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  1. #include <iostream>
  2. #include <tuple>
  3.  
  4. //function traits
  5. template <typename T>
  6. struct function_traits : public function_traits<decltype(&T::operator())>
  7. {};
  8.  
  9. template <typename C, typename R, typename... A>
  10. struct function_traits<R(C::*)(A...) const>
  11. {
  12.    template <size_t i>
  13.    struct arg
  14.    {
  15.       typedef typename std::tuple_element<i, std::tuple<A...>>::type type;
  16.    };
  17. };
  18.  
  19. template<typename InArg, typename Function>
  20. class selfCompose {
  21.   Function f;
  22.  public:
  23.   selfCompose(Function f): f(f) {}
  24.   auto operator() (InArg x) -> decltype(f(f(x))) 
  25.   {
  26.     return f(f(x));                              
  27.   }
  28. };
  29.  
  30. template<typename Fun>
  31. selfCompose<typename function_traits<Fun>::template arg<0>::type, Fun>
  32. make_selfCompose(Fun f)
  33. {
  34.   typedef typename function_traits<Fun>::template arg<0>::type InArg;
  35.   return selfCompose<InArg, decltype(f)>(f);
  36. }
  37.  
  38. int main() {
  39.   auto f = [](int x){return x*x;};
  40.   std::cout << make_selfCompose(f)(4)
  41.             << std::endl;
  42.   return 0;
  43. }
  44.  
  45.  
Success #stdin #stdout 0s 2884KB
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https://ideone.com/1sNy9Z
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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