fork download
copy 
  1. #include <type_traits>
  2. //User types
  3. struct field_t{};
  4. struct other_t{};
  5. struct StructA{field_t field;};
  6. struct StructB{other_t other;};
  7. struct StructC:public StructA{};
  8. struct StructD:public StructB{};
  9. struct StructT{other_t field;field_t other;};
  10. //Implementation detail
  11. template<typename T,typename FIELD_T>
  12. struct type_has_field{
  13.   struct ok_type{char padding[1];};
  14.   struct no_type{char padding[8];};
  15.   template<class U>
  16.   struct has_field{
  17.     struct Fallback{int field;};
  18.     struct Derived:U,Fallback{};
  19.     template<typename C, C> struct ChT;
  20.     template<typename C> static no_type f(ChT<int Fallback::*,&C::field>(*)=nullptr);
  21.     template<typename C> static ok_type f(...);
  22.     static const bool value=sizeof(ok_type)==sizeof(f<Derived>(0));
  23.   };
  24.   struct FalseStruct{
  25.     template<class U>struct next{typedef FalseStruct type;};
  26.     static const bool value=false;
  27.   };
  28.   struct TrueStruct{
  29.     static const bool value=true;
  30.   };
  31.   struct NextStruct1{
  32.     template<class U>
  33.     struct next{
  34.       static const bool flag=(
  35.         std::is_same<
  36.           FIELD_T,
  37.           decltype((*(U*)nullptr).field)
  38.         >::value
  39.       );
  40.       typedef typename std::conditional<
  41.         flag,
  42.         TrueStruct,FalseStruct
  43.       > type_expr;
  44.       typedef typename type_expr::type type;
  45.     };
  46.   };
  47.   struct NextStruct0{
  48.     template<class U>
  49.     struct next{
  50.       static const bool flag=(
  51.         has_field<U>::value
  52.       );
  53.       typedef typename std::conditional<
  54.         flag,
  55.         NextStruct1,FalseStruct
  56.       > type_expr;
  57.       typedef typename type_expr::type type;
  58.     };
  59.   };
  60.   typedef typename std::conditional<
  61.     std::is_class<T>::value,
  62.     NextStruct0,FalseStruct
  63.   > type_expr0;
  64.   typedef typename type_expr0::type type_expr1;
  65.   typedef typename type_expr1::template next<T> type_expr2;
  66.   typedef typename type_expr2::type type_expr3;
  67.   typedef typename type_expr3::template next<T> type_expr4;
  68.   typedef typename type_expr4::type type_expr5;
  69.   static const bool value=type_expr5::value;
  70. };
  71. //Experiment
  72. static_assert(
  73.   false==type_has_field<float,field_t>::value,
  74.   "field_t float::field - should not exist"
  75. );
  76. static_assert(
  77.   false==type_has_field<char,field_t>::value,
  78.   "field_t char::field - should not exist"
  79. );
  80. static_assert(
  81.   true==type_has_field<StructA,field_t>::value,
  82.   "field_t StructA::field - should exist"
  83. );
  84. static_assert(
  85.   false==type_has_field<StructB,field_t>::value,
  86.   "field_t StructB::field - should not exist"
  87. );
  88. static_assert(
  89.   true==type_has_field<StructC,field_t>::value,
  90.   "field_t StructC::field - should exist"
  91. );  
  92. static_assert(
  93.   false==type_has_field<StructD,field_t>::value,
  94.   "field_t StructD::field - should not exist"
  95. );
  96. static_assert(
  97.   false==type_has_field<StructT,field_t>::value,
  98.   "field_t StructT::field - should not exist"
  99. );
  100. static_assert(
  101.   true==type_has_field<StructT,other_t>::value,
  102.   "other_t StructT::field - should exist"
  103. );
  104. int main(){return 0;}
Success #stdin #stdout 0s 3336KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
Standard output is empty
https://ideone.com/T8R062
http://w...content-available-to-author-only...v.ru/code/forum/?id=152200&page=2
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
secret

Share or Embed source code

Discover > Sphere Engine API

The brand new service which powers Ideone!

Discover > IDE Widget

Widget for compiling and running the source code in a web browser!