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  1. #include <iostream>
  2. #include <type_traits>
  3.  
  4. enum class Mutability {Const, Non_Const};
  5.  
  6. template<Mutability T>
  7. struct ConstMutability: std::true_type{};
  8.  
  9. template<>
  10. struct ConstMutability<Mutability::Non_Const>: std::false_type{};
  11.  
  12. template <typename T, Mutability U>
  13. class Obj
  14. {
  15. public:
  16.     template <Mutability V, typename std::enable_if<
  17.     std::is_same<ConstMutability<U>, ConstMutability<V>>::value || 
  18.     (ConstMutability<V>::value && !ConstMutability<U>::value)
  19.     >::type* = nullptr>
  20.     class Ref
  21.     {
  22.     public:
  23.         Ref() {std::cout << "Successfully created a Ref object" << std::endl;}
  24.  
  25.         friend class Obj;
  26.     };
  27.  
  28.     Obj() {}
  29. };
  30.  
  31.  
  32. int main()
  33. {
  34.     Obj<int, Mutability::Const>::Ref<Mutability::Const> test1; //pass
  35.     //Obj<int, Mutability::Const>::Ref<Mutability::Non_Const> test2; // fail
  36.     Obj<int, Mutability::Non_Const>::Ref<Mutability::Const> test3; // pass
  37.     Obj<int, Mutability::Non_Const>::Ref<Mutability::Non_Const> test4; // pass
  38. }
Success #stdin #stdout 0s 3456KB
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Standard input is empty
stdout
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Successfully created a Ref object
Successfully created a Ref object
Successfully created a Ref object
https://ideone.com/JEiOX9
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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