draconisplusplus/include/rfl/Box.hpp

#ifndef RFL_BOX_HPP_
#define RFL_BOX_HPP_

#include <memory>
#include <stdexcept>

#include "Result.hpp"

namespace rfl {

  /// The Box class behaves very similarly to the unique_ptr, but unlike the
  /// unique_ptr, it is 100% guaranteed to be filled at all times (unless the
  /// user tries to access it after calling std::move does something else that
  /// is clearly bad practice).
  template <class T>
  class Box {
   public:
    /// The only way of creating new boxes is
    /// Box<T>::make(...).
    template <class... Args>
    static Box<T> make(Args&&... _args) {
      return Box<T>(std::make_unique<T>(std::forward<Args>(_args)...));
    }

    /// You can generate them from unique_ptrs as well, in which case it will
    /// return an Error, if the unique_ptr is not set.
    static Result<Box<T>> make(std::unique_ptr<T>&& _ptr) {
      if (!_ptr) {
        return Error("std::unique_ptr was a nullptr.");
      }
      return Box<T>(std::move(_ptr));
    }

    Box() : ptr_(std::make_unique<T>()) {}

    Box(const Box<T>& _other) = delete;

    Box(Box<T>&& _other) = default;

    template <class U>
    Box(Box<U>&& _other) noexcept : ptr_(std::forward<std::unique_ptr<U>>(_other.ptr())) {}

    ~Box() = default;

    /// Returns a pointer to the underlying object
    T* get() const { return ptr_.get(); }

    /// Copy assignment operator
    Box<T>& operator=(const Box<T>& _other) = delete;

    /// Move assignment operator
    Box<T>& operator=(Box<T>&& _other) noexcept = default;

    /// Move assignment operator
    template <class U>
    Box<T>& operator=(Box<U>&& _other) noexcept {
      ptr_ = std::forward<std::unique_ptr<U>>(_other.ptr());
      return *this;
    }

    /// Returns the underlying object.
    T& operator*() { return *ptr_; }

    /// Returns the underlying object.
    T& operator*() const { return *ptr_; }

    /// Returns the underlying object.
    T* operator->() { return ptr_.get(); }

    /// Returns the underlying object.
    T* operator->() const { return ptr_.get(); }

    /// Returns the underlying unique_ptr
    std::unique_ptr<T>& ptr() { return ptr_; }

    /// Returns the underlying unique_ptr
    const std::unique_ptr<T>& ptr() const { return ptr_; }

   private:
    /// Only make is allowed to use this constructor.
    explicit Box(std::unique_ptr<T>&& _ptr) : ptr_(std::move(_ptr)) {}

   private:
    /// The underlying unique_ptr_
    std::unique_ptr<T> ptr_;
  };

  /// Generates a new Ref<T>.
  template <class T, class... Args>
  auto make_box(Args&&... _args) {
    return Box<T>::make(std::forward<Args>(_args)...);
  }

  template <class T1, class T2>
  inline auto operator<=>(const Box<T1>& _b1, const Box<T2>& _b2) {
    return _b1.ptr() <=> _b2.ptr();
  }

  template <class CharT, class Traits, class T>
  inline std::basic_ostream<CharT, Traits>&
  operator<<(std::basic_ostream<CharT, Traits>& _os, const Box<T>& _b) {
    _os << _b.get();
    return _os;
  }

} // namespace rfl

namespace std {

  template <class T>
  struct hash<rfl::Box<T>> {
    size_t operator()(const rfl::Box<T>& _b) const { return hash<unique_ptr<T>>()(_b.ptr()); }
  };

  template <class T>
  inline void swap(rfl::Box<T>& _b1, rfl::Box<T>& _b2) {
    return swap(_b1.ptr(), _b2.ptr());
  }

} // namespace std

#endif