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  1. #include <iostream>
  2. #include <map>
  3. #include <type_traits>
  4. #include <utility>
  5.  
  6. using namespace std;
  7.  
  8. struct one {
  9.   int foo(const int) { return 0; }
  10.   int bar() { return 0; }
  11. };
  12.  
  13. struct two {
  14.   int foo(const int) { return 1; }
  15. };
  16.  
  17. struct three {
  18.   int foo(const int) { return 2; }
  19.   int bar() { return 2; }
  20. };
  21.  
  22. struct owner {
  23.   map<int, one> ones;
  24.   map<int, two> twos;
  25.   map<int, three> threes;
  26.  
  27.   template<class T, class... Ts>
  28.   struct overload : T, overload<Ts...> {
  29.     using T::operator();
  30.     using overload<Ts...>::operator();
  31.     overload(T t, Ts... ts) : T(t), overload<Ts...>(ts...) {}
  32.   };
  33.  
  34.   template<class T>
  35.   struct overload<T> : T {
  36.     using T::operator();
  37.     overload(T t) : T(t) {}
  38.   };
  39.  
  40.   template<class...Ts>
  41.   overload<Ts...> make_overload(Ts... ts) {
  42.     return overload<Ts...>(ts...);
  43.   }
  44.  
  45.   struct fallback_t { template<class T> fallback_t(T&&) {} };
  46.  
  47.   template <typename T, typename Func>
  48.   int callFunc(T& param, const Func& func) {
  49.     return func(param);
  50.   }
  51.  
  52.   template <typename T>
  53.   int findObject(int key, const T& func) {
  54.     if(ones.count(key) != 0U) {
  55.       return callFunc(ones[key], func);
  56.     }
  57.     else if(twos.count(key) != 0U) {
  58.       return callFunc(twos[key], func);
  59.     }
  60.     else {
  61.       return callFunc(threes[key], func);
  62.     }
  63.   }
  64.  
  65.   int foo(const int key, const int param) { return findObject(key, [&](auto& value) { return value.foo(param); }); }
  66.   int bar(const int key) {
  67.     return findObject(key, make_overload(
  68.       [](const auto& value) -> decltype(int(value.bar())) { return value.bar(); },
  69.       [](fallback_t) -> int { std::cout << "fallback\n"; return 13; }
  70.       ));
  71.   }
  72. };
  73.  
  74. int main() {
  75.   owner myOwner;
  76.  
  77.   myOwner.ones.insert(make_pair(0, one()));
  78.   myOwner.twos.insert(make_pair(1, two()));
  79.   myOwner.threes.insert(make_pair(2, three()));
  80.  
  81.   myOwner.foo(2, 1);
  82.   cout << myOwner.bar(1) << endl;
  83.   cout << myOwner.bar(2) << endl;
  84. }
Success #stdin #stdout 0s 4264KB
comments (?)
stdin
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Standard input is empty
stdout
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fallback
13
fallback
13
https://ideone.com/PB8SRN
language:
C++14 (gcc 8.3)
created:
6 years ago
visibility:
public

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