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  1. #include <type_traits>
  2. #include <iterator>
  3.  
  4. // Do NOT define key_type YET (note the "basic" in the name).
  5. template<class R>
  6. struct range_basic_type_traits
  7. {
  8.     // "iterator": The type of the iterators of a range
  9.     using iterator = decltype(std::begin(std::declval<R>()));
  10.  
  11.     // "value_type": The type of the values of a range
  12.     using value_type = typename std::remove_reference<decltype(*(std::declval<iterator>()))>::type;
  13. };
  14.  
  15.  
  16. // Define a "has_key" type (std::true/false_type) for the existence member functions (not checking their return value)
  17. struct _test_has_key {
  18.     template<class TARGET> static auto test(TARGET* p) -> decltype(p->key(), std::true_type());
  19.     template<class>        static auto test(...) -> std::false_type;
  20. };
  21. template<class TARGET> struct has_key : public decltype(_test_has_key::test<TARGET>(0)) {};
  22.  
  23.  
  24. // Adding the key_type if the iterator of R has key() defined
  25. template<class R>
  26. struct range_type_traits : public range_basic_type_traits<R>
  27. {
  28.     using key_type = typename std::enable_if<
  29.         has_key<typename range_basic_type_traits<R>::iterator>::value,
  30.         typename std::remove_reference<decltype(std::declval<typename range_basic_type_traits<R>::iterator>().key())>
  31.     >::type;
  32. };
  33.  
  34.  
  35. //------------------------------------------------------------------------------
  36.  
  37. template<class K, class T>
  38. struct Test {
  39.     struct iterator {
  40.         const T &  operator*() const;
  41.         const K &  key() const;
  42.     };
  43.     iterator begin() const;
  44. };
  45.  
  46. int main()
  47. {
  48.     // test it:
  49.     using K = int;
  50.     using T = float;
  51.     using R = Test<K,T>;
  52.     static_assert(std::is_same<typename range_type_traits<R>::value_type, const T>::value, "value_type is incorrect");
  53.     static_assert(std::is_same<typename range_type_traits<R>::key_type, const K>::value, "key_type is incorrect");
  54. }
Compilation error #stdin compilation error #stdout 0s 2892KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In function ‘int main()’:
prog.cpp:53:5: error: static assertion failed: key_type is incorrect
stdout
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Standard output is empty
https://ideone.com/LJnRQ3
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
secret

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