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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. struct Action
  5. {
  6. 	template <typename T>
  7. 	static bool func_real(T a){return func(a, special_());}
  8.  
  9. private:
  10.  
  11.     struct general_ {};
  12.     struct special_ : general_ {};
  13.     template<typename> struct bool_ { typedef bool type; };
  14.  
  15.     template<typename S, typename bool_<decltype(std::declval<S>().Y())>::type = 0>
  16.     static bool func(S a, special_) {
  17.     	cout<<"Y() exists"<<endl;
  18.     	if(a.X()){
  19.     		return a.Y();
  20.     	}
  21.   		return false;
  22.     }
  23.  
  24.     template<typename S>
  25.     static bool func(S a, general_) {
  26.         cout<<"Y() does not exist"<<endl;
  27.         return false;
  28.     }
  29. };
  30.  
  31.  
  32. struct Has{
  33. 	bool X(){return true;}
  34. 	bool Y(){return true;}
  35. };
  36.  
  37. struct Has2{
  38. 	bool X(){return true;}
  39. 	bool Y(){return false;}
  40. };
  41.  
  42. struct HasNo{
  43. 	bool X(){return false;}
  44. };
  45.  
  46. int main() {
  47. 	Has has;
  48. 	Has2 has2;
  49. 	HasNo no;
  50. 	cout<< Action::func_real<Has>(has) <<endl;
  51. 	cout<< Action::func_real<Has2>(has2) <<endl;
  52. 	cout<< Action::func_real<HasNo>(no) <<endl;
  53. 	return 0;
  54. }
Success #stdin #stdout 0s 3296KB
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stdin
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Standard input is empty
stdout
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Y() exists
1
Y() exists
0
Y() does not exist
0
https://ideone.com/Dchh4b
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
secret

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