C++ function doesn't change my argument [duplicate]

Answer 1

void fillInArray(vector<int> someVector);

Your function takes a vector by value. This means the function gets a copy of the vector you pass to it. It can operate on that copy, but those changes aren't visible outside.

You should be passing a reference if you want that function to be able to modify the caller's vector:

void fillInArray(vector<int>& someVector);

(Same change needs to be made in the function definition.)

Answer 2

If you pass an argument by copy, you should not wonder why the callee fails to change the object copied:
It never knew about it.

C++ has constant references to avoid copies (const T&), rvalue-references to enable moving (T&&), and lvalue-references to enable modification of the passed argument (T&).
(Be aware what loks like an rvalue-reference in a template might be a forwarding-reference.)

Of course, you can go the C-route and pass by pointer too.