Help with use of extern


I am missing something ?
Workbench, deviceOS1.2.1, Argon, local build

in .ino file (not inside any function) I define and declare

bool restartFlag = false;                             // used to signal a restart is needed
const int sizeOfMessageText = 60;
char tempMsg[sizeOfMessageText];

in UTILS.ccp file I define

extern bool restartFlag;
extern const int sizeOfMessageText;
extern char tempMsg[];

When I compile I get

.../src/UTILS.cpp:91: undefined reference to `sizeOfMessageText'

yet in the same UTILS.cpp I get no errors with restartFlag


The reason is that it’s a const.
Since a const in C++ defaults to internal linkage, you can’t just define a const in one file and reference it as an extern in another file. To give a const external linkage so it can be referenced from another file, you must explicitly define it as extern

extern const int sizeOfMessageText = 60;

Or you put your extern const in a header file which you also include in the source file where you actually define the variable creating something like this

// --- this would be the part included from the header file ---
extern const int sizeOfMessageText;
// ------------------------------------------------------------

// --- literal code in the source file ---
const int SizeOfMessageText = 60;

achieving the same as in the former one-liner.


I would also suggest you to use #define instead of the extern approach, and on top of that, since it’s a constant across your code, you may want to make the variable name all caps like MSG_TXT_SIZE, it’ll help you while debugging and you can know it’s constant just by looking at it.

other option is using “static const int”