When I say dangerously naive, I mean in a way that people can go to jail¹ for "loosing" or "inventing" cents. Which your software will do if you use floats.
¹IANAL. But this was told when legal people looked at our architecture.
¹IANAL. But this was told when legal people looked at our architecture.
The goal is not to avoid rounding errors (which would be quite difficult when the true account value can be an irrational number, as with 3% APR compounding monthly), but to have the exact same rounding errors that are prescribed by the accounting practices. Which may vary depending on legislation.
A decimal floating point is usually a better starting point than integers are.
Which is why passing around ints is not the solution. And why I specifically mention Domain Models and/or Value Object.
A domain model would throw an exeption or otherwise dissalow certain divisions for example. What I often do, is something like `expense.amount.divide_over(3, leftover_to_last)` or `savings.balance_at(today).percentage_of(3.1337)`.
Sometimes, in simpler setups and when the language allows, I'll re-implement operators like *, / and even + and -. But when actual business logic is needed, I'll avoid these and implement actual domain methods that use the language the business uses.
But never, ever, do I allow just math-ing over the inner values.
So, I disagree: Both decimal floating point and integers are just as "bad". Maybe for the inner values in the domain model or value object, they are fine, but often there integers are a slightly better starting point because they make rounding and leftovers very explicit.