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::istringstream 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;