Phil Endecott wrote:
Dear All,
I have recently written some code for fixed point arithmetic which I would be happy to contribute to Boost, if it is considered worthy. Please let me know if this is something that you would use, especially if you would be able to help with testing on platforms other than Linux and other tasks necessary for Boostification. Here's a very quick summary:
You declare a fixed-point variable by specifying the number of whole-number bits and the number of fraction bits, e.g.
fixed<8,23> x;
I have a very similar library that I've been developing/using for work for some months. I believe number of whole-number bits is commonly called magnitude bits.
This has a range of about -256 to +256 with a precision of 1/(2^23), and occupies 32 bits. boost::integer is used to choose an implementation type with sufficient bits.
Interesting. How do you decide what type to promote to for fixed-point multiply and divides. AFAIK Boost.Integer doesn't support 64-bit types which forced me to roll my own solution.
The usual arithmetic operators are implemented with some help from boost::operators, and should have no overhead; note that overflow is not detected (like the built-in integral types).
Platforms where binary fixed-point math is useful unfortunately tend to use compilers that are a little dated. In particular EBO is missed by RVCT 2.2 in some cases which makes using Boost.Operators a no go for this type of project IMO.
Implicit conversions are provided to and from the built-in integral and floating-point types (well, actually I have only added the ones that I have needed; the others should only need copy&paste). Implicit conversion between fixed types of different sizes is also possible. I'm starting to think that perhaps I should make some of these conversion explicit: I worry that code using this might be doing an "invisible" conversion via a floating point type that I didn't want. Any thoughts about this would be appreciated.
These are way too dangerous. An explicit syntax is necessary.
Operations between fixed-point values of different sizes are possible, and return a fixed-point value with sufficient bits to store the result. For example:
fixed<10,4> a; fixed<15,2> b; fixed<15,4> c; c = a + b;
..except that actually one more bit is needed in the maximum-value case. There are no doubt more opportunities for refinement in this area.
Interesting. The only mixed-type arithmetic I support currently is multiplication through a very cheesy expression template that delays the calculation until it is assigned to a result type.
You can see the code here:
I'll check it out. I think a Boost worthy implementation (which mine certainly isn't in its present form) would require at least: - optional overflow/underflow detection - fixed-point math routines for the common standard library math functions.