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  1. #include <iostream>
  2. #include <type_traits>
  3. #include <experimental/optional>
  4.  
  5. template <class C>
  6. using opt = std::experimental::optional<C>;
  7.  
  8. auto nullopt =  std::experimental::nullopt;
  9.  
  10. class Nothing {};
  11.  
  12. template <class T>
  13. class Maybe
  14. {
  15.     opt<T> t;
  16.     public:
  17.     Maybe(const T& tt) : t(tt) {}
  18.     Maybe(const Maybe& mt) : t(mt.t) {}
  19.     Maybe(Nothing n) : t(nullopt) {}
  20.     Maybe() : t(nullopt) {}
  21.  
  22.     template<class M, class Q = T>
  23.     typename std::enable_if<std::is_class<T>::value, Maybe<M>>::type 
  24.     fetch(M Q::*memptr)
  25.     {
  26.         if (t)
  27.             return Maybe<M>((*t).*memptr);
  28.         else
  29.             return Maybe<M>(Nothing());
  30.     }
  31.  
  32.     template<class M, class Q = T>
  33.     typename std::enable_if<std::is_class<T>::value, Maybe<M>>::type 
  34.     fetch(Maybe<M> Q::*memptr)
  35.     {
  36.         if (t)
  37.             return Maybe<M>((*t).*memptr);
  38.         else
  39.             return Maybe<M>(Nothing());
  40.     }
  41.  
  42.     operator bool() const { return bool(t); }
  43.     T& operator*() { return *t; }
  44.     T* operator->() { return &*t; }
  45.     const T& operator*() const { return *t; }
  46.     const T* operator->() const { return &*t; }
  47.  
  48. };
  49.  
  50.  
  51. struct C
  52. {
  53.     int d;
  54. };
  55.  
  56. struct B
  57. {
  58.     Maybe<C> c;
  59.     B(C c) : c(c) {}
  60.     B() : c() {}
  61. };
  62.  
  63. struct A
  64. {
  65.     Maybe<B> b;
  66.     A(B b) : b(b) {}
  67.     A() : b() {}
  68. };
  69.  
  70. void test(Maybe<A> a)
  71. {
  72.     Maybe<int> d = a.fetch(&A::b).fetch(&B::c).fetch(&C::d);
  73.     if (!d)
  74.         std::cout << "empty\n";
  75.    else
  76.         std::cout << *d << "\n";
  77. };
  78.  
  79. int main()
  80. {
  81.     Maybe<A> empty1{Nothing{}};
  82.     Maybe<A> empty2{A()};
  83.     Maybe<A> empty3{A(B())};
  84.     Maybe<A> full  (A(B(C{42})));
  85.  
  86.     test(empty1);
  87.     test(empty2);
  88.     test(empty3);
  89.     test(full);
  90. }
  91.  
Success #stdin #stdout 0s 4492KB
comments (?)
stdin 
Standard input is empty
stdout 
empty
empty
empty
42
https://ideone.com/46aLXM
language:
C++14 (gcc 8.3)
created:
7 years ago
visibility:
public

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