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  1.     #include <iostream>
  2.     #include <string>
  3.     #include <type_traits>
  4.  
  5.     namespace has_insertion_operator_impl {
  6.       typedef char no;
  7.       typedef char yes[2];
  8.  
  9.       struct any_t {
  10.         template<typename T> any_t( T const& );
  11.       };
  12.  
  13.       no operator<<( std::ostream const&, any_t const& );
  14.  
  15.       yes& test( std::ostream& );
  16.       no test( no );
  17.  
  18.       template<typename T>
  19.       struct has_insertion_operator {
  20.         static std::ostream &s;
  21.         static T const &t;
  22.         static bool const value = sizeof( test(s << t) ) == sizeof( yes );
  23.       };
  24.     }
  25.  
  26.     template<typename T>
  27.     struct has_insertion_operator :
  28.       has_insertion_operator_impl::has_insertion_operator<T> {
  29.     };
  30.  
  31.     template <typename T,
  32.         typename std::enable_if<has_insertion_operator<T>::value, T>::type* = nullptr>
  33.     void print(T obj) {
  34.         std::cout << "from print()" << std::endl;
  35.     }
  36.  
  37.     template <typename T,
  38.         typename std::enable_if<!has_insertion_operator<T>::value, T>::type* = nullptr>
  39.     void print(T obj) {
  40.         std::cout << obj.to_string() << std::endl;
  41.     }
  42.  
  43.     struct Foo
  44.     {
  45.     public:
  46.         friend std::ostream& operator<<(std::ostream & os, Foo const& foo);
  47.     };
  48.  
  49.     struct Bar
  50.     {
  51.     public:
  52.         std::string to_string() const
  53.         {
  54.             return "from to_string()";
  55.         }
  56.     };
  57.  
  58.     int main()
  59.     {
  60.         print<Foo>(Foo());
  61.         print<Bar>(Bar());
  62.  
  63.         //print<Bar>(Foo()); doesn't compile
  64.         //print<Foo>(Bar()); doesn't compile
  65.  
  66.         print(Foo());
  67.         print(Bar());
  68.  
  69.         print(42);
  70.         print('a');
  71.         print(std::string("Hi"));
  72.         print("Hey");
  73.         //print({1, 2, 3}); doesn't compile
  74.         return 0;
  75.     }
  76.  
Success #stdin #stdout 0s 3028KB
comments (?)
stdin 
Standard input is empty
stdout 
from print()
from to_string()
from print()
from to_string()
from print()
from print()
from print()
from print()
https://ideone.com/VnkwcW
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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