How best to implement BCD as an exercise?

I'm a beginner (self-learning) programmer learning C++, and recently I decided to implement a binary-coded decimal (BCD) class as an exercise, and so I could handle very large numbers on Project Euler. I'd like to do it as basically as possible, starting properly from scratch.

I started off using an array of ints, where every digit of the input number was saved as a separate int. I know that each BCD digit can be encoded with only 4 bits, so I thought using a whole int for this was a bit overkill. I'm now using an array of bitset<4>'s.

Is using a library class like this overkill as well?
Would you consider it cheating?
Is there a better way to do this?

EDIT: The primary reason for this is as an exercise - I wouldn't want to use a library like GMP because the whole point is making the class myself. Is there a way of making sure that I only use 4 bits for each decimal digit?

Solution

Just one note, using an array of bitset<4>'s is going to require the same amount of space as an array of long's. bitset is usually implemented by having an array of word sized integers be the backing store for the bits, so that bitwise operations can use bitwise word operations, not byte ones, so more gets done at a time.

Also, I question your motivation. BCD is usually used as a packed representation of a string of digits when sending them between systems. There isn't really anything to do with arithmetic usually. What you really want is an arbitrary sized integer arithmetic library like GMP.