Boost-style Optional

1#include <stdexcept>
2#include <type_traits>
3 
4struct nullopt_t {
5  explicit constexpr nullopt_t(int) {}
6};
7inline constexpr nullopt_t nullopt{0};
8 
9template<typename T>
10class optional {
11  alignas(T) unsigned char storage_[sizeof(T)];
12  bool has_value_;
13 
14public:
15  optional() noexcept : has_value_(false) {}
16  optional(nullopt_t) noexcept : has_value_(false) {}
17  
18  optional(const T& value) : has_value_(true) {
19    new (storage_) T(value);
20  }
21 
22  ~optional() {
23    if (has_value_) reinterpret_cast<T*>(storage_)->~T();
24  }
25 
26  bool has_value() const noexcept { return has_value_; }
27  explicit operator bool() const noexcept { return has_value_; }
28 
29  T& value() {
30    if (!has_value_) throw std::runtime_error("bad optional access");
31    return *reinterpret_cast<T*>(storage_);
32  }
33 
34  T value_or(const T& default_value) const {
35    return has_value_ ? *reinterpret_cast<const T*>(storage_) : default_value;
36  }
37};