tools/doc/cstdfloat.qbk

[book Standardized Floating-Point typedefs for C and C++

    [quickbook 1.7]
    [copyright 2014  Christopher Kormanyos, John Maddock, Paul A. Bristow]
    [license
        Distributed under the Boost Software License, Version 1.0.
        (See accompanying file LICENSE_1_0.txt or copy at
        [@http://www.boost.org/LICENSE_1_0.txt])
    ]
    [authors [Kormanyos, Christopher],  [Maddock, John], [Bristow, Paul A.] ]
    [last-revision $Date$]
    [/version 1.8.3]
]

[template tr1[] [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf Technical Report on C++ Library Extensions]]
[template C99[] [@http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf C99 Standard ISO/IEC 9899:1999]]

[def __gsl [@http://www.gnu.org/software/gsl/ GSL-1.9]]
[def __glibc [@http://www.gnu.org/software/libc/ GNU C Lib]]
[def __hpc [@http://docs.hp.com/en/B9106-90010/index.html HP-UX C Library]]
[def __cephes [@http://www.netlib.org/cephes/ Cephes]]
[def __NTL [@http://www.shoup.net/ntl/ NTL A Library for doing Number Theory]]
[def __NTL_RR [@http://shoup.net/ntl/doc/RR.txt NTL::RR]]
[def __NTL_quad_float [@http://shoup.net/ntl/doc/quad_float.txt NTL::quad_float]]
[def __MPFR [@http://www.mpfr.org/ GNU MPFR library]]
[def __GMP [@http://gmplib.org/ GNU Multiple Precision Arithmetic Library]]
[def __multiprecision [@http://www.boost.org/doc/libs/1_53_0_beta1/libs/multiprecision/doc/html/index.html Boost.Multiprecision]]
[def __cpp_dec_float [@http://www.boost.org/doc/libs/1_53_0_beta1/libs/multiprecision/doc/html/boost_multiprecision/tut/floats/cpp_dec_float.html cpp_dec_float]]
[def __R [@http://www.r-project.org/ The R Project for Statistical Computing]]
[def __godfrey [link godfrey Godfrey]]
[def __pugh [link pugh Pugh]]
[def __NaN [@http://en.wikipedia.org/wiki/NaN NaN]]
[def __errno [@http://en.wikipedia.org/wiki/Errno `::errno`]]
[def __Mathworld [@http://mathworld.wolfram.com Wolfram MathWorld]]
[def __Mathematica [@http://www.wolfram.com/products/mathematica/index.html Wolfram Mathematica]]
[def __WolframAlpha [@http://www.wolframalpha.com/ Wolfram Alpha]]
[def __TOMS748 [@http://portal.acm.org/citation.cfm?id=210111 TOMS Algorithm 748: enclosing zeros of continuous functions]]
[def __TOMS910 [@http://portal.acm.org/citation.cfm?id=1916469 TOMS Algorithm 910: A Portable C++ Multiple-Precision System for Special-Function Calculations]]
[def __why_complements [link why_complements why complements?]]
[def __complements [link math_toolkit.stat_tut.overview.complements complements]]
[def __performance [link perf performance]]
[def __building [link math_toolkit.building building libraries]]
[def __e_float [@http://calgo.acm.org/910.zip e_float (TOMS Algorithm 910)]]
[def __Abramowitz_Stegun M. Abramowitz and I. A. Stegun, Handbook of Mathematical Functions, NBS  (1964)]
[def __DMLF [@http://dlmf.nist.gov/ NIST Digital Library of Mathematical Functions]]
[def __IEEE754  [@http://en.wikipedia.org/wiki/IEEE_floating_point IEEE_floating_point]]
[def __N3626  [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3626.pdf N3626]]
[def __N1703 [@http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1703.pdf N1703]]

[/ Some composite templates]
[template super[x]'''<superscript>'''[x]'''</superscript>''']
[template sub[x]'''<subscript>'''[x]'''</subscript>''']
[template floor[x]'''&#x230A;'''[x]'''&#x230B;''']
[template floorlr[x][lfloor][x][rfloor]]
[template ceil[x] '''&#x2308;'''[x]'''&#x2309;''']

[/template header_file[file] [@../../../../[file] [file]]]

[note A printer-friendly PDF version of this manual is also available.]

[section:overview Overview]

The header `<boost/cstdfloat.hpp>` provides optional standardized
floating-point `typedef`s having specified widths.
These are useful for writing portable code because they
should behave identically on all platforms.
All `typedef`s are in `namespace boost`.

The `typedef`s include `float16_t, float32_t, float64_t, float128_t`,
their corresponding least and fast types,
and the corresponding maximum-width type.
The `typedef`s are based on underlying built-in types
such as `float`, `double`, or `long double`, or based on other compiler-specific
non-standardized types such as `__float128`.
The underlying types of these typedef's must conform with
the corresponding specifications of binary16, binary32, binary64,
and binary128 in __IEEE754 floating-point format
[@http://en.wikipedia.org/wiki/IEEE_floating_point].

The typedef's are based on __N3626
proposed for a new C++14 standard header `<cstdfloat>` and
__N1703 proposed for a new C language standard header `<stdfloat.h>`.

The 128-bit floating-point type, of great interest in scientific and
numeric programming, is not required in the boost header,
and may not be supplied for all platforms/compilers, because compiler
support for a 128-bit floating-point type is not mandated by either
the C standard or the C++ standard.

The following code uses `<boost/cstdfloat.hpp>` in combination with
`<boost/math/special_functions.hpp>` to compute a simplified
version of the Jahnke-Emden-Lambda function. Here, we use
a floating-point type with exactly 64 bits (i.e., `float64_t`).
If we were to use, for instance, built-in `double`,
then there would be no guarantee that the code would
behave identically on all platforms. With `float64_t` from
`<boost/cstdfloat.hpp>`, however, this is very likely.
Using `float64_t`, we know that
this code is portable and uses a floating-point type
with approximately 15 decimal digits of precision.

  #include <cmath>
  #include <boost/cstdfloat.hpp>
  #include <boost/math/special_functions.hpp>

  boost::float64_t jahnke_emden_lambda(boost::float64_t v, boost::float64_t x)
  {
    const boost::float64_t gamma_v_plus_one = boost::math::tgamma(v + 1);
    const boost::float64_t x_half_pow_v     = std::pow(x / 2, v);

    return gamma_v_plus_one * boost::math::cyl_bessel_j(x, v) / x_half_pow_v;
  }

See `cstdfloat_test.cpp` for a more detailed test program.

[endsect] [/section:overview Overview]

[section:rationale Rationale]

The implementation of `<boost/cstdfloat.hpp>` is designed to utilize `<float.h>`,
defined in the 1989 C standard. The preprocessor is used to query certain
preprocessor definitions in `<float.h>` such as FLT_MAX, DBL_MAX, etc.
Based on the results of these queries, an attempt is made to automatically
detect the presence of built-in floating-point types having specified widths.
An unequivocal test regarding conformance with __IEEE754 (IEC599) based on
[@ http://en.cppreference.com/w/cpp/types/numeric_limits/is_iec559 `std::numeric_limits<>::is_iec559`]
is performed with `BOOST_STATIC_ASSERT`.

The header `<boost/cstdfloat.hpp>` makes the standardized floating-point
`typedef`s safely available in `namespace boost` without placing any names
in `namespace std`. The intention is to complement rather than compete
with a potential future C++ Standard Library that may contain these `typedef`s.
Should some future C++ standard include `<stdfloat.h>` and `<cstdfloat>`,
then `<boost/cstdfloat.hpp>` will continue to function, but will become redundant
and may be safely deprecated.

Because `<boost/cstdfloat.hpp>` is a boost header, its name conforms to the
boost header naming conventions, not the C++ Standard Library header
naming conventions.

[note
<boost/cstdfloat.hpp> [*cannot synthesize or create
a `typedef` if the underlying type is not provided by the compiler].
For example, if a compiler does not have an underlying floating-point
type with 128 bits (highly sought-after in scientific and numeric programming),
then `float128_t` and its corresponding least and fast types are not
provided by `<boost/cstdfloat.hpp`>.]

[warning
As an implementation artifact, certain C macro names from `<float.h>`
may possibly be visible to users of `<boost/cstdfloat.hpp>`.
Don't rely on using these macros; they are not part of any Boost-specified interface.
Use `std::numeric_limits<>` for floating-point ranges, etc. instead.]

[endsect] [/section:rationale Rationale]

[section:exact_typdefs Exact-Width Floating-Point `typedef`s]

The `typedef float#_t`, with # replaced by the width, designates a
floating-point type of exactly # bits. For example `float32_t` denotes
a single-precision floating-point type with approximately
7 decimal digits of precision (equivalent to binary32 in __IEEE754).

Floating-point types specified in C and C++ are allowed to have
implementation-specific widths and formats.
However, if a platform supports underlying floating-point types
(conformant with __IEEE754) with widths of 16, 32, 64, 128 bits,
or any combination thereof,
then `<boost/cstdfloat.hpp>` does provide the corresponding `typedef`s
`float16_t, float32_t, float64_t, float128_t,`
their corresponding least and fast types,
and the corresponding maximum-width type

The absence of `float128_t` is indicated by the macro `BOOST_NO_FLOAT128_T`.

[endsect] [/section:exact_typdefs Exact-Width Floating-Point `typedef`s]


[section:fastest_typdefs Fastest minimum-width floating-point `typedef`s]

The `typedef float_least#_t`, with # replaced by the width, designates a
floating-point type with a [*width of at least # bits], such that no
floating-point type with lesser size has at least the specified width.
Thus, `float_least32_t` denotes the smallest floating-point type with
a width of at least 32 bits.

Minimum-width floating-point types are provided for all existing
exact-width floating-point types on a given platform.

For example, if a platform supports `float32_t` and `float64_t`,
then `float_least32_t` and `float_least64_t` will also be supported, etc.

[endsect] [/section:fastest_typdefs Fastest minimum-width floating-point `typedef`s]

[section:fastest_typdefs Fastest minimum-width floating-point `typedef`s]

The typedef `float_fast#_t`, with # replaced by the width, designates
the [*fastest] floating-point type with a width of at least # bits.

There is no absolute guarantee that these types are the fastest for all purposes.
In any case, however, they satisfy the precision and width requirements.

Fastest minimum-width floating-point types are provided for all existing
exact-width floating-point types on a given platform.

For example, if a platform supports `float32_t` and `float64_t`,
then `float_fast32_t` and `float_fast64_t` will also be supported, etc.

[endsect] [/section:fastest_typdefs Fastest minimum-width floating-point `typedef`s]

[section:greatest_typdefs Greatest-width floating-point typedef]

The `typedef floatmax_t` designates a floating-point type capable of representing
any value of any floating-point type in a given platform.

The greatest-width typedef is provided for all platforms.

[endsect] [/section:greatest_typdefs Greatest-width floating-point typedef]

[section:macros Floating-Point Constant Macros]

All macros of the type `BOOST_FLOAT16_C, BOOST_FLOAT32_C, BOOST_FLOAT64_C,
BOOST_FLOAT128_C, BOOST_FLOATMAX_C` are always defined after inclusion of
`<boost/cstdfloat.hpp>`. These allow floating-point constants of at
least the specified width to be declared.

For example:

  #include <boost/cstdfloat.hpp>

  // Declare Pythagoras' constant with approximately 7 decimal digits of precision.
  static const boost::float32_t pi = BOOST_FLOAT32_C(3.1415926536);

  // Declare the Euler-gamma constant with approximately 34 decimal digits of precision.
  static const boost::float128_t euler = BOOST_FLOAT128_C(0.57721566490153286060651209008240243104216);

[endsect] [/section:macros Floating-Point Constant Macros]


[/ cstdfloat.qbk
  Copyright 2014 Christopher Kormanyos, John Maddock and Paul A. Bristow.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]