Justin wrote:
Craig Ringer wrote:
`double' in C++ refers to double precision floating point. `double' is
subject to all the usual fun with rational decimals being irrational
binary floats (and vice versa).
Not according to MS specific if i'm reading it correctly
*Microsoft Specific >*
The double type contains 64 bits: 1 for sign, 11 for the exponent, and
52 for the mantissa. Its range is +/--1.7E308 with at least 15 digits of
precision
I take it you're referring to:
http://msdn.microsoft.com/en-us/library/e02ya398(VS.80).aspx ?
See how it says "The format is similar to the float format...." ?
As you can see from:
http://msdn.microsoft.com/en-us/library/hd7199ke(VS.80).aspx
the `double' type is a binary floating point representation, just like
float. It just has a bigger exponent and a bigger mantissa, so it can
represent more extreme values and do so with more precision.
Being a binary floating point representation it's subject to all the
usual problems with comparison for equality, rounding oddities, etc.
Here's one of the many explanations out there on the 'net. I haven't
read this particular one, it's just a viable looking Google hit:
http://www.cprogramming.com/tutorial/floating_point/understanding_floating_point.html
By the way, there was at least a proposal for a numeric/decimal type for
C++0x . It doesn't seem to have made the cut.
http://209.85.173.104/search?q=cache:D0Iqhgz7X1QJ:www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2041.pdf+%22c%2B%2B0x%22+decimal+OR+numeric&hl=en&ct=clnk&cd=1&gl=au&client=firefox-a
http://en.wikipedia.org/wiki/C%2B%2B0x
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/
It looks like ISO C might adopt a decimal type or library though:
http://www2.hursley.ibm.com/decimal/
Note in particular the support in gcc 4.2 or newer.
There's also a library:
http://www2.hursley.ibm.com/decimal/dfpal/
that might be useful.
--
Craig Ringe
