linker problems with extern const struct

Projects Programming
1

In a header file, x.h, I have struct Ff {int a; int b;};

In one cpp file, a.cpp I have
#include <x.h>
const struct Ff abc[] = { {3,4},{4,5} };

In sp.ino file I have
#include <x.h>

extern const struct Ff abc[];
...
Serial.println(abc[0].a);

When I compile and link I get:
c:/users/jonathan/appdata/local/arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-4-b40a506/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld.exe: C:\Users\jonathan\AppData\Local\Temp\arduino_build_511602\sketch\sp.ino.cpp.o:(.text.setup+0x8): undefined reference to `abc'.

If I remove the keyword Const I get no problems but I would like the variable to be Const.
Any help appreciated.

2

Constants and variables are not quite the same
Be careful to distinguish between variables and constants:
By default, in C++ all const objects declared at global namespace scope have internal linkage. That is, they are, in effect, static const.

i tried what you said and got the same issue. I then put the const struct abc definition in the x.h file and changed abc to def in a.cpp, thinking there would now be multiple declarations of abc in a.cpp and sp.cpp, there weren't.

it seemed like const struct Ff abc was more like a #define

not exactly sure if that is what the above says. "static" would limit its scope to the file

3

Thanks for reply. The link was very useful. It stated:
If we apply the extern qualifier explicitly to the definition of the constant K above, then we give it "external linkage" and it becomes available in other files linked to the file in which its extern definition appears:

So giving the definition extern attribute as well, solved the problem. So in a.cpp, I have the line:
extern const struct Ff abc[] = { {3,4},{4,5} };

4

The IDE takes your .ino file (and any other .ino files) and combines them together into essentially one .cpp file to compile. Since this is separate from the other 'a.cpp' file, those are separate compilation units and the 'const' keyword makes constants declared in one not visible to the other. That is what is meant by internal linkage

5

If you already have an extern object defined in an included header, you can't redeclare it in your ino. That is, you can't call "extern const struct Ff abc[];" in your sketch (and you don't need to).

6

Nothing about this issue is in the header file.
file: a.cpp

extern const struct Ff abc[] = { {3,4},{4,5} };  // force const object to have external linkage

file: sp.ino

extern const struct Ff abc[];  // declaration to external object

The point is that

int myVariable = 0;

defined at the global level of a.cpp has external linkage by default, but

const int myVariable = 0;

defined at the global level of a.cpp does not since it is a const. C++ requires you to include 'external' in order to get external linkage