35 lines
1.7 KiB
Plaintext
35 lines
1.7 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:rounding Rounding Rules for Conversions]
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As a general rule, all conversions between unrelated types are performed using basic arithmetic operations, therefore
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conversions are either exact, or follow the same rounding rules as arithmetic for the type in question.
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The following table summarises the situation for conversions from native types:
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[table
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[[Backend][Rounding Rules]]
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[[__cpp_int][Conversions from integer types are exact if the target has sufficient precision, otherwise they
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truncate to the first 2^MaxBits bits (modulo arithmetic). Conversions from floating-point types
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are truncating to the nearest integer.]]
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[[__gmp_int][Conversions are performed by the GMP library except for conversion from `long double` which is truncating.]]
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[[__tom_int][Conversions from floating-point types are truncating, all others are performed by libtommath and are exact.]]
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[[__gmp_float][Conversions are performed by the GMP library except for conversion from `long double` which should be exact
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provided the target type has as much precision as a `long double`.]]
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[[__mpfr_float_backend][All conversions are performed by the underlying MPFR library.]]
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[[__cpp_dec_float][All conversions are performed using basic arithmetic operations and are truncating.]]
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[[__gmp_rational][See __gmp_int]]
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[[__cpp_rational][See __cpp_int]]
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[[__tommath_rational][See __tom_int]]
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]
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[endsect] [/section:rounding Rounding Rules for Conversions]
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