value_or.hpp

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//          Copyright Dominic Koepke 2019 - 2023.
// Distributed under the Boost Software License, Version 1.0.
//    (See accompanying file LICENSE_1_0.txt or copy at
//          https://www.boost.org/LICENSE_1_0.txt)
 
#ifndef SL_UTILITY_NULLABLES_VALUE_OR_HPP
#define SL_UTILITY_NULLABLES_VALUE_OR_HPP
 
#pragma once
 
#include "Simple-Utility/functional/bind_back.hpp"
#include "Simple-Utility/nullables/base.hpp"
 
namespace sl::nullables::detail
{
    template <input_nullable TInputNullable, std::convertible_to<value_t<TInputNullable>> TAlternative>
    [[nodiscard]]
    constexpr value_t<TInputNullable> value_or(TInputNullable&& inputNullable, TAlternative&& alternative)
    {
        if (inputNullable != null_v<TInputNullable>)
        {
            return value_t<TInputNullable>{unwrap(std::forward<TInputNullable>(inputNullable))};
        }
        return value_t<TInputNullable>{std::forward<TAlternative>(alternative)};
    }
 
    class value_or_caller_fn
    {
    public:
        template <input_nullable TInputNullable, std::convertible_to<value_t<TInputNullable>> TAlternative>
        [[nodiscard]]
        constexpr value_t<TInputNullable> operator ()(
            TInputNullable&& inputNullable,
            TAlternative&& alternative
        ) const
            noexcept(noexcept(value_or(std::declval<TInputNullable>(), std::declval<TAlternative>())))
        {
            return value_or(
                std::forward<TInputNullable>(inputNullable),
                std::forward<TAlternative>(alternative)
            );
        }
    };
}
 
namespace sl::nullables
{
    inline constexpr auto value_or = []<class Value>(
        Value&& value
    ) noexcept(std::is_nothrow_constructible_v<std::remove_cvref_t<Value>, Value>)
    {
        return Algorithm{functional::bind_back(detail::value_or_caller_fn{}, std::forward<Value>(value))};
    };
 
}
 
#endif