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  1. #include <iostream>
  2. #include <string>
  3. #include <map>
  4.  
  5. // define some tags to create uniqueness 
  6. struct portal_tag {};
  7. struct cake_tag {};
  8.  
  9. // a string-like identifier that is typed on a tag type   
  10. template<class Tag, class Base>
  11. struct strong_type
  12. {
  13.     // needs to be default-constuctable because of use in map[] below
  14.     strong_type(Base s) : _value(std::move(s)) {}
  15.     strong_type() : _value() {}
  16.  
  17.     // provide access to the underlying string value        
  18.     const Base& value() const { return _value; }
  19. private:
  20.     Base _value;
  21.  
  22.     // will only compare against same type of id.
  23.     friend bool operator < (const strong_type& l, const strong_type& r) {
  24.         return l._value < r._value;
  25.     }
  26. };
  27.  
  28.  
  29. // create some type aliases for ease of use    
  30. using PortalId = strong_type<portal_tag, std::string>;
  31. using CakeId = strong_type<cake_tag, std::string>;
  32.  
  33. using namespace std;
  34.  
  35. // confirm that requirements are met
  36. auto main() -> int
  37. {
  38.     PortalId portal_id("2");
  39.     CakeId cake_id("is a lie");
  40.     std::map<CakeId, PortalId> p_to_cake; // OK
  41.  
  42.     p_to_cake[cake_id]   = portal_id; // OK
  43.     //p_to_cake[portal_id] = cake_id;   // COMPILER ERROR
  44.  
  45.     //portal_id = cake_id;        // COMPILER ERROR
  46.     //portal_id = "1.0";          // COMPILER ERROR
  47.     portal_id = PortalId("42"); // OK
  48.     return 0;
  49. }
Success #stdin #stdout 0s 3456KB
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https://ideone.com/IVlefk
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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