How to use "+" character as a String

Question

Well I am trying to save phone number as a string. For ease of use I want to add "+" character programmatically. Like this:

String cepNo ="+" + "901234567";

But program gives this error.

exit status 1
invalid operands of types 'const char [2]' and 'const char [13]' to binary 'operator+'

How to add "+" beginning of the string.

Answer 1

You cannot add C strings (i.e. NUL-terminated arrays of char) with the + operator. It only works with String objects. You can, though, add a String object and a C-string, because in this case the compiler implicitly converts the C string into a String object:

String phone_number = "901234567";
phone_number = "+" + phone_number;

The last line is interpreted by the compiler as

phone_number = String("+") + phone_number;

That being said, I encourage you to try to avoid String objects if possible. These use dynamic memory allocation which carries the risk of heap fragmentation. The same thing can be achieved with plain C strings:

char phone_number[] = "901234567";
char full_phone_number[strlen(phone_number) + 2];
strcpy(full_phone_number, "+");
strcat(full_phone_number, phone_number);

Note the size of the second string: strlen(phone_number) + 2. You need one extra char for the + sign, and another one for the NUL string terminator.

Now, with C strings you can do better, and avoid copying: reserve enough room in an array for the full number (“+” included), and then fill the digits starting from the second slot in the char array. You can then add the + sign later:

char phone_number[16];
strcpy(phone_number + 1, "901234567");
phone_number[0] = '+';

Although I personally would write the “+” sign and a NUL while initializing the array:

char phone_number[16] = "+";
strcat(phone_number, "901234567");

Answer 2

use String cepNo = String("+") + "901234567"; or String a = "+"; String cepNo = a + "901234567";