Forward declarations for variables?

struct A Unit[10] is not a forward declaration of a variable. The term "forward declaration" normally refers to non-defining declarations, while struct A Unit[10] is a definition. So in your code you are defining Unit multiple times in the same source file. In C language it is allowed, since in C definitions without an initializer are tentative definitions. They may occur many times in the same translation unit. In C++ there's no such thing as tentative definition. In C++ multiple definitions are always illegal.

If you want a genuine forward declaration for a variable, you have to use the keyword extern

extern struct A Unit[10];

This will work in both C and C++. However, as a side effect, this will give Unit external linkage. If you need a variable with internal linkage, then you are out of luck in C++, since in C++ it is not possible to forward-declare a variable with internal linkage. Meanwhile, in C tentative definitions will still help you to achieve that.

C allows variables to be tenatively declared (I guess). C++ does not. Once 'Unit' has been defined, it cannot be redefined in the same scope

In C++, a variable declaration must be prefixed with extern:

extern A Unit[10];

// ...

A Unit[10] = { ... };

(Note that in C++ you can omit the leading struct.)