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* Update Jamfile.v2 * Update introduction.qbk * Update tutorial.qbk * Update tutorial_cpp_int.qbk * Update tutorial_gmp_int.qbk * Update tutorial_tommath.qbk * Update integer_examples.cpp * Update tutorial_cpp_bin_float.qbk * Update tutorial_cpp_dec_float.qbk * Update tutorial_gmp_float.qbk * Update tutorial_mpfr_float.qbk * Update tutorial_float128.qbk * Update tutorial_float_builtin_ctor.qbk * Update big_seventh.cpp * Update tutorial_float_eg.qbk * Update floating_point_examples.cpp * Update mpfr_precision.cpp * Update gauss_laguerre_quadrature.cpp * Update tutorial_interval_mpfi.qbk * Update tutorial_cpp_complex.qbk * Update tutorial_mpc_complex.qbk * Update tutorial_float128_complex.qbk * Update tutorial_complex_adaptor.qbk * Update tutorial_rational.qbk * Update tutorial_tommath_rational.qbk * Update tutorial_logged_adaptor.qbk * Update tutorial_debug_adaptor.qbk * Update tutorial_visualizers.qbk * Update tutorial_fwd.qbk * Update tutorial_conversions.qbk * Update tutorial_random.qbk * Update random_snips.cpp * Update tutorial_constexpr.qbk * Update tutorial_import_export.qbk * Update cpp_int_import_export.cpp * Update tutorial_mixed_precision.qbk * Update tutorial_variable_precision.qbk * Update scoped_precision_example.cpp * Update tutorial_numeric_limits.qbk * Update tutorial_numeric_limits.qbk * Update numeric_limits_snips.cpp * Update numeric_limits_snips.cpp * Update tutorial_numeric_limits.qbk * Update numeric_limits_snips.cpp * Update numeric_limits_snips.cpp * Update tutorial_io.qbk * Update reference_number.qbk * Update reference_cpp_bin_float.qbk * Update reference_cpp_double_fp_backend.qbk * Update reference_internal_support.qbk * Update reference_backend_requirements.qbk * Update performance.qbk * Update performance_overhead.qbk * Update performance_real_world.qbk * Update performance_integer_real_world.qbk * Update performance_rational_real_world.qbk * Update reference_number.qbk * Update tutorial_numeric_limits.qbk * Update reference_backend_requirements.qbk
71 lines
3.8 KiB
Plaintext
71 lines
3.8 KiB
Plaintext
[/
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Copyright 2011 - 2020 John Maddock.
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Copyright 2013 - 2019 Paul A. Bristow.
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Copyright 2013 Christopher Kormanyos.
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt).
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]
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[section:cpp_dec_float cpp_dec_float]
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`#include <boost/multiprecision/cpp_dec_float.hpp>`
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namespace boost { namespace multiprecision {
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template <unsigned Digits10, class ExponentType = std::int32_t, class Allocator = void>
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class cpp_dec_float;
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typedef number<cpp_dec_float<50> > cpp_dec_float_50;
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typedef number<cpp_dec_float<100> > cpp_dec_float_100;
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} } // namespaces
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The `cpp_dec_float` back-end is used in conjunction with `number`: It acts as an entirely C++ (header only and dependency free)
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floating-point number type that is a drop-in replacement for the native C++ floating-point types, but with
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much greater precision.
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Type `cpp_dec_float` can be used at fixed precision by specifying a non-zero `Digits10` template parameter.
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The typedefs `cpp_dec_float_50` and `cpp_dec_float_100` provide arithmetic types at 50 and 100 decimal digits precision
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respectively. Optionally, you can specify an integer type to use for the exponent, this defaults to a 32-bit integer type
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which is more than large enough for the vast majority of use cases, but larger types such as `long long` can also be specified
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if you need a truly huge exponent range. In any case the ExponentType must be a __fundamental signed integer type at least 2 bytes
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and 16-bits wide.
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Normally `cpp_dec_float` allocates no memory: all of the space required for its digits are allocated
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directly within the class. As a result care should be taken not to use the class with too high a digit count
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as stack space requirements can grow out of control. If that represents a problem then providing an allocator
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as the final template parameter causes `cpp_dec_float` to dynamically allocate the memory it needs: this
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significantly reduces the size of `cpp_dec_float` and increases the viable upper limit on the number of digits
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at the expense of performance. However, please bear in mind that arithmetic operations rapidly become ['very] expensive
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as the digit count grows: the current implementation really isn't optimized or designed for large digit counts.
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There is full standard library and `std::numeric_limits` support available for this type.
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Things you should know when using this type:
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* Default constructed `cpp_dec_float`s have a value of zero.
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* The radix of this type is 10. As a result it can behave subtly differently from base-2 types.
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* The type has a number of internal guard digits over and above those specified in the template argument.
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Normally these should not be visible to the user.
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* The type supports both infinities and NaNs. An infinity is generated whenever the result would overflow,
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and a NaN is generated for any mathematically undefined operation.
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* There is a `std::numeric_limits` specialisation for this type.
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* Any `number` instantiated on this type is convertible to any other `number` instantiated on this type -
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for example you can convert from `number<cpp_dec_float<50> >` to `number<cpp_dec_float<SomeOtherValue> >`.
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Narrowing conversions are truncating and `explicit`.
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* Conversion from a string results in a `std::runtime_error` being thrown if the string can not be interpreted
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as a valid floating-point number.
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* The actual precision of a `cpp_dec_float` is always slightly higher than the number of digits specified in
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the template parameter, actually how much higher is an implementation detail but is always at least 8 decimal
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digits.
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* Operations involving `cpp_dec_float` are always truncating. However, note that since there are guard digits
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in effect, in practice this has no real impact on accuracy for most use cases.
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[h5 cpp_dec_float example:]
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[cpp_dec_float_eg]
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[endsect]
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