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340 lines
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<title>Endian Conversion Functions</title>
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<b>
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<font size="6">Endian Conversion Functions</font></b></td>
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<table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" bgcolor="#D7EEFF" width="100%">
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<tr>
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<td><b><a href="../../../index.htm">Boost Home</a>
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<a href="index.html">Endian Home</a>
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<a href="conversion.html">Conversion Functions</a>
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<a href="arithmetic.html">Arithmetic Types</a>
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<a href="buffers.html">Buffer Types</a></b></td>
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<p></p>
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<td width="100%" bgcolor="#D7EEFF" align="center">
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<i><b>Contents</b></i></td>
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</tr>
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<tr>
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<td width="100%" bgcolor="#E8F5FF">
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<a href="#Introduction">Introduction</a><br>
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<a href="#Reference">Reference</a><br>
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<a href="#Synopsis">Synopsis</a><br>
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<a href="#Requirements">Requirements</a><br>
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<code><a href="#Reversible">Reversible</a></code><br>
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<a href="#Customization-points">Customization for
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UDTs</a><br>
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<a href="#Functions">Functions</a><br>
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<a href="#FAQ">FAQ</a><br>
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<a href="#Acknowledgements">Acknowledgements</a></td>
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</tr>
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<tr>
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<td width="100%" bgcolor="#D7EEFF" align="center">
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<b><i>Headers</i></b></td>
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</tr>
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<tr>
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<td width="100%" bgcolor="#E8F5FF">
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<a href="../include/boost/endian/conversion.hpp"><boost/endian/conversion.hpp></a><br>
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<a href="../include/boost/endian/buffers.hpp"><boost/endian/buffers.hpp></a><br>
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<a href="../include/boost/endian/arithmetic.hpp"><boost/endian/arithmetic.hpp></a></td>
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</tr>
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</table>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>Header <a href="../include/boost/endian/conversion.hpp">boost/endian/conversion.hpp</a>
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provides byte order reversal and conversion functions that convert objects of
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the built-in
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integer types, and also types <code>float</code> and <code>double,</code>
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between native, big, or little endian byte
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ordering. User defined types are also supported.</p>
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<h2><a name="Reference">Reference</a></h2>
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<p>Functions are implemented <code>inline</code> if appropriate.<code> noexcept</code> is
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elided for compilers that do not support it.
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Boost scoped enum emulation is used so that the library still works for compilers that do not support scoped enums. </p>
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<h3>
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Header <code><boost/endian/conversion.hpp></code>
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<a name="Synopsis">Synopsis</a></h3>
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<pre>#define BOOST_ENDIAN_INTRINSIC_MSG "<b><font face="Arial"><i>message describing presence or absence of intrinsics</i></font></b>"
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namespace boost
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{
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namespace endian
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{
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enum class <a name="order">order</a>
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{
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big, // big-endian
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little, // little-endian
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native = <b><i>implementation-defined</i></b> // same as order::big or order::little<b><i>
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</i></b>};
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// reverse byte order (i.e. endianness)
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int8_t <a href="#reverse_endianness">reverse_endianness</a>(int8_t x) noexcept;
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int16_t <a href="#reverse_endianness">reverse_endianness</a>(int16_t x) noexcept;
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int32_t <a href="#reverse_endianness">reverse_endianness</a>(int32_t x) noexcept;
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int64_t <a href="#reverse_endianness">reverse_endianness</a>(int64_t x) noexcept;
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uint8_t <a href="#reverse_endianness">reverse_endianness</a>(uint8_t x) noexcept;
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uint16_t <a href="#reverse_endianness">reverse_endianness</a>(uint16_t x) noexcept;
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uint32_t <a href="#reverse_endianness">reverse_endianness</a>(uint32_t x) noexcept;
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uint64_t <a href="#reverse_endianness">reverse_endianness</a>(uint64_t x) noexcept;
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float <a href="#reverse_endianness">reverse_endianness</a>(float x) noexcept;
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double <a href="#reverse_endianness">reverse_endianness</a>(double x) noexcept;
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template <class Value>
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void <a href="#reverse">reverse</a>(Value& x) noexcept;
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// reverse byte order unless native endianness is big
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template <class ReversibleValue >
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ReversibleValue <a href="#big_endian_value">big_endian_value</a>(ReversibleValue x) noexcept;
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template <class Reversible>
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void <a href="#big_endian">big_endian</a>(Reversible& x) noexcept;
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// reverse byte order unless native endianness is little
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template <class ReversibleValue >
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ReversibleValue <a href="#little_endian_value">little_endian_value</a>(ReversibleValue x) noexcept;
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template <class Reversible>
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void <a href="#little_endian">little_endian</a>(Reversible& x) noexcept;
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// generic byte order conversion
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template <order From, order To, class ReversibleValue>
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ReversibleValue <a href="#convert_value_generic">convert_value</a>(ReversibleValue from) noexcept;
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template <order From, order To, class Reversible>
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void <a href="#convert_generic">convert</a>(Reversible& x) noexcept;
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// runtime byte-order conversion
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template <class ReversibleValue>
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ReversibleValue <a href="#convert_value_runtime">convert_value</a>(ReversibleValue from,
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order from_order, order to_order) noexcept;
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template <class Reversible>
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void <a href="#convert_runtime">convert</a>(Reversible& x,
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order from_order, order to_order) noexcept;
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} // namespace endian
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} // namespace boost</pre>
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<p dir="ltr">The implementation is required to define the <code>enum class order</code>
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constant <code>native</code> as
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<code>big</code> on big endian platforms and <code>little</code> on little
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endian platforms.</p>
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<h3><a name="Requirements">Requirements</a></h3>
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<h4>Template argument <code><a name="Reversible">Reversible</a></code> requirements</h4>
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<p>This subsection describes names that are used to specify constraints on
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template arguments.</p>
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<p>User-defined types may be used in the function templates in this header only
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if they meet these requirements.</p>
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<p><code>Reversible</code> is an object type to be supplied by a C++ program
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instantiating a template.</p>
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<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
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<tr>
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<td width="160"><b>Expression</b></td>
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<td width="160"><b>Meaning of x</b></td>
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<td width="347"><b>Requirements</b></td>
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</tr>
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<tr>
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<td valign="top">
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<p><code>reverse_endianness(x)</code></td>
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<td valign="top">
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<code>x</code> is a value of a possibly <code>const</code> type convertible
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to <code>Reversible</code>.</td>
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<td>
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<p>The value of <code>x</code> with the
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order of its constituent bytes reversed is returned.</td>
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</tr>
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<tr>
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<td valign="top">
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<code>reverse_endianness_in_place(x)</code></td>
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<td valign="top">
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<code>x</code> is a
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modifiable lvalue of type <code>Reversible</code>.</td>
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<td>
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The order of the constituent bytes of <code>x</code> are
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reversed.</td>
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</tr>
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</table>
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<blockquote>
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<p> [<i>Note:</i> A user-defined type meets these requirements by defining a
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non-member function in the same namespace as the UDT itself so that the function
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can be found by argument dependent lookup (ADL). <i>—end note</i>]</p>
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</blockquote>
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<h4> <a name="Customization-points">Customization points</a> for user-defined types (<a name="UDT">UDT</a>s)</h4>
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<p> This subsection describes requirements on the endian library's own
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implementation.</p>
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<p> The endianness conversion function templates that return values are
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required to perform reversal of endianness if needed by making an unqualified
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call to <code>reverse_endianness(<i>argument</i>)</code>, as described in the
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preceding table.</p>
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<p> The endianness conversion function templates that modify their argument in
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place are required to perform reversal of endianness if needed by making an
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unqualified call to <code>reverse_endianness_in_place(<i>argument</i>)</code>,
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as described in the preceding table.</p>
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<p> See <a href="../example/udt_conversion_example.cpp">
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udt_conversion_example.cpp</a> for an example user-defined type.</p>
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<h3><a name="Functions">Functions</a></h3>
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<pre><a name="reverse_endianness"></a>int8_t reverse_endianness(int8_t x) noexcept;
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int16_t reverse_endianness(int16_t x) noexcept;
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int32_t reverse_endianness(int32_t x) noexcept;
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int64_t reverse_endianness(int64_t x) noexcept;
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uint8_t reverse_endianness(uint8_t x) noexcept;
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uint16_t reverse_endianness(uint16_t x) noexcept;
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uint32_t reverse_endianness(uint32_t x) noexcept;
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uint64_t reverse_endianness(uint64_t x) noexcept;
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float reverse_endianness(float x) noexcept;
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double reverse_endianness(double x) noexcept;</pre>
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<blockquote>
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<p><i>Returns:</i> <i><code>x</code></i>, with the order of its
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constituent bytes reversed.</p>
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</blockquote>
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<pre><a name="reverse"></a>template <class Value>
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void reverse(Value& x) noexcept;</pre>
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<blockquote>
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<p><i>Postconditions:</i> The order of the constituent bytes of
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<code>x</code> are reversed.</p>
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</blockquote>
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<pre><a name="big_endian_value"></a>template <class ReversibleValue >
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ReversibleValue big_endian_value(ReversibleValue x) noexcept;
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<a name="big_endian"></a>template <class Reversible>
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void big_endian(Reversible& x) noexcept;</pre>
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<blockquote>
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<p><i>Returns (first form)</i>: <code>x</code> if the native byte order is big
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endian, otherwise <code>reverse_endianness(x)</code>.</p>
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<p><i>Effects (second form):</i> None if the native byte order is big
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endian, otherwise <code>reverse(x)</code>.</p>
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<p><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x = <b><i>some-value</i></b>;
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big_endian(x); // reverses the byte order of x, unless
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// the native byte order is big-endian</pre>
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</blockquote>
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</blockquote>
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<pre><a name="little_endian_value"></a>template <class ReversibleValue >
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ReversibleValue little_endian_value(ReversibleValue x) noexcept;
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<a name="little_endian"></a>template <class Reversible>
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void little_endian(Reversible& x) noexcept;</pre>
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<blockquote>
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<p><i>Returns (first form)</i>: <code>x</code> if the native byte order is little
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endian, otherwise <code>reverse_endianness(x)</code>.</p>
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<p><i>Effects (second form):</i> None if the native byte order is little
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endian, otherwise <code>reverse(x)</code>.</p>
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<p><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x = <b><i>some-value</i></b>;
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int32_t y(little_endian(x));
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// y has been set to x; the byte order is reversed unless
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// the native byte order is little-endian.</pre>
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</blockquote>
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</blockquote>
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<pre><a name="convert_value_generic"></a>template <order From, order To, class ReversibleValue>
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ReversibleValue convert_value(ReversibleValue from) noexcept;
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<a name="convert_generic"></a>template <order From, order To, class Reversible>
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void convert(Reversible& x) noexcept;
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</pre>
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<blockquote>
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<p>The <b><i>effective order</i></b> of an order template parameter
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is the same as the order template parameter if the parameter is not <code>
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order::native</code>, otherwise it is the constant <code>order::big</code> or
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<code>order::little</code> that represents the actual native byte order.</p>
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<p><i>Returns (first form)</i>: <code>from</code> if <code>From</code>
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and <code>To</code> have the same effective order, otherwise <code>
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reverse_endianness(from)</code>.</p>
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<p><i>Effects (second form):</i> None if <code>From</code> and <code>
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To</code> have the same effective order, otherwise <code>reverse(x)</code>.</p>
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<p><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x;
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<i>... read an external big-endian value into x</i>
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convert<order::big, order::native>(x); // more generic equivalent of big_endian(x);</pre>
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</blockquote>
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</blockquote>
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<pre><a name="convert_value_runtime"></a>template <class ReversibleValue>
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ReversibleValue convert_value(ReversibleValue from,
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order from_order, order to_order) noexcept;
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<a name="convert_runtime"></a>template <class Reversible>
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void convert(Reversible& x,
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order from_order, order to_order) noexcept;</pre><blockquote><p><i>Returns (first form)</i>: <code>from</code> if <code>effect_order(from_order) == effective_order(to_order)</code>, otherwise <code>reverse_endianness(from)</code>.</p>
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<p><i>Effects (second form):</i> None if <code>effect_order(from_order) == effective_order(to_order)</code>, otherwise <code>reverse(x)</code>.</p>
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<p><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x;
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<i>... read an external value of an endianness know only at runtime into x</i>
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convert(x, some_order, order::native); // convert to native byte order if needed</pre>
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</blockquote>
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</blockquote>
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<h2> <a name="FAQ">FAQ</a></h2>
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<p>See the <a href="index.html#FAQ">Endian home page</a> FAQ for a library-wide
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FAQ.</p>
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<p><b>Why are the template versions of <code>reverse()</code> and <code>reverse_endianness()</code>
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in a detail namespace?</b></p>
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<blockquote>
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<p>They are unsafe for general use. Consider reversing
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the bytes of a <code>std::pair</code> as a whole - the bytes from <code>first</code>
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would end up in <code>second</code> and visa versa, and this is totally
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wrong!</p>
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</blockquote>
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<p><b>Why are both value returning and modify-in-place functions provided?</b></p>
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<blockquote>
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<p>Returning the result by value is the standard C and C++ idiom for functions that compute a
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value from an argument. Modify-in-place functions allow cleaner code in many real-world
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endian use cases and are more efficient for user defined types that have
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members such as string data that do not need to be reversed. Thus both forms are
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provided.</p>
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</blockquote>
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<h2><a name="Acknowledgements">Acknowledgements</a></h2><p>Tomas Puverle was instrumental in identifying and articulating the need to
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support endian conversion as separate from endian integer types. Phil Endecott suggested the form of the value returning signatures. Vicente Botet and other reviewers suggested supporting floating point types and user defined types. General reverse template implementation approach using std::reverse suggested by Mathias Gaunard. Portable implementation approach for 16, 32, and 64-bit integers suggested by tymofey, with avoidance of undefined behavior as suggested by Giovanni Piero Deretta, and a further refinement suggested by Pyry Jahkola. Intrinsic builtins implementation approach for 16, 32, and 64-bit integers suggested by several reviewers, and by David Stone, who provided his Boost licensed macro implementation that became the starting point for <a href="../include/boost/endian/detail/intrinsic.hpp">boost/endian/detail/intrinsic.hpp</a>.
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Pierre Talbot provided the <code>int8_t reverse_endianness()</code> and templated
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<code>reverse()</code> implementations.</p>
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<hr>
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<p>Last revised:
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->07 December, 2014<!--webbot bot="Timestamp" endspan i-checksum="38646" --></p>
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<p>© Copyright Beman Dawes, 2011, 2013</p>
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<p>Distributed under the Boost Software License, Version 1.0. See <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a></p>
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