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  1. #include <cstdio>
  2.  
  3. namespace intern {
  4.  
  5.   template<char... NN> struct string {
  6.  
  7.     static constexpr char const value[ sizeof...(NN) ]{NN...};
  8.  
  9.     static_assert( value[ sizeof...(NN) - 1 ] == '\0', "interned string was too long" );
  10.  
  11.     static constexpr auto data() { return value; }
  12.   };
  13.  
  14.   template<char... N> constexpr char const string<N...>::value[];
  15.  
  16.   template<int N>
  17.   constexpr char ch ( char const(&s)[N], int i ) { return i < N ? s[i] : '\0'; }
  18.  
  19.   template<typename T> struct is_string {
  20.     static const bool value = false;
  21.   };
  22.  
  23.   template<char... NN> struct is_string< string<NN...> > {
  24.     static const bool value = true;
  25.   };
  26. }
  27.  
  28. //prefixing macros with a $ helps with readability
  29. #define $c(a,b) intern::ch(a, b)
  30.  
  31. //10 characters + '\0', add $c(...) for more
  32. #define $(s) intern::string<$c(s,0),$c(s,1),$c(s,2),$c(s,3),$c(s,4),$c(s,5),$c(s,6),$c(s,7),$c(s,8),$c(s,9),$c(s,10)>
  33.  
  34. struct singleton {
  35.  
  36. 	template<typename T>
  37. 	singleton(T) : name{ T::value } {
  38. 		if( T::value == $("db")::value )
  39. 			printf("Initializing DB singleton: %s\n", T::value);
  40. 		else 
  41. 			printf("Initializing unknown singleton: %s\n", T::value);
  42. 	}
  43.  
  44. 	void do_action() {
  45. 		printf("singleton(%s)::do_action()\n", name);
  46. 	}
  47.  
  48. 	char const *name;
  49. };
  50.  
  51. template<typename K, typename V> struct map {
  52.   static V value;
  53. };
  54.  
  55. template<typename K, typename V> V map<K,V>::value{K{}};
  56.  
  57. template<typename K> using get_singleton = map<K,singleton>;
  58.  
  59. int main() {
  60.  
  61. 	get_singleton< $("db") >::value.do_action();
  62. 	get_singleton< $("map") >::value.do_action();
  63.  
  64. }
  65.  
Success #stdin #stdout 0s 16064KB
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stdin
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Standard input is empty
stdout
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Initializing DB singleton: db
Initializing unknown singleton: map
singleton(db)::do_action()
singleton(map)::do_action()
https://ideone.com/A9Vk6q
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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