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  1. #include <iostream>
  2. #include <memory>
  3. #include <type_traits>
  4.  
  5. struct TagA {};
  6. struct TagB {};
  7.  
  8. template< typename tp_Derived >
  9. struct BaseCRTP;
  10.  
  11. struct Base
  12. {
  13. 	template< typename > friend
  14. 	struct BaseCRTP;
  15.  
  16. 	private: void
  17. 	handler_impl(const std::shared_ptr<Base> &) {}
  18. };
  19.  
  20. template< typename tp_Derived >
  21. struct BaseCRTP: public Base
  22. {
  23. 	public: void
  24. 	handler(const std::shared_ptr<Base> & obj)
  25. 	{
  26. 		static_assert
  27. 		(
  28. 			::std::is_base_of< TagA, tp_Derived >::value
  29. 		,	"\"handler\" method can only be used in classes deriving from TagA"
  30. 		);
  31. 		return(handler_impl(obj));
  32. 	}
  33. };
  34.  
  35. struct DerivedA : BaseCRTP< DerivedA >, TagA
  36. {};
  37.  
  38. struct DerivedB : BaseCRTP< DerivedB >, TagB
  39. {};
  40.  
  41. int main()
  42. {
  43. 	DerivedA a; // OK
  44. 	(void) a; // not used
  45. 	auto pha(&DerivedA::handler); // OK
  46. 	DerivedB b; // OK
  47. 	(void) b; // not used
  48. 	auto phb(&DerivedB::handler); // static_assertion failure
  49. 	return 0;
  50. }
Compilation error #stdin compilation error #stdout 0s 16064KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In instantiation of ‘void BaseCRTP<tp_Derived>::handler(const std::shared_ptr<Base>&) [with tp_Derived = DerivedB]’:
prog.cpp:48:22:   required from here
prog.cpp:26:3: error: static assertion failed: "handler" method can only be used in classes deriving from TagA
   static_assert
   ^~~~~~~~~~~~~
stdout
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Standard output is empty
https://ideone.com/p2CO35
language:
C++ (gcc 8.3)
created:
8 years ago
visibility:
public

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