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<title>Endian RFC</title>
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<table>
<tr>
<td align="left">Doc. no.:</td>
<td align="left">P0803R0</td>
</tr>
<tr>
<td align="left">Date:</td>
<td align="left">
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y-%m-%d" startspan -->2017-10-15<!--webbot bot="Timestamp" endspan i-checksum="12057" --></td>
</tr>
<tr>
<td align="left">Reply to:</td>
<td align="left">Beman Dawes &lt;bdawes at acm dot org&gt;
</tr>
<tr>
<td align="left">Audience:</td>
<td align="left">Library Evolution</td>
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<h1 align="center">Endian Library Request for Comments (R0)</h1>
<h2>Abstract</h2>
<p>The Boost Endian library manipulates the
<a href="https://en.wikipedia.org/wiki/Endianness">endianness</a> of integers
and user-defined types. It provides three approaches to dealing with endianness:
conversion functions, buffer classes, and arithmetic classes. Each approach
has use cases where it is preferred over the other approaches. The purpose of
this RFC is to determine interest in adding all or any of these components to
the C++ standard library or a library TS.</p>
<h2>Introduction</h2>
<p>The Boost Endian documentation (<a href="https://www.boost.org/libs/endian/doc/index.html">www.boost.org/libs/endian/doc</a>)
provides an <a href="https://www.boost.org/libs/endian/doc/index.html">overview</a>
and detailed technical specifications for the
<a href="https://www.boost.org/libs/endian/doc/conversion.html">conversion
functions</a>, <a href="https://www.boost.org/libs/endian/doc/arithmetic.html">
arithmetic types</a>, and
<a href="https://www.boost.org/libs/endian/doc/buffers.html">buffer types</a>.
</p>
<p>A separate discussion covers
<a href="https://www.boost.org/libs/endian/doc/choosing_approach.html">choosing
between the three approaches</a> provided by the library.</p>
<p>For those not interested in plowing through the entire documentation, the
<a href="https://www.boost.org/libs/endian/doc/index.html#FAQ">Overall FAQ</a>,
<a href="https://www.boost.org/libs/endian/doc/conversion.html#FAQ">Conversion
FAQ</a>, <a href="https://www.boost.org/libs/endian/doc/arithmetic.html#FAQ">
Arithmetic FAQ</a>, and
<a href="https://www.boost.org/libs/endian/doc/buffers.html#FAQ">Buffers FAQ</a>
answer a lot of question likely to arise during standardization.</p>
<h2>Motivation for standardization</h2>
<ul>
<li>Lack of any one de facto standard endianness capability in either C or
C++. While most platforms provides some way to handle endianness in C, the
mechanism varies from platform to platform.</li>
<li>Commonly used endian handling functions are extremely error prone. While
the Boost Endian conversion functions have the same problems, the buffer and
arithmetic classes do not.</li>
<li>Surprisingly difficult to write and test yourself in a portable yet
efficient (i.e. using intrinsics) way. </li>
<li>Explicit requests for standardization from Boost Endian users.</li>
<li>The Boost library is existing practice with years of both implementation
and end-user experience.</li>
<li>The committee has already failed once to standardize endian conversion
functions; perhaps now is time to do it right.</li>
</ul>
<h2>Impact on the standard library</h2>
<p>All three proposed approaches to endianness would be a pure additions to the
standard library and would break no existing user code (modulo the usual
namespace caveats).</p>
<h2>Proposed wording</h2>
<p>The Boost Endian reference documentation for
<a href="https://www.boost.org/libs/endian/doc/conversion.html#Reference">
Conversion</a>,
<a href="https://www.boost.org/libs/endian/doc/arithmetic.html#endian">
Arithmetic</a>, and
<a href="https://www.boost.org/libs/endian/doc/buffers.html#endian">Buffer</a>
approaches follows the standard library&#39;s <i>Method of description</i>
[description], so provides a fairly close approximation of what proposed wording
would look like. Actual proposed wording will be provided in a proposal document
for any portions of the library of interest to the committee. Names will need
the usual bikeshedding.</p>
<h2>Questions for the Library Evolution Group</h2>
<ul>
<li>How would you likely vote on a fully-worded and reviewed proposal to add a
standardized version of the Boost Endian conversion functions to the standard
library or a library TS?<br>
&nbsp;</li>
<li>How would you likely vote on a fully-worded and reviewed proposal to add a
standardized version of the Boost Endian buffer classes to the standard
library or a library TS?<br>
&nbsp;</li>
<li>How would you likely vote on a fully-worded and reviewed proposal to add a
standardized version of the Boost Endian arithmetic classes to the standard
library or a library TS?</li>
</ul>
<h2>Acknowledgements</h2>
<p>The original design for the arithmetic classes was developed by Darin Adler
based on work by Mark Borgerding. The Boost documentation acknowledges
<a href="https://www.boost.org/libs/endian/doc/index.html#Acknowledgements">45+
other people</a>.</p>
<hr>
<p>&nbsp;</p>
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