ccls/src/maybe.h

#pragma once

#include <optional>

#include <utility>

// Like std::optional, but the stored data is responsible for containing the
// empty state. T should define a function `bool T::Valid()`.
template <typename T> class Maybe {
  T storage;

public:
  constexpr Maybe() = default;
  Maybe(const Maybe &) = default;
  Maybe(std::nullopt_t) {}
  Maybe(const T &x) : storage(x) {}
  Maybe(T &&x) : storage(std::forward<T>(x)) {}

  Maybe &operator=(const Maybe &) = default;
  Maybe &operator=(const T &x) {
    storage = x;
    return *this;
  }

  const T *operator->() const { return &storage; }
  T *operator->() { return &storage; }
  const T &operator*() const { return storage; }
  T &operator*() { return storage; }

  bool Valid() const { return storage.Valid(); }
  explicit operator bool() const { return Valid(); }
  operator std::optional<T>() const {
    if (Valid())
      return storage;
    return std::nullopt;
  }

  void operator=(std::optional<T> &&o) { storage = o ? *o : T(); }

  // Does not test if has_value()
  bool operator==(const Maybe &o) const { return storage == o.storage; }
  bool operator!=(const Maybe &o) const { return !(*this == o); }
};