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  1. #include <memory>
  2. #include <type_traits>
  3. #include <iostream>
  4.  
  5. template<class BaseT>
  6. class polyval {
  7. public:
  8.   template<class U>
  9.   polyval(const U& val) {
  10.     static_assert(std::is_same<BaseT, U>::value || std::is_base_of<BaseT, U>::value, "U must be BaseT or derived from BaseT.");
  11.     val_ = std::unique_ptr<BaseT>(new U(val));
  12.   }
  13.  
  14.   template<class U>
  15.   polyval(U&& val) {
  16.     static_assert(std::is_same<BaseT, U>::value || std::is_base_of<BaseT, U>::value, "U must be BaseT or derived from BaseT.");
  17.     val_ = std::unique_ptr<BaseT>(new U(std::move(val)));
  18.   }
  19.  
  20.   polyval(const polyval<BaseT>&) = delete;
  21.   polyval(polyval<BaseT>&&) = delete;
  22.  
  23.   polyval<BaseT>& operator=(const polyval<BaseT>&) = delete;
  24.   polyval<BaseT>& operator=(const polyval<BaseT>&&) = delete;
  25.  
  26.   template<class U = BaseT>
  27.   U& get() {
  28.     static_assert(std::is_same<BaseT, U>::value || std::is_base_of<BaseT, U>::value, "U must be BaseT or derived from BaseT.");
  29.     return dynamic_cast<U&>(*val_);
  30.   }
  31.  
  32.   template<class U = BaseT>
  33.   const U& get() const {
  34.     static_assert(std::is_same<BaseT, U>::value || std::is_base_of<BaseT, U>::value, "U must be BaseT or derived from BaseT.");
  35.     return dynamic_cast<U&>(*val_);
  36.   }
  37.  
  38. private:
  39.   std::unique_ptr<BaseT> val_;
  40.  
  41.   friend void swap(polyval<BaseT>& a, polyval<BaseT>& b) {
  42.     using std::swap;
  43.     swap(a.val_, b.val_);
  44.   }
  45. };
  46.  
  47. class B {
  48. public:
  49.   virtual void foo() { std::cout << "B\n"; }
  50. };
  51.  
  52. class D1 : public B {
  53. public:
  54.   virtual void foo() { std::cout << "D1\n"; }
  55. };
  56.  
  57. class D2 : public D1 {
  58. public:
  59.   virtual void foo() { std::cout << "D2\n"; }
  60. };
  61.  
  62. class D3 : public D2 {
  63. public:
  64.   virtual void foo() { std::cout << "cannot happen.\n"; }
  65. };
  66.  
  67. int main() {
  68.   polyval<B> b((B()));
  69.   polyval<B> d1((D1()));
  70.   polyval<B> d2((D2()));
  71.  
  72.   b.get().foo();
  73.   d1.get<D1>().foo();
  74.   d2.get().foo();
  75.   d2.get<D3>().foo();
  76. }
Runtime error #stdin #stdout 0s 2960KB
comments (?)
stdin 
Standard input is empty
stdout 
B
D1
D2
https://ideone.com/4Vu1t
language:
C++14 (gcc 8.3)
created:
13 years ago
visibility:
public

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