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  1. #include <boost/type_traits.hpp> // or <type_traits> for TR1
  2. #include <iostream>
  3.  
  4. struct edge {};
  5. struct weighted_edge : edge          { virtual double get_weight() const { return 3.14; } };
  6. struct derived_edge  : weighted_edge { virtual double get_weight() const { return 42; } };
  7.  
  8. template <typename E, bool is_weighted>
  9. struct edge_weight_impl;
  10.  
  11. template <typename E>
  12. struct edge_weight_impl<E, false>
  13. {
  14.     typedef int result_type;
  15.     int operator()(const E& e) const { return 1; }
  16. };
  17.  
  18. template <typename E>
  19. struct edge_weight_impl<E, true>
  20. {
  21.     // typedef decltype(E().weight()) result_type; // c++0x
  22.     typedef double result_type;
  23.  
  24.     result_type operator()(const E& e) const 
  25.     { 
  26.         return e.get_weight();
  27.     }
  28. };
  29.  
  30. template <typename E>
  31.     typename edge_weight_impl<E, boost::is_base_of<weighted_edge, E>::value>::result_type 
  32.         get_weight(const E& e)
  33. {
  34.     return edge_weight_impl<E, boost::is_base_of<weighted_edge, E>::value>()(e);
  35. }
  36.  
  37. int main()
  38. {
  39.     edge e;
  40.     weighted_edge we;
  41.     derived_edge de;
  42.  
  43.     std::cout << "--- static polymorphism" << std::endl;
  44.     std::cout << "edge:\t"          << get_weight(e) << std::endl;
  45.     std::cout << "weighted_edge:\t" << get_weight(we) << std::endl;
  46.     std::cout << "derived_edge:\t"  << get_weight(de) << std::endl;
  47.     // use some additional enable_if to get rid of this:
  48.     std::cout << "bogus:\t"         << get_weight("bogus") << std::endl;
  49.  
  50.     std::cout << "\n--- runtime polymorphism" << std::endl;
  51.  
  52.     edge* ep = &e;
  53.     std::cout << "edge:\t"          << get_weight(*ep) << std::endl;
  54.     weighted_edge* wep = &we;
  55.     std::cout << "weighted_edge:\t" << get_weight(*wep) << std::endl;
  56.     wep = &de;
  57.     std::cout << "bogus:\t"         << get_weight(*wep) << std::endl;
  58.  
  59. }
  60.  
Success #stdin #stdout 0s 2724KB
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stdout
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--- static polymorphism
edge:	1
weighted_edge:	3.14
derived_edge:	42
bogus:	1

--- runtime polymorphism
edge:	1
weighted_edge:	3.14
bogus:	42
https://ideone.com/AqmsL
language:
C++ (gcc 8.3)
created:
14 years ago
visibility:
public

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