std::less<K>>
  42. struct selector : public default_policy<T>
  43. {		
  44. 	template<typename Value>
  45. 	selector(Value&& def_value) : default_policy<T>(std::forward<Value>(def_value))
  46. 	{
  47. 	}
  48.  
  49. 	selector()
  50. 	{	
  51. 	}
  52.  
  53. 	struct selector_proxy
  54. 	{
  55. 		selector_proxy(std::map<K,T, key_comp>& sel_map, const K& pKey) : selector_map(sel_map), key(pKey)
  56. 		{
  57. 		}
  58.  
  59. 		template<typename Val>
  60. 		selector_proxy& operator=(Val&& pValue)
  61. 		{
  62. 			auto it = selector_map.find(key);
  63. 			if ( it != end(selector_map) )
  64. 			{
  65. 				it->second = std::forward<Val>(pValue);
  66. 			}
  67. 			else
  68. 			{
  69. 				selector_map.insert(std::make_pair(key, std::forward<Val>(pValue)));
  70. 			}
  71. 		}
  72.  
  73. 	private:
  74. 		std::map<K,T,key_comp>& selector_map;	
  75. 		const K& key;
  76. 	};
  77.  
  78. 	selector_proxy at(const K& key)
  79. 	{
  80. 		return selector_proxy(selector_map, key);
  81. 	}
  82.  
  83. 	T& get(const K& key)
  84. 	{
  85. 		return const_cast<T&>(static_cast<const selector*>(this)->get(key));
  86. 	}
  87.  
  88. 	const T& get(const K& key) const
  89. 	{
  90. 		auto it = selector_map.find(key);
  91. 		if ( it != end(selector_map) )
  92. 		{
  93. 			return it->second;
  94. 		}
  95. 		else
  96. 		{
  97. 			return default_policy<T>::get_default();
  98. 		}
  99. 	}
  100.  
  101. private:
  102. 	std::map<K,T,key_comp> selector_map;
  103. };
  104.  
  105. using namespace std;
  106.  
  107. int main()
  108. {
  109.     selector<string, int> d_sel_1(0);
  110.     d_sel_1.at("one") = 1;
  111.     d_sel_1.at("two") = 2;
  112.     cout << d_sel_1.get("one") << endl;
  113.     cout << d_sel_1.get("three") << endl;
  114.  
  115.     selector<string, int, disable_default> nd_sel {};
  116.     try 
  117.     {
  118.         cout << nd_sel.get("one") << endl;
  119.     }
  120.     catch(...)
  121.     {
  122.         cout << "not found" << endl;
  123.     }
  124. }
Success #stdin #stdout 0s 2988KB
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https://ideone.com/ijrb8H
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
secret

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