Is there a way to get a struct from a JSON without throwing an exception when failure？

[HEUBITLYJ]

for example，i have a struct like:

  struct UserInfo
    {
        std::string user_name;                                     
        std::string passwd;                                       
        UserRoles user_roles = UserManager::UserRoles::kOperator;  
        std::string create_time;
    };

when json content is not UserInfo, json.get<UserInfo>() will throw type_error exception,
how should i avoid use try catch to catch the throwed exception, just get the error message when failed to get?

[nlohmann]

Please provide a concrete example.

[HEUBITLYJ]

here i want to get user info from user_info.json：

 std::ifstream stream("user_info.json");
nlohmann::json json;
stream>> json;
UserInfo user_info = json.get<UserInfo>();

if someone accidentally modified the content of user_info.json ， json.get<UserInfo>() will failed and throw exception type_error, i must use try catch to catch the exception, is there any way to just get type_error(maybe some api return error when failed) without using tty catch .

[nlohmann]

I assume you somewhere define a mapping from UserInfo to nlohmann::json?

[HEUBITLYJ]

yes

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(UserInfo, user_name, passwd, user_roles, create_time);

[nlohmann]

OK. So by using this macro, you generate a function like

template<typename BasicJsonType>
void from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)
{
    person nlohmann_json_default_obj;
    nlohmann_json_t.name = nlohmann_json_j.value("name", nlohmann_json_default_obj.name);
    nlohmann_json_t.address = nlohmann_json_j.value("address", nlohmann_json_default_obj.address);
    nlohmann_json_t.age = nlohmann_json_j.value("age", nlohmann_json_default_obj.age);
}

(taken from https://json.nlohmann.me/api/macros/nlohmann_define_type_non_intrusive/#examples - the member names differ here)

The exception you see comes from the value function (https://json.nlohmann.me/api/basic_json/value/). If you want to have a different behavior, you have to implement the from_json function yourself.

[HEUBITLYJ]

ok，now i know how to avoid throw exception，thanks a lot