Create a New Object in a Function - C++

Question

I'm using a library for an MQTT connection and I would like to make a function that creates the object instead of setting it up in the pre-processor. The reason is because I need to grab which server it connects to after the program starts running.

Here is my mqttConnection function:

void mqttConnection(bool checkConnection)   {

if (!checkConnection)   {
    strcpy(mqttServer, mqttUrl);

    PubSubClient client(mqttServer, 1883, callback, wifiClient); // why would it care if this was here?
}

// Connecting to MQTT broker, loop until connected
while (!client.connected()){
    if(!client.connect("device-name")) {
        Serial.println("MQTT Broker Connection Failed. Trying Again...");
    }
    else {
        Serial.println("MQTT Broker Connection Success!");
        if(client.subscribe(mqttTopic)) {
            Serial.println("Subscription Successful! Woot!");
        }
    }
}

}

Here is the Class:

#ifndef PubSubClient_h
#define PubSubClient_h

#include <Arduino.h>
#include "Client.h"

// MQTT_MAX_PACKET_SIZE : Maximum packet size
#define MQTT_MAX_PACKET_SIZE 128

// MQTT_KEEPALIVE : keepAlive interval in Seconds
#define MQTT_KEEPALIVE 15

#define MQTTPROTOCOLVERSION 3
#define MQTTCONNECT     1 << 4  // Client request to connect to Server
#define MQTTCONNACK     2 << 4  // Connect Acknowledgment
#define MQTTPUBLISH     3 << 4  // Publish message
#define MQTTPUBACK      4 << 4  // Publish Acknowledgment
#define MQTTPUBREC      5 << 4  // Publish Received (assured delivery part 1)
#define MQTTPUBREL      6 << 4  // Publish Release (assured delivery part 2)
#define MQTTPUBCOMP     7 << 4  // Publish Complete (assured delivery part 3)
#define MQTTSUBSCRIBE   8 << 4  // Client Subscribe request
#define MQTTSUBACK      9 << 4  // Subscribe Acknowledgment
#define MQTTUNSUBSCRIBE 10 << 4 // Client Unsubscribe request
#define MQTTUNSUBACK    11 << 4 // Unsubscribe Acknowledgment
#define MQTTPINGREQ     12 << 4 // PING Request
#define MQTTPINGRESP    13 << 4 // PING Response
#define MQTTDISCONNECT  14 << 4 // Client is Disconnecting
#define MQTTReserved    15 << 4 // Reserved

#define MQTTQOS0        (0 << 1)
#define MQTTQOS1        (1 << 1)
#define MQTTQOS2        (2 << 1)

class PubSubClient {
private:
   Client* _client;
   uint8_t buffer[MQTT_MAX_PACKET_SIZE];
   uint16_t nextMsgId;
   unsigned long lastOutActivity;
   unsigned long lastInActivity;
   bool pingOutstanding;
   void (*callback)(char*,uint8_t*,unsigned int);
   uint16_t readPacket();
   uint8_t readByte();
   boolean write(uint8_t header, uint8_t* buf, uint16_t length);
   uint16_t writeString(char* string, uint8_t* buf, uint16_t pos);
   uint8_t *ip;
   char* domain;
   uint16_t port;
public:
   PubSubClient(Client& client);
   PubSubClient(uint8_t *, uint16_t, void(*)(char*,uint8_t*,unsigned int),Client& client);
   PubSubClient(char*, uint16_t, void(*)(char*,uint8_t*,unsigned int),Client& client);
   boolean connect(char *);
   boolean connect(char *, char *, char *);
   boolean connect(char *, char *, uint8_t, uint8_t, char *);
   boolean connect(char *, char *, char *, char *, uint8_t, uint8_t, char*);
   void disconnect();
   boolean publish(char *, char *);
   boolean publish(char *, uint8_t *, unsigned int);
   boolean publish(char *, uint8_t *, unsigned int, boolean);
   boolean publish_P(char *, uint8_t *, unsigned int, boolean);
   boolean subscribe(char *);
   boolean loop();
   boolean connected();
};


#endif

The error I get is,

'client' was not declared in this scope

Why does it matter that it's in the function?

Answer 1

PubSubClient client(mqttServer, 1883, callback, wifiClient);

Is inside an if statement scope. Because of the scope rules in C++ the variable can only be accessed until the closing brace of the if statement.

You can change

if (!checkConnection) {
    strcpy(mqttServer, mqttUrl);

    PubSubClient client(mqttServer, 1883, callback, wifiClient); // why would it care if this was here?
}

To

if (!checkConnection) {
    strcpy(mqttServer, mqttUrl);
}
PubSubClient client(mqttServer, 1883, callback, wifiClient);

Answer 2

In addition to the answers regarding the scope of client , if you use the latest version of the PubSubClient library, there are now functions for setting the server details after the client object has been created:

PubSubClient client(wifiClient);

// some time later
client.setServer(mqttServer,1883);

This makes it much easier to change the server details at runtime.

Full api documentation is available here: http://pubsubclient.knolleary.net/

Answer 3

The scope of client is limited to the if statement. You need to move its declaration outside of that scope, like this:

PubSubClient client(/*Some default arguments*/);

if (!checkConnection) {
   client = client(mqttServer, 1883, callback, wifiClient);
}

while (!client.connected()) {
/*...*/
}

Answer 4

i can connect with mQTT server and also i can subscribe from client. but how to get subscribed data from client

it shows 1

str=client.subscribe(testq);
   if(str>0)
    {

       Serial.println(str);
}

which means subscription successfull

but how to print subscribed data

KIndly Help me