How to use extern for sharing variables between C++ files?

Hi! I started learning C++ and I'm trying to split my program into multiple files. I have three files: main.cpp, functions.cpp and functions.h. In functions.h I declared a global variable int global_counter = 0; and several functions. In functions.cpp I defined these functions that modify global_counter. In main.cpp I include functions.h and try to use global_counter, but the compiler gives an error "multiple definition of `global_counter`". I've heard something about the extern keyword, but I don't fully understand how to use it correctly. Can you explain with a simple example? Here's my current code: // functions.h #ifndef FUNCTIONS_H #define FUNCTIONS_H int global_counter = 0; void increment(); void print_counter(); #endif // functions.cpp #include "functions.h" #include

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