Imagine you're developing a system for managing a store's inventory. Each product in the inventory is represented by a class that includes product code and quantity in stock. Your task is to create functionality that allows for easy updating of product details from an input source like a file or standard input, using operator overloading for intuitive data entry.

Assume that a class named Product has been defined, and it has two member variables: a string named productCode and an int named quantityInStock.

Write a function that overloads the $+=$ operator for the Product class. The function should accept an int as its argument. The function should add the int value to the quantityInStock attribute of the Product object. Make sure the function returns the dereferenced this pointer.

Assume that the function has already been declared in the Product class with the following statement:

Product operator$+=($int$)$;