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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. //Forward declarations
  5. template< typename T >
  6. class Container;
  7. template< typename T, typename charT, typename traits >
  8. basic_ostream< charT, traits >& 
  9.     operator<<( basic_ostream< charT, traits >& out, 
  10.                                const Container< T >& a_container ); 
  11.  
  12. //Definitions
  13. template< typename T >
  14. class Container
  15. {
  16. public:
  17.     Container( T a_value ): value( a_value ){}
  18.  
  19. private:
  20.     T value;
  21.  
  22.     template< typename T2, typename charT, typename traits >
  23.     friend basic_ostream< charT, traits >& 
  24.         operator<<( basic_ostream< charT, traits >& out, 
  25.                     const Container< T2 >& a_container );
  26. };
  27.  
  28. template< typename T, typename charT, typename traits >
  29. basic_ostream< charT, traits >& 
  30.     operator<<( basic_ostream< charT, traits >& out, 
  31.                 const Container< T >& a_container )
  32. {
  33.     static_assert(std::is_same<T, int>::value, "Error: T must be an integer");
  34.     out << a_container.value;
  35.     return out;
  36. }
  37.  
  38. //Main
  39. int main( void )
  40. {
  41.     Container< int > my_container( 42 );
  42.     Container<bool> other(100);
  43.  
  44.     cout << my_container;
  45.     cout << other; // ERROR
  46.  
  47.     return 0;
  48. }
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In instantiation of ‘std::basic_ostream<charT, traits>& operator<<(std::basic_ostream<charT, traits>&, const Container<T>&) [with T = bool; charT = char; traits = std::char_traits<char>]’:
prog.cpp:45:13:   required from here
prog.cpp:33:5: error: static assertion failed: Error: T must be an integer
     static_assert(std::is_same<T, int>::value, "Error: T must be an integer");
     ^
stdout
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Standard output is empty
https://ideone.com/bcFZy5
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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