const char *jsstr = R"({"a":true, "b":null})";
flatjson::fjson json{jsstr};
assert(json.is_valid());
assert(json.is_object());
assert(json.size() == 2);

auto a = json.at("a");
assert(a.is_bool());
assert(a.to_bool() == true);

auto b = json.at("b");
assert(b.is_null());