If the constructors are not explicit.
string null_isbn("9-999-99999-9"); // call string ctor
Sales_data item1(null_isbn); // call Sales_data ctor
Sales_data item2("9-999-99999-9");
// first cover "9-999-99999-9" to string, then call Sales_data ctor
If the constructors are explicit, the same operations happen.