Re: [Boost-users] Ann: Floating Point Utilities Review starts today

From: Johan Råde (rade_at_[hidden])
Date: 2008-02-18 12:45:24

John Maddock wrote:

> 3) C++ locale facets: these will read and write non-finite numbers in a 
> portable and round-trippable way: that is not otherwise possible with 
> current C++ std library implementations.  These are particularly useful for 
> number-serialisation for example.
> 
> Since the design is already specified by the C++ standard for these facets, 
> your review here should focus on implementation, testing, documentation, and 
> perhaps where in Boost these should best be placed if accepted.

Actually, there are som interface issues to be discussed here:

1. Are the trap_infinity and trap_nan flags a good idea?
I added them at Robert Ramey's request, but I'm not convinced myself.
The implementation is not entirely satisfactory.
In the ideal implementation both nonfinite_num_put and nonfinite_num_get would report infinity or 
nan by setting the failbit of the stream.
The problem is that the std::num_put facet don't have access to the stream state.
Hence std::num_get reports infinity or nan by setting the failbit of the stream
while std::num_put reports infinity or nan by throwing an exception.

2. I have chosen to implement the flags as arguments to the constructor.
The flags can not be changed after the facet has been constructed.
An alternative design would be to use the xalloc and iword mechanism.
Then one could have an interface for changing the flags using stream manipulators.
But that would require a cpp file to be added to the library.
(I consider this a flaw in the design of the standard iostreams library.
A design that relied on GUIDs instead of xalloc would be more flexible.)

3. nan is formatted as "nan" or "-nan" depending on the signbit.
Maybe the default should be to always format nan as "nan",
and have a signed_nan flag, similar to the signed_zero flag,
to show the signbit when formatting nan.

--Johan