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  1. #include <type_traits> // To use 'std::integral_constant'.
  2. #include <iostream>    // To use 'std::cout'.
  3. #include <iomanip>     // To use 'std::boolalpha'.
  4. #include <map>
  5. #include <vector>
  6.  
  7. #define GENERATE_HAS_MEMBER(member)                                               \
  8.                                                                                   \
  9. template < class T >                                                              \
  10. class HasMember_##member                                                          \
  11. {                                                                                 \
  12. private:                                                                          \
  13.     using Yes = char[2];                                                          \
  14.     using  No = char[1];                                                          \
  15.                                                                                   \
  16.     struct Fallback { int member; };                                              \
  17.     struct Derived : T, Fallback { };                                             \
  18.                                                                                   \
  19.     template < class U >                                                          \
  20.     static No& test ( decltype(U::member)* );                                     \
  21.     template < typename U >                                                       \
  22.     static Yes& test ( U* );                                                      \
  23.                                                                                   \
  24. public:                                                                           \
  25.     static constexpr bool RESULT = sizeof(test<Derived>(nullptr)) == sizeof(Yes); \
  26. };                                                                                \
  27.                                                                                   \
  28. template < class T >                                                              \
  29. struct has_member_##member                                                        \
  30. : public std::integral_constant<bool, HasMember_##member<T>::RESULT>              \
  31. {                                                                                 \
  32. };
  33.  
  34.  
  35. GENERATE_HAS_MEMBER(find) // Creates 'has_member_func'.
  36.  
  37. struct B
  38. {
  39.     double find ( const char* );
  40. };
  41.  
  42.  
  43. int main ( )
  44. {
  45.     std::cout << std::boolalpha
  46.               << "\n" "'find' in 'B' : "
  47.               << has_member_find<B>() // <type_traits>-like interface.
  48.               << "\n" "'find' in 'std::map' : "
  49.               << has_member_find<std::map<int,int> >() // Implicitly convertible to 'bool'.
  50.               << "\n" "'find' in 'std::vector' : "
  51.               << has_member_find<std::vector<int> >() // Implicitly convertible to 'bool'.
  52.               << "\n";
  53. }
Success #stdin #stdout 0s 16048KB
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stdin
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Standard input is empty
stdout
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'find' in 'B' : true
'find' in 'std::map' : true
'find' in 'std::vector' : false
https://ideone.com/KNsNXL
language:
C++ (gcc 8.3)
created:
6 years ago
visibility:
public

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