optional.h

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#ifndef META_OPTIONAL_H_
#define META_OPTIONAL_H_

#include <stdexcept>
#include <type_traits>
#include "util/comparable.h"

namespace meta
{
namespace util
{

constexpr struct trivial_init_t
{
} trivial_init{};

struct nullopt_t
{
    struct init
    {
    };
    constexpr nullopt_t(init) {};
};
constexpr nullopt_t nullopt{nullopt_t::init{}};

struct optional_dummy_t
{
};

template <class T>
union optional_storage
{
    optional_dummy_t dummy_;

    T value_;

    optional_storage(trivial_init_t);

    template <class... Args>
    optional_storage(Args&&... args);

    ~optional_storage()
    {
        /* nothing */
    }
};

template <class T>
class optional : public util::comparable<optional<T>>
{
  public:
    optional();

    optional(nullopt_t);

    optional(const T& value);

    optional(T&& value);

    optional(const optional&);

    optional(optional&&);

    optional& operator=(optional);

    ~optional();

    void swap(optional& other);

    const T& operator*() const;

    T& operator*();

    const T* operator->() const;

    T* operator->();

    explicit operator bool() const;

    void clear();

  private:
    const T* dataptr() const;

    T* dataptr();

    bool initialized_;

    optional_storage<T> storage_;
};

template <class T>
bool operator<(const optional<T>& lhs, const optional<T>& rhs);

class bad_optional_access : public std::runtime_error
{
  public:
    using std::runtime_error::runtime_error;
};
}
}

#include "util/optional.tcc"
#endif