Confused with linking header file in C++

[nuzzle]

But now, I wanna call a "const int" data declared in header1.h from main.cpp.
I tried including header1.h in main.cpp, it shows the error. It says redefined.

Strange. I should say that the "const int" isn't initialized.

A const declaration should look like this,

const int someData = 43;

and you should be able to include it in many compilation units (like main.cpp and function.cpp) you then link.

It's only if someData isn't const you should need to declare it extern. In that case you make sure that this is in one compilation unit only,

int someData;

and that all compilation units who want to use someData incude this,

extern int someData;