C++: Reading a json object from file with nlohmann json

Question

I am using the nlohmann's json library to work with json objects in c++. Ultimately, I'd like to read a json object from a file, e.g. a simple object like this.

{
"happy": true,
"pi": 3.141
}

I'm not quite sure how to approach this. At https://github.com/nlohmann several ways are given to deserialise from a string literal, however it doesn't seem trivial to extend this to read in a file. Does anyone have experience with this?

Answer 1

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("{\"json\": true}");
json j = json::parse(ifs);

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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);

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Original answer for JSON for Modern C++ version 1

Use json::operator>>(std::istream&):

json j;
std::ifstream ifs("{\"json\": true}");
ifs >> j;

Answer 2

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);