How to organize variables if we add a file with a class?

Question

C++ / C#

Good afternoon.
There is a main.cpp file in which variables are declared and used.
There are modul.h and modul.cpp which implement a class that uses these variables.
I connected the header file, but it still shows that the variables in the class are not declared.
I thought in the class constructor to make a class declaration, but for some reason the byte array does not understand.
BYTE* buffer.

Answer 1

There is a main.cpp file in which variables are declared and used.

Get rid of this. If you really need global variables, then use predefinition.

Answer 2

int main() {
    int a = 5;
    int b = 10;
    Modul modul(a, b);
}

Answer 3

Write in C
external variables are included as extern BYTE* var;