optional_ref.hpp

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/*
 * This file is part of Algorithms library toolkit.
 * Copyright (C) 2017 Jan Travnicek (jan.travnicek@fit.cvut.cz)
 
 * Algorithms library toolkit is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 
 * Algorithms library toolkit is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 
 * You should have received a copy of the GNU General Public License
 * along with Algorithms library toolkit.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#pragma once
 
#include <optional>
#include <memory>
#include <compare>
 
namespace ext {
 
template < class T >
class optional_ref {
    T * m_value;
 
public:
    optional_ref ( ) : optional_ref ( std::nullopt ) { }
    optional_ref ( std::nullopt_t ) : m_value ( nullptr ) { }
 
    optional_ref ( T & val ) : m_value ( std::addressof ( val ) ) {
    }
 
    template < class U >
    optional_ref ( const optional_ref < U > & other ) : optional_ref ( other.value ( ) ) {
    }
 
    template < class U >
    optional_ref ( optional_ref < U > && other ) : optional_ref ( std::move ( other ).value ( ) ) {
    }
 
    optional_ref & operator = ( const optional_ref & ) = delete;
    optional_ref & operator = ( optional_ref && ) = delete;
 
    optional_ref ( const optional_ref & other ) = default;
    optional_ref ( optional_ref && other ) noexcept = default;
 
    ~ optional_ref ( ) noexcept = default;
 
    const T * operator-> ( ) const {
        return m_value;
    }
 
    T * operator-> ( ) {
        return m_value;
    }
 
    const T & operator * ( ) const & {
        return value ( );
    }
 
    T & operator * ( ) & {
        return value ( );
    }
 
    const T && operator * ( ) const && {
        return std::move ( value ( ) );
    }
 
    T && operator * ( ) && {
        return std::move ( value ( ) );
    }
 
    explicit operator bool ( ) const noexcept {
        return has_value ( );
    }
 
    bool has_value ( ) const noexcept {
        return m_value != nullptr;
    }
 
    T & value ( ) & {
        return * m_value;
    }
 
    const T & value ( ) const & {
        return * m_value;
    }
 
    T && value ( ) && {
        return std::move ( * m_value );
    }
 
    const T && value ( ) const && {
        return std::move ( * m_value );
    }
 
    const T & value_or ( const T & default_value ) const & {
        if ( has_value ( ) )
            return value ( );
        else
            return default_value;
    }
 
    T & value_or ( T & default_value ) & {
        if ( has_value ( ) )
            return value ( );
        else
            return default_value;
    }
 
    T && value_or ( T && default_value ) && {
        if ( has_value ( ) )
            return std::move ( value ( ) );
        else
            return std::move ( default_value );
    }
 
    bool operator == ( const optional_ref < T >& rhs ) const {
        const optional_ref < T > lhs = * this;
        return lhs.has_value ( ) && rhs.has_value ( ) ? lhs.value ( ) == rhs.value ( ) : lhs.has_value ( ) == rhs.has_value ( );
    }
 
    auto operator <=> ( const optional_ref < T >& rhs ) const {
        const optional_ref < T > lhs = * this;
        return lhs.has_value ( ) && rhs.has_value ( ) ? lhs.value ( ) <=> rhs.value ( ) : lhs.has_value ( ) <=> rhs.has_value ( );
    }
 
};
 
template< class T >
std::ostream & operator << ( std::ostream & out, const ext::optional_ref < T > & optional ) {
    if ( ! optional )
        return out << "void";
    else
        return out << optional.value ( );
}
 
 
} /* namespace ext */