A newbie in Arduino- and I'm trying to concatenate string and chars ( well in python it is quite different ).

I'm trying to post a MQTT message- constructed following format to the client.publish(outTopic, "["+outTopic+"]"+ msg)

while outTopic is a char* variable, and msg is char- but I get an error for such concatenation.

please your help

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


// GPIOs on SONOFF board
int LED_SONOFF = 13;
int REL_SONOFF = 12;
int PIN_SW = 14;

const char* ssid = "HomeNetwork_2.4G";
const char* password = "pswd";
const char* mqtt_server = "192.168.2.113";

WiFiClient espClient;
PubSubClient client(espClient);


char msg[50];
const char* outTopic = "HomePi/Dvir/Messages";
const char* inTopic = "HomePi/Dvir/Windows/SONOFF";
const char* inTopic2 = "HomePi/Dvir/Windows/All";
const char* clientID = "Sonoff#1";
String m1, m2;

void start_wifi() {
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");  
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP()); 
}

void setup() {
  start_wifi(); // start wifi service
  Serial.begin(115200); // start serial service
  delay(10);

  // define sonoff gpios
  pinMode(LED_SONOFF, OUTPUT);
  pinMode(REL_SONOFF, OUTPUT);
  pinMode(PIN_SW, INPUT_PULLUP); // GPIO14 has a pullup resistor
  digitalWrite(LED_SONOFF,HIGH); // LED high is OFF
  digitalWrite(REL_SONOFF,LOW); 

  // start mqtt
  client.setServer(mqtt_server, 1883);
  client.setCallback(callback);  
}

void switch_on() {
  digitalWrite(REL_SONOFF,HIGH);
  digitalWrite(LED_SONOFF, LOW); // LED ON
  pub_mqtt("Switch ON");
}

void switch_off() {
  digitalWrite(REL_SONOFF,LOW);
  digitalWrite(LED_SONOFF, HIGH); // LED OFF
  pub_mqtt("Switch OFF");
}

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

  if ((char)payload[0] == '1') {
    switch_on();   
  } 
  if ((char)payload[0] == '0') {
    switch_off();
    Serial.print("payload: ");
    Serial.println(payload[0]);
  }

}

void reconnect() {
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    if (client.connect("ESP8266Client")) {
      Serial.println("connected");
      client.publish(outTopic, "Connected");
      
      client.subscribe(inTopic);
      m1="Subscribed to: "+inTopic;
      client.publish(outTopic, m1);
      if (inTopic2 !=""){
        client.subscribe(inTopic2);
        m2="Subscribed to: ";//+ String(inTopic2);
        client.publish(outTopic, m2);
      }
      
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      delay(5000);
    }
  }
}

void pub_mqtt(char* msg) {
  client.publish(outTopic, msg);
}

void loop() {
  if (!client.connected()) {
    reconnect();
  }
  client.loop();

  if (digitalRead(PIN_SW)==1) {
    if (digitalRead(REL_SONOFF)!=LOW) {
      switch_off();
    }
  }
  else if (digitalRead(PIN_SW)==0) {
    if (digitalRead(REL_SONOFF)!=HIGH){
      switch_on();
    }
  }
   delay(100);
}