Reading Json data from file using structure object.

[Jhonlothan]

Hi, I have some data like
[{"Name" : "jhon" , "age" : "13" , "section" : "A"},
{"Name" : "Larry" , "age" : "15" , "section" : "B"},
{"Name" : "narin" , "age" : "12" , "section" : "A"}]
.
.
.
.
Like this in my file.json file. How can I read this data using structure object?

[nlohmann]

This would be invalid JSON, as there is no surrounding array or commas between the objects, right?

[jhonabraham22]

See https://json.nlohmann.me/api/basic_json/parse/ (examples https://json.nlohmann.me/api/basic_json/parse/#examples)

Hi in my case file contains individual lines in json format, but the entire file is not in json format. This is incoming stream of data so I can't change the file format. How can I read each line from file and parse using json ? Can you please help.

{"Name" : "jhon" , "age" : "13" , "section" : "A"}
{"Name" : "Larry" , "age" : "15" , "section" : "B"}
{"Name" : "narin" , "age" : "12" , "section" : "A"}

[nlohmann]

I will add an example later.

[Jhonlothan]

It will be helpful for me also... Waiting for your example.

[PreetM07]

If your json object is j, then cout<<j[items][source][text]; will give the required data.

It gives type error while building.
and if used strings as j["items"]["source"]["text"]...
it gives following runtime error
terminate called after throwing an instance of 'nlohmann::detail::type_error'
what(): [json.exception.type_error.305] cannot use operator[] with a string argument with array

[nlohmann]

Here is a branch to add documentation and an example for JSON Lines (newline-separated JSON text): https://github.com/nlohmann/json/pull/3247/files. Please let me know if this is helpful or if you need more details.

[jhonabraham22]

Instead of taking input in code, Can you please directly read from a file. And can you please show how to access particular key value in any one of the json line?.

[Jhonlothan]

Hi,
I used j.value but it's throwing an error 302 type must be a string, but is null. I assigned default value is of same type as key value.
Here is my assignment

Obj.Classmember1 = std::string (j.value("city", "Texas")) ;

[nlohmann]

Then use contains and is_string.

[Jhonlothan]

Can you please show how contains and is_string is used for the following class member assignment

Obj.Classmember1 = std::string (j.value("city", "Texas")) ;

[nlohmann]

if (j.contains("city") && j["city"].is_string()) {
  Obj.Classmember1 = j["city"].get<std::string>();
}

[Jhonlothan]

Thank you very much.It worked . But i have lot of class members to assign. Do I need to use if loop for every class member?

[gregmarr]

This should only be required if you have nulls in your JSON data, or if you are using j["city"] on a non-const json object. This will create a city: null key/value pair. If you can figure out where that is happening and eliminate it, then you can use j.value("city", "Texas"). If your JSON data can have "city": null, then you have to go with contains() and is_string() but you can put that in a function and re-use it.

[Jhonlothan]

Hi, is there any way to convert nlohmann::basic_json<>::string_t to unit32_t?
I want to read data from json file and assign it to a struct member of type unit32_t.

[nlohmann]

No, you have to do this conversion yourself, e.g. via std:: stoul.