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  1. #include <type_traits>
  2. #include <utility>
  3.  
  4. #include <iostream>
  5.  
  6. using namespace std;
  7.  
  8. template <typename T>
  9. class Base
  10. {
  11.   protected:
  12.     template <typename U>
  13.     Base(U&& callable)
  14.     {
  15.     	cout << "called" << endl;
  16.       static_assert(
  17.           std::is_same<
  18.               typename std::remove_reference<decltype(callable())>::type, T
  19.             >::value,
  20.           "Expression does not evaluate to correct type!");
  21.     }
  22.  
  23.   public:
  24.     virtual ~Base(void) =default; // Causes error 
  25.  
  26.     virtual operator T(void) =0;
  27.  
  28.     Base(Base&&)
  29.     {
  30.  
  31.     	cout << "move called" << endl;
  32.     }
  33. };
  34.  
  35. template <typename T, typename U>
  36. class Derived : public Base<T>
  37. {
  38.   public:
  39.     Derived(U&& callable) : Base<T>{std::forward<U>(callable)} {}
  40.  
  41.     operator T(void) override final
  42.     {
  43.       return {};
  44.     }
  45. };
  46.  
  47. void TakesWrappedInt(Base<int>&&) {}
  48.  
  49. template <typename U>
  50. auto MakeLazyInt(U&& callable)
  51. {
  52.   return Derived<
  53.             typename std::remove_reference<decltype(callable())>::type, U>{
  54.       std::forward<U>(callable)};
  55. }
  56.  
  57. int main()
  58. {
  59.   TakesWrappedInt(MakeLazyInt([&](){return 3;}));
  60. }
Success #stdin #stdout 0s 3412KB
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called
https://ideone.com/jdu5f7
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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