Return json in response of apis ?

[Shravan40]

I am using nlohmann json library in my crow project. Below is my sample POST method.

// Add two numbers
    
    CROW_ROUTE(app,"/add")
            .methods("POST"_method)
    ([](const crow::request& req) {
        auto x = json::parse(req.body);
        if (x == nullptr)
            return crow::response(400);
        auto sum = x["a"].get<long double>() + x["b"].get<long double>();
        bool status = true;
        string message = "It doesn't matter, right ?";
        json return_json = {
            {"status", status},
            {"message", message},
            {"sum", sum},
        };
        return crow::response(return_json);
    });

2nd last line of my code produces error because error: call of overloaded ‘response(json&)’ is ambiguous.

When I replace it with return crow::response(return_json.dump()); it works perfectly, because this converts the json to string type. I would like to know, how can I return the actual json in response of my apis.

[moneroexamples]

There are few ways you can do it. Simples is to just dump it as a string: return crow::response(return_json.dump());

You can also define your own respnse:

struct jsonresponse: crow::response
{
    jsonresponse(const nlohmann::json& _body)
            : crow::response {_body.dump()}
    {
        add_header("Access-Control-Allow-Origin", "*");
        add_header("Access-Control-Allow-Headers", "Content-Type");
        add_header("Content-Type", "application/json");
    }
};
}

And then return it.

I'm using the same json library as you. You can have a look how I do it: https://github.com/moneroexamples/onion-monero-blockchain-explorer/blob/master/main.cpp Might find something that will help you.

[Shravan40]

@moneroexamples :I am trying to return json response the way you have suggested. But I am not able to return an object as response. Here is the error I am receiving

static assertion failed: could not find from_json() method in T's namespace

Do you have any idea, how it can be resolved ?

[mrozigor]

Can you show code sample? And maybe full error text?

[Shravan40]

@mrozigor : Sure
Here are the sample block of code

namespace changestreet {
    struct jsonresponse: crow::response {
        jsonresponse(const nlohmann::json& _body): crow::response {_body.dump()} {
            add_header("Access-Control-Allow-Origin", "*");
            add_header("Access-Control-Allow-Headers", "Content-Type");
            add_header("Content-Type", "application/json");
        }
    };
}

class Employer_access_detail {
public:
        size_t user_id = 0;
        string auth_token = "", message;
};

CROW_ROUTE(app,"/v1/login")
        .methods("POST"_method)
([](const crow::request& req) {
        // login do not require any header verification
        json input_json = json::parse(req.body);
        Employer_access_detail login_output = login(input_json);
        changestreet::jsonresponse return_json{login_output};
        return return_json;
});

And Error is coming in the 3rd last line of above code, where I am trying to convert UDT to json.

[mrozigor]

Based on other responses in nlohmann's JSON library you should implement method 'from_json' outside class:

void from_json(const nlohmann::json& j, Employer_access_detail& d) {
    d.user_id = j.at("user_id");
    d.auth_token = j.at("auth_token");
}

[Shravan40]

Sadly nlohmann json library doesn't support user defined datatype of json. One can read more about it here