Hi Chris,
A first draft of this library is available at https://sourceforge.net/projects/stddecimal/ Runs on MSVC10 and gcc 4.6.3.
Wraps the Intel implementation only, In my opinion, this is a good start. But to really improve the portability and usefulness of your proposal, you might want to consider writing your own portable decimal back end.
As tempting as this sounds, I don't have the skill, and certainly don't have the time. If anyone finds this idea appealing, I'm more than happy to help out with testing.
As far as I know, you simply won't get the third party code into boost. Perhaps I'm wrong here. But the best you can do is create a front end C++ implementation in as portable a way as possible and provide a mechanism for linking with prebuilt versions of the 3rd-party library. This is, for example, what we have done with GNU GMP, MPFR and MPIR in association with the proposed Boost.Multiprecision library.
Sounds reasonable. Is the GPL license compatible with the boost license? The decNumber is available under GPL. We should also contact the library authors and see if they will grant permission to include the libraries under terms amenable to the boost license - both libraries were made available for gcc but I'm not sure if both are under the GPL. How would 'boost' go about this - I could contact the authors but I wouldn't like to pose as a boost representative without some sort of permission!
But we would need to figure out how this would best fit into the existing (and potentially future) numerical context of boost. [SNIP] here's what I would suggest:
1) Consider writing your own fully portable, boost-owned decimal. Provide the Intel compatibility as an optimized back-end for selected architecture. You would need a bit more software architecture to support multiple back ends.
As above regarding writing own backend. Regarding software architecture, the rudiments are in place (a half-implementation using decNumber is done but not committed).
2) I'm probably jumping the gun here, but you may want to take a look at the back-end requirements for a number type in the proposed Boost.Multiprecision. You might eventually want to target compatibility with this potential boost library.
I'll take a look, but the goal as I see it is complying with the std::decimal proposal - that is, to fill the gap until it becomes standard and all compilers support it. This doesn't preclude extended functionality being bolted on.
3) You have some amounts of code in cpp source files. In my opinion, a decimal implementation should be header-only. The one exception is the use of library code mentioned earlier. One of the powers of boost is its portability. And this is strongly facilitated with its emphasis on header-only libraries.
I'd have though the implementation of the actual numerics is complex enough to benefit from being compiled into a library - but I have little insight into the pros and cons of the two approaches. As an alternative, would making the back-end C compliant (or C-with-pass-by- reference) be enough to facilitate portability? Both 3rd party backends are C, and I see no reason for a boost owned version to be much different. The wrappers I've provided thus far could certainly be header only, but I'm working on the assumption that's pointless if you are insisting on a compiled back-end.
4) Is there any connection between decimal and fixed-point? If so, do you want to contact Vicente Escriba who is concerned with fixed-point?
I don't think there is a connection apart from optionally providing direct conversion between the two, however I'll take a closer look at the fixed point library.
5) Do you need any elementary transcendental functions such as sin, cos, tan, exp, log, etc. Do you want to support any of these? Do your potential users need them? If so, I could potentially point you in the right direction for getting started on some potentially sensible algorithms in the digit ranges you are targeting.
Yes, I was planning on using the versions of these functions supplied by the 3rd party code. Also, the standard doesn't specify the accuracy of most of these functions, so conversion to and from floating point is an option in the early stages.
Again, to iterate lika a broken record...
Why don't you make a first attempt at writing the code for the *guts* of the decimal types. I would be in a position to assist you with this, if you would like. However, I will be available at the earliest in fall for any significant contribution on a project with this level of complexity.
It'd be a great project, but I don't have the time. My primary goal is to implement this for my own purposes - given C libraries and a blueprint of the C++ interface a freely available this is a fairly trivial task. Even if this library doesn't make it into boost proper, I think it will still be valuable to have a proven wrapper + 3rd party combo that developers can use with some confidence as to it's reliability - even if they have to grab both parts separately. Since it's following a proposed standard, users can expect continuing support - at the very least that something similar will eventually be provided by compilers. At any rate, having the wrapper with support for multiple back-ends and all the tests in place will be a good starting point for anyone who should wish to implement their own version of a floating point decimal! Thanks again for such detailed feedback! Regards, Josh.