Char initialized as concatenation of String +String variables generates error

Projects Programming
1

I am trying to insert a string into a char with concatenation and not getting any love from the Arduino compiler. It is over my head. Here is the code:

String strClient_id = "ESP8266-"+WiFi.macAddress().substring(9,11)
                                +WiFi.macAddress().substring(12,14)
                                +WiFi.macAddress().substring(15);


char mqtt_door_open_topic[60] = "stat/"+strClient_id.c_str()+"/garage/door1/reedswitch";

Here is the error:

 error: invalid operands of types 'const char [6]' and 'const char*' to binary 'operator+'

 char mqtt_door_open_topic[60] = "stat/"+strClient_id.c_str()+"/garage/door1/reedswitch";

                                                            ^

I've search and tried several variations, but I just end up generating different errors. For instance, if I just use strClient_id instead of strClient_id.c_str(), I get this error:

error: array must be initialized with a brace-enclosed initializer

 char mqtt_door_open_topic[60] = "stat/"+strClient_id+"/garage/door1/reedswitch";

                                                      ^

Any help appreciated.

2 
char mqtt_door_open_topic[60]

doesn't define a String object but a character array (also called C string sometimes). On a character array you cannot use the "+" operator but you have to use the corresponding functions to manipulate it. Concatenating is done by the strcat function, you should use the strncat version.

4

@pylon, I will go research the strncat() function now.

@Delta_G, how would you accomplish the above using only char?

6

JSCSJSCS:
@Delta_G, how would you accomplish the above using only char?

With strncat() and maybe some of the other c string functions (you'll find them when you look up strncat() )

7

Thank you all, I will go research char data type concatenation.

8

I was not able to get done what I wanted with strcat or strncat. I could not find a way to concatenate char and Strings using these functions because they require the source and destination to be of type char and I could not figure out how to change a char to a String or vice versa. Very confusing. Then I found the toCharArray() method. It turned out to be the problem solver.

#include <ESP8266WiFi.h>

char client_id[15];

createClientId();

void createClientId(){
  strcat(client_id,"ESP8266-");
  char temp[3];
  WiFi.macAddress().substring(9,11).toCharArray(temp,3);
  strcat(client_id,temp);
  WiFi.macAddress().substring(12,14).toCharArray(temp,3);
  strcat(client_id,temp);
  WiFi.macAddress().substring(15).toCharArray(temp,3);
  strcat(client_id,temp);
}

Any suggestions to clean up/shorten, correct mistakes in this noobie code welcomed! Thanks again for the help everyone.

10

I can't do anything about that as far as I know. The wifi.macaddress() call returns a String. If Arduino had another library that give a wifi device's MAC address as a char array, I would love to hear about it.

11

I can't do anything about that as far as I know. The wifi.macaddress() call returns a String. If Arduino had another library that give a wifi device's MAC address as a char array, I would love to hear about it.

To be exact: The Arduino WiFi class doesn't have the macaddress() method so don't blame Arduino for someone else's fail.

Using the String class on the ESP8266 may be less of a problem than on the Arduino because the ESP8266 has much more RAM so wasting a few bytes for that class may be acceptable.

12

@pylon, Thank you for the clarification. I looked into this further and you are correct. I am using the WiFi.macAddress() from ESP8266WiFi.h.

Arduino does have it's own WiFi.macAddress():

Arduino's WiFi.macAddress() returns a byte array however (better than a string as you say), while ESP8266WiFi.h returns a string.

I have edited my previous post's code to make this clear.

13

ESP8266WiFi.h returns a string.

Do you mean a string or a String ?

14

I meant String. From ESP8266WiFi.cpp

String ESP8266WiFiSTAClass::macAddress(void) {
    uint8_t mac[6];
    char macStr[18] = { 0 };
    wifi_get_macaddr(STATION_IF, mac);

    sprintf(macStr, "%02X:%02X:%02X:%02X:%02X:%02X", mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
    return String(macStr);
}