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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.  
  38.     struct Bar
  39.     {
  40.         std::string print()
  41.         {
  42.             return "The answer...";
  43.         }
  44.     };
  45.  
  46.     struct Baz
  47.     {
  48.         int print() { return 1729; };
  49.         operator int() const
  50.         {
  51.             return 42;
  52.         }
  53.     };
  54.  
  55.     int main()
  56.     {
  57.         Foo foo;
  58.         Bar bar;
  59.         Baz baz;
  60.  
  61.         static_assert(std::is_same<decltype(print(foo)), int>::value, "!");
  62.  
  63.         std::cout << print(foo) << std::endl;
  64.         std::cout << print(bar) << std::endl;
  65.         std::cout << print(baz) << std::endl;
  66.         std::cout << print(42) << std::endl;
  67.     }
  68.  
Success #stdin #stdout 0s 2984KB
comments (?)
stdin
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Standard input is empty
stdout
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42
The answer...
1729
42
https://ideone.com/EDa20G
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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