calculator.cpp:6:17: error: ‘int Calculator::selectOption’ is not a static data member of ‘class Calculator’
int Calculator::selectOption;
^~~~~~~~~~~~
calculator.cpp:7:1: error: expected unqualified-id before ‘{’ token
{
int Calculator::selectOption;
should be int Calculator::selectOption()
However, if you're planning to return the result of a division, you'd better to return a double.
At first glance, it seems you're not passing the user input to your class instance - I don't think it can work that way.