Subject: Re: [boost] [xint] Boost.XInt formal review
From: Peter Dimov (pdimov_at_[hidden])
Date: 2011-03-06 21:25:00
Jeffrey Lee Hellrung, Jr. wrote:
> It *can* be made thread-safe if you use atomic operations (as far as I
> know; someone correct me if this I'm wrong here). However, that likely
> introduces overhead even when you only need single threaded usage.
Well, it doesn't really matter whether it introduces overhead compared to
unsafe CoW if we operate under the constraint that the library must be
thread safe (which, quite frankly, it must be in this day and age). The
competition there is only between CoW with atomic inc/decrements and deep
copy. Move, by the way, eliminates an atomic increment and decrement in the
same cases in which it would eliminate a deep copy.
I probably should note that expression templates require CoW, as far as I
can see; if you return an expression from a*b instead of computing the
product, the expression needs to keep a and b alive. Having it store copies
would defeat the purpose. Storing references wouldn't work well in generic
code such as a forwarding function. Even something as simple as auto x = f()
+ y; would be a subtle bug.
I wouldn't worry much about the branch in this context. :-)
CoW aside, I do worry about the apparent overtemplatization of the library
(possibly as a result of Boost feedback) and the calls that it isn't
templatized enough. There are many potential users of an unlimited precision
integer library who do not care in the slightest about implementation
details and just need an integer class that provides the appropriate
operations. For the people who do care, the library should provide free
function templates that operate on a well-defined Integer concept which is
"operationless" on its own and only stores bits (in chunks of some
element_type perhaps). The supplied 'integer' class would be our best effort
of implementing this concept in the manner we think is best for the users
who want us to make this decision for them. The rest should be able to
define their own integer classes and get the operations "for free".