How to parse a possibly null json string in C++! need help!!!!!!!!!

[aijyo]

Just as the title, server give me a JSON string maybe a value like:
{"label": null}

My C++ struct :
class LabelItem
{
public：
std::string label;
// etc ...

NLOHMANN_DEFINE_TYPE_INTRUSIVE(LabelItem, label);
}

when I parse data from server：
LabelItem item;
from_json(message, item);

My program crashed!
I've tried wrapping my string to handle null, but that failed。code is:
class JsonString
{
public:
JsonString(const std::string& data)
: value(data)
{

	}
	JsonString& operator=(const std::string& val) 
	{
		this->value = val;
		return *this;
	}

	friend void to_json(nlohmann::json& j, const JsonString& data)
	{
		j = data.value;
	}

	friend void from_json(const nlohmann::json& j, JsonString& data)
	{
		if (!j.is_null())
		{
			data.value = j;
		}
	}

	std::string value;
};

error is :
1>xxx-proto.h(189,1): error C2672: “nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to”: 未找到匹配的重载函数
1>xxx-proto.h(189,1): error C2783: “ValueType &nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to(ValueType &) noexcept() const”: 未能为“__formal”推导 模板 参数

Can anyone help me?

[gregmarr]

NLOHMANN_DEFINE_TYPE_INTRUSIVE creates functions that requires that all the values are there in the JSON and have the proper types. If you want it to be able to use a default value when the value isn't in the JSON, you can use NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT instead. However, I'm pretty sure that won't handle the null case, and you'll need to write your own from_json function for that.

[aijyo]

After my define my own type to process null string， nested struct cannot pass compile：
JsonString is:
class JsonString
{
public:
JsonString(const std::string& data)
: value(data)
{

	}
	JsonString& operator=(const std::string& val) 
	{
		this->value = val;
		return *this;
	}

	friend void to_json(nlohmann::json& j, const JsonString& data)
	{
		j = data.value;
	}

	friend void from_json(const nlohmann::json& j, JsonString& data)
	{
		if (!j.is_null())
		{
			data.value = j;
		}
	}

	std::string value;
};

Nested class:
class SubscribeRequest
{
public:
class SubPublisherItem
{
public:
class SubOptions
{
public:
class SubSimulcast
{
public:
JsonString profile;

			public:
				NLOHMANN_DEFINE_TYPE_INTRUSIVE(SubSimulcast, profile);
			};

		public:
			SubSimulcast simulcast;

		public:
			NLOHMANN_DEFINE_TYPE_INTRUSIVE(SubOptions, simulcast);
		};
	public:
		JsonString sessionId;
		JsonString userId;
		SubOptions options;

	public:
		NLOHMANN_DEFINE_TYPE_INTRUSIVE(SubPublisherItem, sessionId, userId);
	};

public:
	JsonString event = SUBSCRIBE_MSG;
	JsonString roomToken;
	JsonString userId;
	JsonString subscriberType;
	JsonString txid;
	std::vector<SubPublisherItem> publishers;
public:
	NLOHMANN_DEFINE_TYPE_INTRUSIVE(SubscribeRequest, event, roomToken, userId
		, subscriberType, publishers, txid);
};

error:
严重性 代码 说明 项目 文件 行 禁止显示状态
错误 C2672 “nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to”: 未找到匹配的重载函数 xxx xxx-proto.h 417
错误 C2783 “ValueType &nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>>::get_to(ValueType &) noexcept() const”: 未能为“__formal”推导 模板 参数 xxx xxx-proto.h 417

[gregmarr]

I'm not sure why it doesn't pick up the from_json in the JsonString for the SubPublisherItem, but I wasn't suggesting you create a wrapper type for std::string. I was suggesting that you implement the from_json in the container classes, as the method you're trying won't handle a missing value. With the latest version you can use NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE to do the to_json part.

Having said that, using NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT in the containing classes works once you add a default constructor to JsonString. https://www.godbolt.org/z/W61P8jfeE

[aijyo]

Thanks for your help，I decided to implement from_json myself, but I think it is also necessary to leave a method that can convert null to empty string, because there are really many places where this string needs to be used, and I cannot ask the server to modify the code. Others may also want this situation。