I'm not even sure I understand the contract. How can you round to a number of decimal places when the result is not necessarily representable?
Good question. Not sure I have an answer. I just evolved these algorithms becouse I couldnt find them anywhere else.
Rather than an implementation, could you provide an example of a documentation, stating precisely what the return value should be?
Sure. I could do that.
Also, if the intention is float/double/long double, why templates, as opposed to three overloads? Thanks for any clarification.
Well, this is boost after all. What tradeoffs do you see?
Equally useful as rounding, perhaps, would be printing to a string with guaranteed rounding. You know that printing or reading a binary floating point to a decimal string representation, and back, may not result in the same number. This goes with printf and scanf, as well as I/O streams (perhaps this has been corrected but I haven't seen anything very sophisticated in the STL implementations I lurk in). There are known algorithms to do this with guaranteed rounding, e.g.:
I'm not an expert here. If you would be willing to advise me on these issues, I might be persuaded to put together some test cases and example files. I know there alot of subtle issues involved with rounding floating point numbers.