multi_stage_value.hpp

// Copyright (C) 2022-2023 by Camden Mannett.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "arg_router/policy/policy.hpp"
 
#include <functional>
#include <optional>
 
namespace arg_router::policy
{
template <typename ResultType, typename ValueType>
class multi_stage_value
{
public:
    using merge_fn = std::function<void(std::optional<ResultType>&, ValueType&&)>;
 
    constexpr explicit multi_stage_value(merge_fn fn) noexcept : fn_{std::move(fn)} {}
 
    constexpr void merge(std::optional<ResultType>& result, ValueType&& value) const
    {
        fn_(result, std::forward<ValueType>(value));
    }
 
private:
    merge_fn fn_;
};
 
template <typename ResultType, typename ValueType>
struct is_policy<multi_stage_value<ResultType, ValueType>> : std::true_type {
};
 
template <typename T>
struct has_multi_stage_value {
private:
    template <typename... Ts>
    constexpr static std::true_type test(const multi_stage_value<Ts...>*);
 
    constexpr static std::false_type test(...);
 
public:
    // NOLINTNEXTLINE(hicpp-vararg)
    constexpr static bool value = decltype(test(std::declval<T*>()))::value;
};
 
template <typename T>
constexpr bool has_multi_stage_value_v = has_multi_stage_value<T>::value;
}  // namespace arg_router::policy