nlohmann json version 3.2.0, runtime error - json

Question

I have several json files from a game I am modding that I want to be able to process using an external application.
as far as I can tell this code is within the specifications but I get a runtime error:
terminate called after throwing an instance of 'nlohmann::detail::type_error'
what(): [json.exception.type_error.304] cannot use at() with string
this is the stripped code to reproduce the error:
#include "json.hpp"
using json = nlohmann::json;
using namespace std;
namespace ns
{
class info
{
public:
std::string id;
};
void to_json(json& j, const info& mi);
void from_json(const json& j, info& mi);
}
int main()
{
json j ="[{\"id\": \"identifier\"}]";
ns::info info = j;
return 0;
}
void ns::to_json(json& j, const ns::info& mi)
{
j = json{
{"id",mi.id},
};
}
void ns::from_json(const json& j, ns::info& mi)
{
mi.id = j.at("id").get<std::string>();
}
and here is the compiler output:
-------------- Build: Debug in jsontest (compiler: GNU GCC Compiler)---------------
mingw32-g++.exe -Wall -std=c++11 -fexceptions -g -c C:\Users\UserOne\Documents\c++\jsontest\main.cpp -o obj\Debug\main.o
mingw32-g++.exe -o bin\Debug\jsontest.exe obj\Debug\main.o
Output file is bin\Debug\jsontest.exe with size 2.82 MB
Process terminated with status 0 (0 minute(s), 3 second(s))
0 error(s), 0 warning(s) (0 minute(s), 3 second(s))

Answer 1

There are two problems:
json j = "..."; initializes j with a JSON string value. It doesn't try to parse the contents. For that, you need to make it a json literal instead: json j = "..."_json;
After fixing that, you have a JSON array and but you're trying to access a field of a JSON object in ns::from_json().
So, fix both of those:
json j ="{\"id\": \"identifier\"}"_json;
and it'll work.
You might also consider using raw strings to avoid escaping all the quotes:
json j = R"({"id": "identifier"})"_json;
or just using an initializer list instead of parsing a string of json:
json j = { {"id", "identifier" } };
And if your source is a string being provided as a function argument or whatever, instead of a literal known at compile time:
std::string s = R"({"id": "identifier"})";
json j = json::parse(s);