Most efficient implementation of a large number class

Question

When doing calculations on very large numbers where integral data types such as double or int64 falls short, a separate class to handle such large numbers may be needed.

Answer 1

Using the built-in features of a language work for me.

Java has BigInteger and BigDecimal, and Python automagicaly switches to an object similar to Java's if a number gets out of the range of an integer or whatnot.

As for other languages though, I have no idea.

I hate re-inventing the wheel.