...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
#include <boost/multiprecision/cpp_bin_float.hpp>
namespace boost{ namespace multiprecision{ enum digit_base_type { digit_base_2 = 2, digit_base_10 = 10 }; template <unsigned Digits, digit_base_type base = digit_base_10, class Allocator = void, class Exponent = int, ExponentMin = 0, ExponentMax = 0> class cpp_bin_float; typedef number<cpp_bin_float<50> > cpp_bin_float_50; typedef number<cpp_bin_float<100> > cpp_bin_float_100; typedef number<backends::cpp_bin_float<24, backends::digit_base_2, void, boost::int16_t, -126, 127>, et_off> cpp_bin_float_single; typedef number<backends::cpp_bin_float<53, backends::digit_base_2, void, boost::int16_t, -1022, 1023>, et_off> cpp_bin_float_double; typedef number<backends::cpp_bin_float<64, backends::digit_base_2, void, boost::int16_t, -16382, 16383>, et_off> cpp_bin_float_double_extended; typedef number<backends::cpp_bin_float<113, backends::digit_base_2, void, boost::int16_t, -16382, 16383>, et_off> cpp_bin_float_quad; typedef number<backends::cpp_bin_float<237, backends::digit_base_2, void, boost::int32_t, -262142, 262143>, et_off> cpp_bin_float_oct; }} // namespaces
The cpp_bin_float
back-end
is used in conjunction with number
:
It acts as an entirely C++ (header only and dependency free) floating-point
number type that is a drop-in replacement for the native C++ floating-point
types, but with much greater precision.
Type cpp_bin_float
can
be used at fixed precision by specifying a non-zero Digits
template parameter. The typedefs cpp_bin_float_50
and cpp_bin_float_100
provide
arithmetic types at 50 and 100 decimal digits precision respectively.
Optionally, you can specify whether the precision is specified in decimal
digits or binary bits - for example to declare a cpp_bin_float
with exactly the same precision as double
one would use number<cpp_bin_float<53, digit_base_2> >
.
The typedefs cpp_bin_float_single
,
cpp_bin_float_double
,
cpp_bin_float_quad
, cpp_bin_float_oct
and cpp_bin_float_double_extended
provide software analogues of the IEEE single, double, quad and octuple
float data types, plus the Intel-extended-double type respectively. Note
that while these types are functionally equivalent to the native IEEE types,
but they do not have the same size or bit-layout as true IEEE compatible
types.
Normally cpp_bin_float
allocates no memory: all of the space required for its digits are allocated
directly within the class. As a result care should be taken not to use
the class with too high a digit count as stack space requirements can grow
out of control. If that represents a problem then providing an allocator
as a template parameter causes cpp_bin_float
to dynamically allocate the memory it needs: this significantly reduces
the size of cpp_bin_float
and increases the viable upper limit on the number of digits at the expense
of performance. However, please bear in mind that arithmetic operations
rapidly become very expensive as the digit count grows:
the current implementation really isn't optimized or designed for large
digit counts. Note that since the actual type of the objects allocated
is completely opaque, the suggestion would be to use an allocator with
void
value_type
,
for example: number<cpp_bin_float<1000, digit_base_10, std::allocator<void> > >
.
The final template parameters determine the type and range of the exponent:
parameter Exponent
can
be any signed integer type, but note that MinExponent
and MaxExponent
can not
go right up to the limits of the Exponent
type as there has to be a little extra headroom for internal calculations.
You will get a compile time error if this is the case. In addition if MinExponent
or MaxExponent are zero, then the library will choose suitable values that
are as large as possible given the constraints of the type and need for
extra headroom for internal calculations.
There is full standard library and numeric_limits
support available for this type.
Things you should know when using this type:
cpp_bin_float
s
have a value of zero.
std::numeric_limits
specialisation for
this type.
number
instantiated
on this type, is convertible to any other number
instantiated on this type - for example you can convert from number<cpp_bin_float<50> >
to number<cpp_bin_float<SomeOtherValue> >
.
Narrowing conversions round to nearest and are explicit
.
std::runtime_error
being thrown if the string can not be interpreted as a valid floating-point
number.
sqrt
function are also correctly rounded, but transcendental functions (sin,
cos, pow, exp etc) are not.
#include <boost/multiprecision/cpp_bin_float.hpp> #include <boost/math/special_functions/gamma.hpp> #include <iostream> int main() { using namespace boost::multiprecision; // Operations at fixed precision and full numeric_limits support: cpp_bin_float_100 b = 2; std::cout << std::numeric_limits<cpp_bin_float_100>::digits << std::endl; std::cout << std::numeric_limits<cpp_bin_float_100>::digits10 << std::endl; // We can use any C++ std lib function, lets print all the digits as well: std::cout << std::setprecision(std::numeric_limits<cpp_bin_float_100>::max_digits10) << log(b) << std::endl; // print log(2) // We can also use any function from Boost.Math: std::cout << boost::math::tgamma(b) << std::endl; // These even work when the argument is an expression template: std::cout << boost::math::tgamma(b * b) << std::endl; // And since we have an extended exponent range we can generate some really large // numbers here (4.0238726007709377354370243e+2564): std::cout << boost::math::tgamma(cpp_bin_float_100(1000)) << std::endl; return 0; }