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  1. #include <cstdint>
  2. #include <type_traits>
  3.  
  4. #define DEFINE_HAS_MEMBER(traitsName, memberName)                             \
  5.     template <typename U>                                                     \
  6.     class traitsName                                                          \
  7.     {                                                                         \
  8.     private:                                                                  \
  9.         struct Fallback { int memberName; };                                  \
  10.         struct Dummy {};                                                      \
  11.         template<typename T, bool is_a_class = std::is_class<T>::value>       \
  12.         struct identity_for_class_or_dummy { using type = Dummy; };           \
  13.         template<typename T>                                                  \
  14.         struct identity_for_class_or_dummy<T, true> { using type = T; };      \
  15.                                                                               \
  16.         template <typename Base>                                              \
  17.         struct Derived : Base, Fallback {};                                   \
  18.         template<typename T, T> struct helper;                                \
  19.         template<typename T>                                                  \
  20.         static std::uint8_t                                                   \
  21.         check(helper<int (Fallback::*),                                       \
  22.               &Derived<typename identity_for_class_or_dummy<T>::type>::memberName>*); \
  23.         template<typename T> static std::uint16_t check(...);                 \
  24.     public:                                                                   \
  25.         static                                                                \
  26.         constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint16_t);  \
  27.     }
  28.  
  29. DEFINE_HAS_MEMBER(has_foo, foo);
  30.  
  31. class C{ public: char foo; };
  32. class D : public C {};
  33. class E {};
  34.  
  35. static_assert(has_foo<C>::value, "");
  36. static_assert(has_foo<D>::value, "");
  37. static_assert(!has_foo<E>::value, "");
  38. static_assert(!has_foo<int>::value, "");
  39.  
  40. int main()
  41. {
  42. 	return 0;
  43. }
  44.  
Success #stdin #stdout 0s 3292KB
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https://ideone.com/LArNVO
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
secret

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