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  1.     #include <string>
  2.     #include <iostream>
  3.     #include <type_traits>
  4.  
  5.     namespace detail
  6.     {
  7.         template<typename T, typename = void>
  8.         struct print_helper
  9.         {
  10.             static T print(T t) {
  11.                 return t;
  12.             }
  13.         };
  14.  
  15.         template<typename T>
  16.         struct print_helper<T, decltype(std::declval<T>().print(), (void)0)>
  17.         {
  18.             static auto print(T t) -> decltype(t.print()) {
  19.                 return t.print();
  20.             }
  21.         };
  22.     }
  23.  
  24.     template<typename T>
  25.     auto print(T t) -> decltype(detail::print_helper<T>::print(t))
  26.     {
  27.         return detail::print_helper<T>::print(t);
  28.     }
  29.  
  30.     struct Foo
  31.     {
  32.         int print()
  33.         {
  34.             return 42;
  35.         }
  36.  
  37.         operator int() const
  38.         {
  39.             return 32;
  40.         }
  41.     };
  42.  
  43.     struct Bar
  44.     {
  45.         std::string print()
  46.         {
  47.             return "The answer...";
  48.         }
  49.  
  50.         operator int() const
  51.         {
  52.             return (int)Foo();
  53.         }
  54.     };
  55.  
  56.     struct Baz
  57.     {
  58.         operator std::string() const
  59.         {
  60.             return std::string("The answer...");
  61.         }
  62.     };
  63.  
  64.     int main()
  65.     {
  66.         std::cout << print(Foo()) << std::endl;
  67.         std::cout << print(Bar()) << std::endl;
  68.         std::cout << print(42) << std::endl;
  69.         std::cout << print((int)Foo()) << std::endl;
  70.         std::cout << print("The answer...") << std::endl;
  71.         std::cout << print(std::string("The answer...")) << std::endl;
  72.         std::cout << print((int)Bar()) << std::endl;
  73.         std::cout << print((std::string)Baz()) << std::endl;
  74.     }
  75.  
Success #stdin #stdout 0s 3028KB
comments (?)
stdin 
Standard input is empty
stdout 
42
The answer...
42
32
The answer...
The answer...
32
The answer...
https://ideone.com/coQY6O
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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