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  1. #include <memory>
  2.  
  3. template<
  4.     class T,
  5.     class Deleter = std::default_delete<T>
  6. > class transitive_ptr : public std::unique_ptr<T,Deleter>
  7. {
  8. public:
  9.     // inherit typedefs for the sake of completeness
  10.     typedef
  11.         typename std::unique_ptr<T,Deleter>::pointer
  12.         pointer;
  13.     typedef
  14.         typename std::unique_ptr<T,Deleter>::element_type
  15.         element_type;
  16.     typedef
  17.         typename std::unique_ptr<T,Deleter>::deleter_type
  18.         deleter_type;
  19.  
  20.     // extra typedef
  21.     typedef
  22.         const typename std::remove_pointer<pointer>::type*
  23.         const_pointer;
  24.  
  25.     // inherit constructor
  26.     using std::unique_ptr<T,Deleter>::unique_ptr;
  27.  
  28.     // add transitively const version of get()
  29.     pointer get() {
  30.         return std::unique_ptr<T,Deleter>::get();
  31.     }
  32.     const_pointer get() const {
  33.         return std::unique_ptr<T,Deleter>::get();
  34.     }
  35.  
  36.     // add transitively const version of operator*()
  37.     typename std::add_lvalue_reference<T>::type
  38.     operator*() {
  39.         return *get();
  40.     }
  41.     typename std::add_lvalue_reference<const T>::type
  42.     operator*() const {
  43.         return *get();
  44.     }
  45.  
  46.     // add transitively const version of operator->()
  47.     pointer operator->() {
  48.         return get();
  49.     }
  50.     const_pointer operator->() const {
  51.         return get();
  52.     }
  53. };
  54.  
  55. struct A {
  56.     transitive_ptr<int> x;
  57.     A(): x{new int} {}
  58. };
  59.  
  60. int main() {
  61.     const A a;
  62.     *a.x = 3;
  63. }
Compilation error #stdin compilation error #stdout 0s 3424KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In function ‘int main()’:
prog.cpp:62:10: error: assignment of read-only location ‘a.A::x.transitive_ptr<T, Deleter>::operator*<int, std::default_delete<int> >()’
     *a.x = 3;
          ^
stdout
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Standard output is empty
https://ideone.com/0RUr3V
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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