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P1272R4 Byteswapping for fun&&nuf #5018

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Merged
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Oct 19, 2021
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P1272R4 Byteswapping for fun&&nuf #5018

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41 changes: 41 additions & 0 deletions source/numerics.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -1290,6 +1290,10 @@
template<class To, class From>
constexpr To bit_cast(const From& from) noexcept;

// \ref{bit.byteswap}, \tcode{byteswap}
template<class T>
constexpr T byteswap(T value) noexcept;

// \ref{bit.pow.two}, integral powers of 2
template<class T>
constexpr bool has_single_bit(T x) noexcept;
Expand Down Expand Up @@ -1355,6 +1359,15 @@
if there is no value of the object's type corresponding to the
value representation produced, the behavior is undefined.
If there are multiple such values, which value is produced is unspecified.
A bit in the value representation of the result is indeterminate if
it does not correspond to a bit in the value representation of \tcode{from} or
corresponds to a bit of an object that is not within its lifetime or
has an indeterminate value\iref{basic.indet}.
For each bit in the value representation of the result that is indeterminate,
the smallest object containing that bit has an indeterminate value;
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issue with paper: Is "smallest" sufficiently clear here? The spec doesn't define size comparisons of objects afaikt.

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I assume "smallest" refers to subobject inclusion.

the behavior is undefined unless that object is
of unsigned ordinary character type or \tcode{std::byte} type.
The result does not otherwise contain any indeterminate values.

\pnum
\remarks
Expand All @@ -1370,6 +1383,34 @@
\end{itemize}
\end{itemdescr}

\rSec2[bit.byteswap]{\tcode{byteswap}}

\indexlibraryglobal{byteswap}%
\begin{itemdecl}
template<class T>
constexpr T byteswap(T value) noexcept;
\end{itemdecl}

\begin{itemdescr}
\pnum
\constraints
\tcode{T} models \libconcept{integral}.

\pnum
\mandates
\tcode{T} does not have padding bits\iref{basic.types.general}.

\pnum
Let the sequence $R$ comprise
the bytes of the object representation of \tcode{value} in reverse order.

\pnum
\returns
An object \tcode{v} of type \tcode{T}
such that each byte in the object representation of \tcode{v} is equal to
the byte in the corresponding position in $R$.
\end{itemdescr}

\rSec2[bit.pow.two]{Integral powers of 2}

\indexlibraryglobal{has_single_bit}%
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1 change: 1 addition & 0 deletions source/support.tex
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Expand Up @@ -579,6 +579,7 @@
#define @\defnlibxname{cpp_lib_bounded_array_traits}@ 201902L // also in \libheader{type_traits}
#define @\defnlibxname{cpp_lib_boyer_moore_searcher}@ 201603L // also in \libheader{functional}
#define @\defnlibxname{cpp_lib_byte}@ 201603L // also in \libheader{cstddef}
#define @\defnlibxname{cpp_lib_byteswap}@ 202110L // also in \libheader{bit}
#define @\defnlibxname{cpp_lib_char8_t}@ 201907L
// also in \libheader{atomic}, \libheader{filesystem}, \libheader{istream}, \libheader{limits}, \libheader{locale}, \libheader{ostream}, \libheader{string}, \libheader{string_view}
#define @\defnlibxname{cpp_lib_chrono}@ 201907L // also in \libheader{chrono}
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