Accessing json data from .qrc file using std::ifstream and nlohmann json

[shreya_agrawal]

I am trying to parse a json file, placed in the .qrc directory of my project. When I use QFile and QJsonDocument, and QJsonObject; it works fine. But, when I try to use std::ifstream and nlohmann::json, the file is not parsed.
Also, if::stream and nlohmann::json function parses the file if I give a hard coded path, instead of including the file in the .qrc folder of the project.
I am confident about the format of my json file.
So, is there any way to parse the json file, present in the .qrc directory using std::ifstream and nlohmann::json?

This is how I am parsing the json data:

std::ifstream file1(jsonFilePath1);
jsonObject1 = nlohmann::json::parse(file1);

This is the exception I am getting:
terminate called after throwing an instance of 'nlohmann::json_abi_v3_11_3::detail::type_error'
what(): [json.exception.type_error.304] cannot use at() with null

[Bonnie]

Qrc files are only accessible by Qt classes. How should ifstream know how to read a qrc file?
You need to read the file using Qt classes and pass the content data to other classes.

[Paul Colby]

Hi @shreya_agrawal,

As @Bonnie said, only QFIle knows how to access files embedded via the Qt Resource System, so your options are:

1. Use QFile::copy() to copy the file out of qrc, onto disk somewhere; or
2. Use QFile to read the contents of the file, then pass the contents to nlohmann::json::parse() (there's an overload the accepts a std::string::iterator, for example); or
3. If you really need to parse in a streaming mode, with nlohmann::json, then you'll need to use a QFile wrapper that implements std::istream; something like this: https://stackoverflow.com/questions/5204335/how-to-use-a-qfile-with-stdiostream#answers

Cheers.

[shreya_agrawal]

@Paul-Colby
Thank you so much for such a detailed solution!
I think I would go with option 3 in my case.