why the program dosent know about "void" function?

Question

main.cpp: In function ‘int main()’:
main.cpp:27:5: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream}’ and ‘void’)
 cout<<arithmetic(difference,first_num);

Answer 1

Because your function should return something to use cout <<, but it returns nothing. Also if function returns not built-in type you should to overload << operator for it