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  1. #include <utility> 
  2. #include <iostream>
  3. #include <functional>
  4.  
  5. using namespace std;
  6.  
  7. class Function
  8. {
  9.   struct Concept {
  10.     typedef Concept * (*cloneFunType)(const Concept *);
  11.     cloneFunType m_cloneFun = nullptr;
  12.  
  13.     virtual ~Concept() = default;
  14.   };
  15.  
  16.   template<typename F>
  17.   struct Model final : Concept {
  18.     static Concept* clone(const Concept *c)  { 
  19.       return new Model(static_cast<const Model*>(c)->data); 
  20.     }
  21.  
  22.     explicit Model(F &&f) : data(move(f)) {
  23.       this->m_cloneFun = &Model::clone;
  24.     }
  25.  
  26.     explicit Model(const F &f) : data(f) {
  27.       this->m_cloneFun = &Model::clone;
  28.     }
  29.  
  30.     F data;
  31.   };
  32.  
  33.   Concept* object;
  34.  
  35.  
  36. public:
  37.   ~Function() {
  38.     delete object;
  39.   }
  40.  
  41.  
  42.   template<typename F>
  43.   explicit Function(F&& f) 
  44.     : object(new Model<typename remove_reference<F>::type>(forward<F>(f)))
  45.   {} 
  46.  
  47.   Function(Function const& that) 
  48.     : object((*(that.object->m_cloneFun))(that.object))
  49.   {}
  50.  
  51.   Function(Function && that) 
  52.     : object(that.object)
  53.   {
  54.     that.object = nullptr;
  55.   }
  56.  
  57.     //...
  58. };
  59.  
  60. struct A {
  61.   A() = default;
  62.   A(const A&) {
  63.     cout << "calling As copy constructor" << endl;
  64.   }
  65.   A(A&&) {
  66.     cout << "calling As move constructor" << endl;
  67.   }
  68.  
  69.   };
  70.  
  71. struct B { 
  72.   B() = default;
  73.   B(const B&) = delete;
  74.   B(B&&) {
  75.     cout << "calling Bs move constructor" << endl;
  76.   }
  77.  
  78. };
  79.  
  80. int main(){
  81.   auto fA = Function(A()); //at runtime, will only call move constructor.
  82.   //auto fB = Function(B()); //will not compile.
  83.  
  84.   cout << "is_copy_constructible<Function>::value: "
  85.  << is_copy_constructible<Function>::value << endl;
  86.  
  87.   cout << "is_copy_constructible<function<void()>>::value: "
  88.  << is_copy_constructible<function<void()>>::value << endl;
  89. }
Success #stdin #stdout 0s 3428KB
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Standard input is empty
stdout
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calling As move constructor
is_copy_constructible<Function>::value: 1
is_copy_constructible<function<void()>>::value: 1
https://ideone.com/FKFktK
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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