C++: Reading a json object from file with nlohmann json
Update 2017-07-03 for JSON for Modern C++ version 3
Since version 3.0, json::json(std::ifstream&) is deprecated. One should use json::parse() instead:
std::ifstream ifs("test.json");json jf = json::parse(ifs);std::string str(R"({"json": "beta"})");json js = json::parse(str);For more basic information on how to use nlohmann's json library, see nlohmann FAQ.
Update for JSON for Modern C++ version 2
Since version 2.0, json::operator>>() id deprecated. One should use json::json() instead:
std::ifstream ifs("{\"json\": true}");json j(ifs);Original answer for JSON for Modern C++ version 1
Use json::operator>>(std::istream&):
json j;std::stringstream ifs("{\"json\": true}");ifs >> j;
The constructor json j(ifs) is deprecated and will be removed in version 3.0.0. Since version 2.0.3 you should write:
std::ifstream ifs("test.json");json j = json::parse(ifs);