This is an "operator overload" member function, specifically a conversion operator.
This function is called when you treat the value of the object instance (in this case, a
string, as a different type (in this case, an
For example, if there is a function that expects an unsigned long as an argument, but you pass a string object instead, this operator member will be called to convert the string to match the type.
There are many types of conversions, such as casting:
string a = ...;
unsigned long b;
b = a; //or:
b = (unsigned long)a;
You can find more information here: