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  1. #include <iostream>
  2. #include <type_traits>
  3.  
  4. struct Coordinate
  5. {
  6.     double x;
  7.     double y;
  8. };
  9.  
  10. template <typename T> class val
  11. {
  12. public:
  13.     val(const T& t) : t(t) {}
  14.  
  15.     val(const val&) = default;
  16.     val& operator = (const val&) = delete;
  17.  
  18.     const T* operator->() const { return &t; }
  19.     const T& get() const { return t; }
  20.  
  21.     T* operator->() { return &t; }
  22.     T& get() { return t; }
  23.  
  24. private:
  25.     T t;
  26. };
  27.  
  28. template <typename T> class var
  29. {
  30. public:
  31.     var(const T& t) : t(t) {}
  32.  
  33.     var(const var&) = default;
  34.     var& operator = (const var&) = default;
  35.     var& operator = (const T&) { this->t = t; return *this; };
  36.  
  37.     const T* operator->() const { return &t; }
  38.     const T& get() const { return t; }
  39.  
  40.     T* operator->() { return &t; }
  41.     T& get() { return t; }
  42.  
  43. private:
  44.     std::remove_const_t<T> t;
  45. };
  46.  
  47. int main()
  48. {
  49.     val<const Coordinate> immutable_val({42, 51});
  50.     val<Coordinate> mutable_val({42, 51});
  51.     var<const Coordinate> immutable_var({42, 51});
  52.     var<Coordinate> mutable_var({42, 51});
  53.  
  54.     //immutable_val->x = 42; // error: cannot assign to return value because function 'operator->' returns a const value
  55.     mutable_val->x = 42;
  56.     //immutable_var->x = 42; // error: cannot assign to return value because function 'operator->' returns a const value
  57.     mutable_var->x = 42;
  58.  
  59.     //immutable_val = {42, 42}; // error: overload resolution selected deleted operator '='
  60.     //mutable_val = {42, 42}; // error: overload resolution selected deleted operator '='
  61.     immutable_var = {42, 42};
  62.     mutable_var = {42, 42};
  63.  
  64.     //mutable_val = mutable_val; // error: overload resolution selected deleted operator '='
  65.     //immutable_val = immutable_val; // error: overload resolution selected deleted operator '='
  66.     immutable_var = immutable_var; 
  67.     mutable_var = mutable_var;
  68. }
  69.  
Success #stdin #stdout 0s 3464KB
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https://ideone.com/BfzOt6
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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