When the compiler encounters a user-defined literal with a user-defined suffix S (it always has a leading underscore for third-party suffixes, as the suffixes without a leading underscore are reserved for the standard library) it does an unqualified name lookup in order to identify a function with the name operator "operator "" S. If it finds one, then it calls it according to the type of the literal and the type of the literal operator. Otherwise, the compiler will yield and error.
In the example from the How to do it... section, the literal operator is called operator "" _KB and has an argument of type unsigned long long int. This is the only integral type possible for literal operators for handling integral types. Similarly, ...
