Mqtt use ip in topic - error


I want to send data from multiple esp units over mqtt
to make this "intelligent" i want to include the ip of the sender.
With some copy paste work i can send now the ip as message.
But i want it to be part of the topic.
When i add the ip with + to the topic i get the error below
I am really no pro in programming, i tried allmost all google solutions like string(ip) and so on but that creates only more new errors.

invalid operands of types 'const char [10]' and 'char [(<anonymous> + 1)]' to binary 'operator+'

my sketch is

 Basic ESP8266 MQTT example

 This sketch demonstrates the capabilities of the pubsub library in combination
 with the ESP8266 board/library.

 It connects to an MQTT server then:
  - publishes "hello world" to the topic "outTopic" every two seconds
  - subscribes to the topic "inTopic", printing out any messages
    it receives. NB - it assumes the received payloads are strings not binary
  - If the first character of the topic "inTopic" is an 1, switch ON the ESP Led,
    else switch it off

 It will reconnect to the server if the connection is lost using a blocking
 reconnect function. See the 'mqtt_reconnect_nonblocking' example for how to
 achieve the same result without blocking the main loop.

 To install the ESP8266 board, (using Arduino 1.6.4+):
  - Add the following 3rd party board manager under "File -> Preferences -> Additional Boards Manager URLs":
  - Open the "Tools -> Board -> Board Manager" and click install for the ESP8266"
  - Select your ESP8266 in "Tools -> Board"


#include <ESP8266WiFi.h>
#include <PubSubClient.h>

// Update these with values suitable for your network.

const char* ssid = "Wifi";
const char* password = "xxxxxxxxxx";
const char* mqtt_server = "";

WiFiClient espClient;
PubSubClient client(espClient);
long lastMsg = 0;
char msg[50];
int value = 0;

void setup_wifi() {

  // We start by connecting to a WiFi network
  Serial.print("Connecting to ");

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {


  Serial.println("WiFi connected");
  Serial.println("IP address: ");

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print("] ");
  for (int i = 0; i < length; i++) {

  // Switch on the LED if an 1 was received as first character
  if ((char)payload[0] == '1') {
    digitalWrite(BUILTIN_LED, LOW);   // Turn the LED on (Note that LOW is the voltage level
    // but actually the LED is on; this is because
    // it is active low on the ESP-01)
  } else {
    digitalWrite(BUILTIN_LED, HIGH);  // Turn the LED off by making the voltage HIGH


void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Create a random client ID
    String clientId = "ESP8266Client-";
    clientId += String(random(0xffff), HEX);
    // Attempt to connect
    if (client.connect(clientId.c_str())) {
      // Once connected, publish an announcement...
      client.publish("outTopic", "hello world");
      // ... and resubscribe
    } else {
      Serial.print("failed, rc=");
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying

void setup() {
  pinMode(BUILTIN_LED, OUTPUT);     // Initialize the BUILTIN_LED pin as an output
  client.setServer(mqtt_server, 1883);

void loop() {

  if (!client.connected()) {

String ipaddress = WiFi.localIP().toString();
char ipchar[ipaddress.length()+1];

  long now = millis();
  if (now - lastMsg > 2000) {
    lastMsg = now;
    snprintf (msg, 50, "hello world #%ld", value);
    Serial.print("Publish message: ");
    client.publish("outTopic/"+ipchar, msg );

I make a unique client ID by using a few digits of the mac address to create source message identifiers.

void connectToMQTT()
  byte mac[5]; // create client ID from mac address
  WiFi.macAddress(mac); // get mac address
  String clientID = String(mac[0]) + String(mac[4]) ; // use mac address to create clientID
  while ( !MQTTclient.connected() )
    MQTTclient.connect( clientID.c_str(), mqtt_username, mqtt_password );
    vTaskDelay( 250 );
  MQTTclient.setCallback( mqttCallback );
  MQTTclient.subscribe  ( topicOK );
  MQTTclient.subscribe  ( topicRemainingMoisture_0 );
  MQTTclient.subscribe  ( topicWindSpeed );
  MQTTclient.subscribe  ( topicWindDirection );
  MQTTclient.subscribe  ( topicRainfall );
  MQTTclient.subscribe  ( topicWSVolts );
  MQTTclient.subscribe  ( topicWSCurrent );
  MQTTclient.subscribe  ( topicWSPower );
  MQTTclient.subscribe  ( topicDPnWI );
} //void connectToMQTT()

@rogierv, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advise on) your project :wink: See About the Installation & Troubleshooting category.

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