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  1. #include <string>
  2. #include <type_traits>
  3.  
  4. class Object{};
  5.  
  6. enum my_conv {
  7.   string, const_object, object
  8. };
  9.  
  10. template<typename T, typename V = void>
  11. struct deducer;
  12.  
  13. template<typename T>
  14. struct deducer<T, typename std::enable_if< std::is_constructible<std::string, T>::value >::type > {
  15.   static const my_conv value = my_conv::string;
  16. }; // (1) test for string                                                                                              
  17.  
  18. template<typename T>
  19. struct deducer<T, typename std::enable_if< std::is_assignable<Object*&, T>::value >::type > {
  20.   static const my_conv value = my_conv::object;
  21. }; // (2) test for Object derived                                                                                      
  22.  
  23. template<typename T>
  24. struct deducer<const T, typename std::enable_if< std::is_assignable<Object*&, T>::value >::type > {
  25.   static const my_conv value = my_conv::const_object;
  26. }; // (3) should test for const Object derived                                                                         
  27.  
  28. class Test : public Object {
  29. public:
  30.   Test() = default;
  31. };
  32.  
  33. int main() {
  34.   std::string   str;
  35.   Test*         t = new Test;
  36.   const Test*   tconst = static_cast<const Test*>(t);
  37.  
  38.   deducer<decltype(t)>::value;// deduce as (1)                                                                         
  39.   deducer<decltype(str)>::value;//deduce as (2)                                                                        
  40.   deducer<decltype(tconst)>::value;//fail to deduce as (1)... why?                                                     
  41. }
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In function 'int main()':
prog.cpp:40:3: error: incomplete type 'deducer<const Test*>' used in nested name specifier
   deducer<decltype(tconst)>::value;//fail to deduce as (1)... why?                                                     
   ^
stdout
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Standard output is empty
https://ideone.com/N9sLsk
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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