i think the documentation on boost::rational is misleading, and it cannot be used with finite precision (e.g. the built-in) types at all: whenever the result of an operation cannot be represented exactly, it returns something totally bogus for example with rational<short> let's try to add 191/181 (=1.055...) and 197/193 (=1.021...) the exact result is 72520/34933 (=2.076...), and we get 6984/-30603 (=-0.229...)! i don't think the intended audience can "code in anticipation of such issues" (the task of determining when these issues come up has the same complexity as doing the actual arithmetic) imho this also means that the pre-calculated gcd() trick in op+= doesn't buy much: finite precision calculations will still fail randomly and my guess would be that it only makes unlimited precision calculations slower (2 gcd() calls + 4 op/()s instead of 1 gcd() call) fixing rational<> to work with finite precision integers so that for calculations like the above we get the best representable approximation (in this case 3197/1540 = 2.076, precise to 7 digits) is pretty straightforward -- the gcd() function implicitly calculates all representable approximations -- but would require rewriting the whole class br, andras