Read JSON with RapidJSON

Question

Using RapidJSON, I can read a local json file by:

std::ifstream ifs { R"(/home//am_v2.json)" };
IStreamWrapper isw { ifs };
Document doc {};
doc.ParseStream( isw );

However, I want to read a variable received from:

auto msg = mqttClient->consume_message();

The content is shown by msg->to.string()

However, I cannot get this content to be read using:

std::ifstream ifs { msg }; 

Any clue?

Here a more complete code:

void MqttApplication::send()
{
    
    try {
        
        mqttClient->start_consuming();
        mqttClient->subscribe(TOPIC, QOS)->wait();        
    }
    catch (const mqtt::exception& exc) {
        cerr << exc.what() << endl;
        return;
    }
    
    while (true) {
        auto msg = mqttClient->consume_message();
        

        if (!msg) {
            if (!mqttClient->is_connected()) {
                cout << "MQTT: mqtt_application lost connection. Attempting reconnect" << endl;
                if (mqttTryReconnect(*mqttClient)) {
                    mqttClient->subscribe(TOPIC, QOS);
                    cout << "MQTT: mqtt_application reconnected" << endl;
                    continue;
                }
                else {
                    cout << "MQTT: mqtt_application reconnect failed." << endl;
                }
            }
            else {
                cout << "MQTT: mqtt_application an error occurred retrieving messages." << endl;
            }
            break;
        }
        

        if (msg->get_topic() == "command" &&
                msg->to_string() == "exit") {
            cout << "Exit command received" << endl;
            break;
        }
        
        cout << msg->get_topic() << ": " << msg->to_string() << endl;
        Document doc;
        doc.Parse(msg->to.string());
    }
}

Thanks

Answer 1

First, make sure you

#define RAPIDJSON_HAS_STDSTRING 1

before you include the Rapidjson headers.

Parsing the contents of a message from mqtt is then as simple as:

Document doc;
doc.Parse(msg->to.string());