libcryfs/either.h

#pragma once
#ifndef MESSMER_CPP_UTILS_EITHER_H
#define MESSMER_CPP_UTILS_EITHER_H

#include <boost/optional.hpp>

namespace cpputils {

    template<class Left, class Right>
    class Either final {
    public:
        //TODO Try allowing construction with any type that std::is_convertible to Left or Right.
        Either(const Left &left): _side(Side::left) {
            _construct_left(left);
        }
        Either(Left &&left): _side(Side::left) {
            _construct_left(std::move(left));
        }
        Either(const Right &right): _side(Side::right) {
            _construct_right(right);
        }
        Either(Right &&right): _side(Side::right) {
            _construct_right(std::move(right));
        }
        //TODO Try allowing copy-construction when Left/Right types are std::is_convertible
        Either(const Either<Left, Right> &rhs): _side(rhs._side) {
            if(_side == Side::left) {
                _construct_left(rhs._left);
            } else {
                _construct_right(rhs._right);
            }
        }
        Either(Either<Left, Right> &&rhs): _side(rhs._side) {
            if(_side == Side::left) {
                _construct_left(std::move(rhs._left));
            } else {
                _construct_right(std::move(rhs._right));
            }
        }

        ~Either() {
            _destruct();
        }

        //TODO Try allowing copy-assignment when Left/Right types are std::is_convertible
        Either<Left, Right> &operator=(const Either<Left, Right> &rhs) {
            _destruct();
            _side = rhs._side;
            if (_side == Side::left) {
                _construct_left(rhs._left);
            } else {
                _construct_right(rhs._right);
            }
            return *this;
        }

        Either<Left, Right> &operator=(Either<Left, Right> &&rhs) {
            _destruct();
            _side = rhs._side;
            if (_side == Side::left) {
                _construct_left(std::move(rhs._left));
            } else {
                _construct_right(std::move(rhs._right));
            }
            return *this;
        }

        //TODO fold, map_left, map_right, left_or_else(val), right_or_else(val), left_or_else(func), right_or_else(func)

        bool is_left() const {
            return _side == Side::left;
        }

        bool is_right() const {
            return _side == Side::right;
        }

        const Left &left() const {
            return _left;
        }
        Left &left() {
            return const_cast<Left&>(const_cast<const Either<Left, Right>*>(this)->left());
        }

        const Right &right() const {
            return _right;
        }
        Right &right() {
            return const_cast<Right&>(const_cast<const Either<Left, Right>*>(this)->right());
        }

        boost::optional<const Left&> left_opt() const {
            if (_side == Side::left) {
                return _left;
            } else {
                return boost::none;
            }
        }
        boost::optional<Left&> left_opt() {
            if (_side == Side::left) {
                return _left;
            } else {
                return boost::none;
            }
        }

        boost::optional<const Right&> right_opt() const {
            if (_side == Side::right) {
                return _right;
            } else {
                return boost::none;
            }
        }
        boost::optional<Right&> right_opt() {
            if (_side == Side::right) {
                return _right;
            } else {
                return boost::none;
            }
        }

    private:
        union {
            Left _left;
            Right _right;
        };
        enum class Side : unsigned char {left, right} _side;

        void _construct_left(const Left &left) {
            new(&_left)Left(left);
        }
        void _construct_left(Left &&left) {
            new(&_left)Left(std::move(left));
        }
        void _construct_right(const Right &right) {
            new(&_right)Right(right);
        }
        void _construct_right(Right &&right) {
            new(&_right)Right(std::move(right));
        }
        void _destruct() {
            if (_side == Side::left) {
                _left.~Left();
            } else {
                _right.~Right();
            }
        }
    };

    template<class Left, class Right>
    bool operator==(const Either<Left, Right> &lhs, const Either<Left, Right> &rhs) {
        if (lhs.is_left() != rhs.is_left()) {
            return false;
        }
        if (lhs.is_left()) {
            return lhs.left() == rhs.left();
        } else {
            return lhs.right() == rhs.right();
        }
    }

    template<class Left, class Right>
    bool operator!=(const Either<Left, Right> &lhs, const Either<Left, Right> &rhs) {
        return !operator==(lhs, rhs);
    }

    template<class Left, class Right>
    std::ostream &operator<<(std::ostream &stream, const Either<Left, Right> &value) {
        if (value.is_left()) {
            stream << "Left(" << value.left() << ")";
        } else {
            stream << "Right(" << value.right() << ")";
        }
        return stream;
    }

    //TODO Test make_either<>
    //TODO make_either<>
}


#endif