How can I get the penultimate element in a list?

Solution 1:

Rather than decrementing rbegin, you should increment it, as shown here:¹

double penultimate = *++foo.rbegin();

as rbegin() returns a reverse iterator, so ++ is the operator to move backwards in the container. Note that I've also dropped the superfluous parentheses: that's not to everyone's taste.

Currently the behaviour of your program is undefined since you are actually moving to end(), and you are not allowed to dereference that. The arbitrary nature of the output is a manifestation of that undefined behaviour.

¹Do retain the minimum size check that you currently have.

Solution 2:

The clearest way, in my mind, is to use the construct designed for this purpose (C++11):

double penultimate = *std::prev(foo.end(), 2)

Solution 3:

I would just do *--(--foo.end()); no need for reverse iterators. It's less confusing too.