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  1. #include <cstdint>
  2. #include <type_traits>
  3.  
  4. #define DEFINE_HAS_SIGNATURE(traitsName, funcName, signature)               \
  5.     template <typename U>                                                   \
  6.     class traitsName                                                        \
  7.     {                                                                       \
  8.     private:                                                                \
  9.         template<typename T, T> struct helper;                              \
  10.         template<typename T>                                                \
  11.         static std::uint8_t check(helper<signature, &funcName>*);           \
  12.         template<typename T> static std::uint16_t check(...);               \
  13.     public:                                                                 \
  14.         static                                                              \
  15.         constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint8_t); \
  16.     }
  17.  
  18. DEFINE_HAS_SIGNATURE(has_ref_swap, T::swap, void (T::*)(T&));
  19. DEFINE_HAS_SIGNATURE(has_value_swap, T::swap, void (T::*)(T));
  20.  
  21. template <typename T>
  22. struct MetaHelperType_impl
  23. {
  24.     static_assert(!has_value_swap<T>::value,
  25.                   "Incorrect implementation of T::swap, "
  26.                   "signature should be void T::swap(T&) instead of T::swap(T)");
  27.  
  28.     using type = std::conditional_t<has_ref_swap<T>::value,
  29.                                     std::true_type,
  30.                                     std::false_type>;
  31. };
  32.  
  33. template <typename T>
  34. using MetaHelperType = typename MetaHelperType_impl<T>::type;
  35.  
  36. struct NonReferenceSwap
  37. {
  38.     void swap(NonReferenceSwap) {}
  39. };
  40.  
  41. struct ReferenceSwap
  42. {
  43.     void swap(ReferenceSwap &) {}
  44. };
  45.  
  46. struct NoSwapFunction {};
  47.  
  48. int main() {
  49. 	static_assert(MetaHelperType<ReferenceSwap>::value, "unexpected result");
  50. 	static_assert(!MetaHelperType<NoSwapFunction>::value, "unexpected result");	
  51. 	static_assert(!MetaHelperType<NonReferenceSwap>::value, "unexpected result"); // Doesn't compile as expected	
  52. }
Compilation error #stdin compilation error #stdout 0s 3408KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In instantiation of 'struct MetaHelperType_impl<NonReferenceSwap>':
prog.cpp:34:61:   required by substitution of 'template<class T> using MetaHelperType = typename MetaHelperType_impl::type [with T = NonReferenceSwap]'
prog.cpp:51:48:   required from here
prog.cpp:24:5: error: static assertion failed: Incorrect implementation of T::swap, signature should be void T::swap(T&) instead of T::swap(T)
     static_assert(!has_value_swap<T>::value,
     ^
stdout
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Standard output is empty
https://ideone.com/szu4f3
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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