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  1. #include <functional>
  2. #include <iostream>
  3. #include <type_traits>
  4.  
  5. template<typename T>
  6. struct Provider final {
  7. 	Provider() = delete;
  8. 	Provider(const Provider& other) : _callback{ other._callback } {}
  9. 	Provider(Provider&& other) : _callback{ std::move(other._callback) } {}
  10.  
  11. 	Provider& operator=(Provider&& other) {
  12. 		std::swap(other._callback, _callback);
  13. 		return *this;
  14. 	}
  15.  
  16. 	Provider& operator=(const Provider& other) {
  17. 		_callback = other._callback;
  18. 		return *this;
  19. 	}
  20.  
  21. 	template<typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type, typename = typename std::enable_if<!std::is_same<U, T>::value>::type>
  22. 	Provider<T>& operator=(Provider<U>&& other) {
  23. 		std::swap(other._callback, _callback);
  24. 		return *this;
  25. 	}
  26.  
  27. 	template<typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type, typename = typename std::enable_if<!std::is_same<U, T>::value>::type>
  28. 	Provider<T>& operator=(const Provider<U>& other) {
  29. 		_callback = other._callback;
  30. 		return *this;
  31. 	}
  32.  
  33. 	template<typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type, typename = typename std::enable_if<!std::is_same<U, T>::value>::type>
  34. 	Provider(const Provider<U>& other) : _callback{ other._callback } {}
  35.  
  36. 	template<typename U, typename = typename std::enable_if<std::is_convertible<U, T>::value>::type, typename = typename std::enable_if<!std::is_same<U, T>::value>::type>
  37. 	Provider(Provider<U>&& other) : _callback{ std::move(other._callback) } {}
  38.  
  39. 	template<typename U, typename = typename std::enable_if<std::is_constructible<std::function<T()>, U>::value>::type>
  40. 	Provider(U callback) : _callback{ callback } {}
  41.  
  42. 	template<typename = typename std::enable_if<!std::is_constructible<std::function<T()>, T>::value, T>::type>
  43. 	Provider(T value) : _callback{[=] { return value; }} {}
  44.  
  45. 	template<typename U>
  46. 	friend struct Provider;
  47.  
  48. 	T operator()() const {
  49. 		return _callback();
  50. 	}
  51.  
  52. private:
  53. 	std::function<T()> _callback;
  54. };
  55.  
  56. template<typename T>
  57. void doSomething(Provider<T> p) {
  58. 	std::cout << "My value is:" << p() << std::endl;
  59. }
  60.  
  61. int main()
  62. {
  63. 	Provider<int> p1 = 9;
  64. 	Provider<double> p2 = [] { return 9.4; };
  65. 	Provider<unsigned int> p3{9};
  66. 	Provider<float> p4{[]{ return 9.4f; }};
  67.  
  68. 	doSomething<unsigned int>(5);
  69. 	doSomething<float>([] { return 9.5f; });
  70. 	doSomething<int>(p1);
  71. 	doSomething<double>(p2);
  72.  
  73.     return 0;
  74. }
  75.  
  76.  
Success #stdin #stdout 0s 3416KB
comments (?)
stdin
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Standard input is empty
stdout
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My value is:5
My value is:9.5
My value is:9
My value is:9.4
https://ideone.com/aANd04
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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