SerializationTraits.h

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/*
 * MIT License
 *
 * Copyright (c) 2025 nfx
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
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 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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 * SOFTWARE.
 */

 
#pragma once
 
#include <nfx/json/Builder.h>
#include <nfx/json/Document.h>
 
#include <stdexcept>
#include <variant>
#include <string>
#include <type_traits>
#include <vector>
 
using namespace nfx::json;
 
namespace nfx::serialization::json
{
    //=====================================================================
    // Forward declarations
    //=====================================================================
 
    template <typename T>
    class Serializer;
 
    template <typename T>
    struct SerializationTraits;
 
    //=====================================================================
    // SFINAE detectors
    //=====================================================================
 
    namespace detail
    {
        template <typename T, typename = void>
        struct has_streaming_serialization : std::false_type
        {
        };

        template <typename T>
        struct has_streaming_serialization<
            T,
            std::void_t<decltype( SerializationTraits<T>::serialize(
                std::declval<const T&>(), std::declval<nfx::json::Builder&>() ) )>> : std::true_type
        {
        };

        template <typename T>
        inline constexpr bool has_streaming_serialization_v = has_streaming_serialization<T>::value;

        template <typename T, typename = void>
        struct has_factory_deserialization : std::false_type
        {
        };

        template <typename T>
        struct has_factory_deserialization<
            T,
            std::void_t<decltype( SerializationTraits<T>::fromDocument( std::declval<const Document&>() ) )>>
            : std::true_type
        {
        };

        template <typename T>
        inline constexpr bool has_factory_deserialization_v = has_factory_deserialization<T>::value;
    } // namespace detail
 
    //=====================================================================
    // SerializationTraits - Read/Write serialization interface
    //=====================================================================

    template <typename T>
    struct SerializationTraits
    {
        static void fromDocument( const Document& doc, T& obj )
        {
            Serializer<T> serializer;
            obj.fromDocument( doc, serializer );
        }
 
        // No default implementation for serialize() - must be specialized if needed
        // SFINAE detector will check if serialize() is available
    };

    template <>
    struct SerializationTraits<std::monostate>
    {
        static void serialize( const std::monostate&, Builder& builder )
        {
            builder.write( nullptr );
        }

        static void fromDocument( const Document& doc, std::monostate& )
        {
            // monostate is always empty - no data to deserialize
            // Just verify the JSON is null
            if( !doc.isNull( "" ) )
            {
                throw std::runtime_error{ "Expected null for std::monostate" };
            }
        }
    };
} // namespace nfx::serialization::json