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  1. #include <iostream>
  2. #include <memory>
  3.  
  4. template <typename T>
  5. struct propagating_unique_ptr : std::unique_ptr<T> {
  6.     using std::unique_ptr<T>::unique_ptr;
  7.     using std::unique_ptr<T>::operator =;
  8.  
  9.     const T *get() const noexcept {
  10.         return std::unique_ptr<T>::get();
  11.     }
  12.     T *get() noexcept {
  13.         return std::unique_ptr<T>::get();
  14.     }
  15.  
  16.     const T &operator *() const noexcept {
  17.         return std::unique_ptr<T>::operator *();
  18.     }
  19.     T &operator *() noexcept {
  20.         return std::unique_ptr<T>::operator *();
  21.     }
  22.  
  23.     const T *operator -> () const noexcept {
  24.         return std::unique_ptr<T>::get();
  25.     }
  26.     T *operator -> () noexcept {
  27.         return std::unique_ptr<T>::get();
  28.     }
  29. }; 
  30.  
  31. struct foo {
  32.     foo() : value_ptr_(std::make_unique<int>(3)) {}
  33.     void increment() {
  34.         ++(*value_ptr_); // OK
  35.     }
  36.     int get_value() const {
  37.         return *value_ptr_;
  38.     }
  39.     propagating_unique_ptr<int> value_ptr_;
  40. };
  41.  
  42. int main() {
  43.     const foo my_foo;
  44.     std::cout << my_foo.get_value() << std::endl;
  45.     my_foo.increment(); // compiler error
  46.     std::cout << my_foo.get_value() << std::endl;
  47. }
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:45:22: error: passing ‘const foo’ as ‘this’ argument discards qualifiers [-fpermissive]
     my_foo.increment(); // compiler error
                      ^
prog.cpp:33:10: note:   in call to ‘void foo::increment()’
     void increment() {
          ^~~~~~~~~
stdout
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Standard output is empty
https://ideone.com/YRN1Lb
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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