diff options
Diffstat (limited to 'libs/endian/doc/arithmetic.html')
| -rw-r--r-- | libs/endian/doc/arithmetic.html | 643 |
1 files changed, 643 insertions, 0 deletions
diff --git a/libs/endian/doc/arithmetic.html b/libs/endian/doc/arithmetic.html new file mode 100644 index 000000000..1066d14b0 --- /dev/null +++ b/libs/endian/doc/arithmetic.html @@ -0,0 +1,643 @@ +<html> + +<head> +<meta http-equiv="Content-Language" content="en-us"> +<meta name="GENERATOR" content="Microsoft FrontPage 5.0"> +<meta name="ProgId" content="FrontPage.Editor.Document"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<title>Endian Arithmetic Types</title> +<link href="styles.css" rel="stylesheet"> +</style> +</head> + +<body> + + +<table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="100%"> + <tr> + <td> +<a href="../../../index.html"> +<img src="../../../boost.png" alt="Boost logo" align="middle" border="0" width="277" height="86"></a></td> + <td align="middle"> + <b> + <font size="6">Endian Arithmetic Types</font> </b> + </td> + </tr> +</table> + +<table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" bgcolor="#D7EEFF" width="100%"> + <tr> + <td><b> + <a href="index.html">Endian Home</a> + <a href="conversion.html">Conversion Functions</a> + <a href="arithmetic.html">Arithmetic Types</a> + <a href="buffers.html">Buffer Types</a> + <a href="choosing_approach.html">Choosing Approach</a></b></td> + </tr> +</table> + +<p></p> + +<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" align="right"> + <tr> + <td width="100%" bgcolor="#D7EEFF" align="center"> + <i><b>Contents</b></i></td> + </tr> + <tr> + <td width="100%" bgcolor="#E8F5FF"> + <a href="#Introduction">Introduction</a><br> + <a href="#Example">Example</a><br> + <a href="#Limitations">Limitations</a><br> + <a href="#Feature-set">Feature set</a><br> + <a href="#Types">Enums and typedefs</a><br> + <a href="#Class_template_endian">Class template <code>endian</code></a><br> + + <a href="#Synopsis">Synopsis</a><br> + <a href="#Members">Members</a><br> + <a href="#Stream-inserter">Stream inserter</a><br> + <a href="#Stream-extractor">Stream extractor</a><br> + <a href="#FAQ">FAQ</a><br> + <a href="#Design">Design</a><br> + <a href="#Experience">Experience</a><br> + <a href="#Motivating-use-cases">Motivating use cases</a><br> + <a href="#C++0x">C++11</a><br> + <a href="#Compilation">Compilation</a><br> + <a href="#Acknowledgements">Acknowledgements</a> + </td> + </tr> + </table> +<h2><a name="Introduction">Introduction</a></h2> +<p>Header <a href="arithmetic.html">boost/endian/arithmetic.hpp</a> +provides integer binary types with control over +byte order, value type, size, and alignment. Typedefs provide easy-to-use names +for common configurations.</p> +<p>These types provide portable byte-holders for integer data, independent of +particular computer architectures. Use cases almost always involve I/O, either via files or +network connections. Although data portability is the primary motivation, these +integer byte-holders may +also be used to reduce memory use, file size, or network activity since they +provide binary integer sizes not otherwise available.</p> +<p>Such integer byte-holder types are traditionally called <b><i> +endian</i></b> types. See the +<a href="http://en.wikipedia.org/wiki/Endian" name="endianness">Wikipedia</a> for +a full +exploration of <b><i>endianness</i></b>, including definitions of <i><b>big +endian</b></i> and <i><b>little endian</b></i>.</p> +<p>Boost endian integers provide the same full set of C++ assignment, +arithmetic, and relational operators as C++ standard integral types, with +the standard semantics.</p> +<p>Unary arithmetic operators are <b> <code><font face="Courier New">+</font></code></b>, +<b> <code>-</code></b>, <b> <code>~</code></b>, <b> +<code>!</code></b>, plus both prefix and postfix <b> <code>--</code></b> and <b> <code>++</code></b>. Binary +arithmetic operators are <b> <code>+</code></b>, <b> <code>+=</code></b>, <b> <code>-</code></b>, +<b> <code> +-=</code></b>, <b> <code>*</code></b>, <b> <code>*=</code></b>, <b> <code>/</code></b>, +<b> <code>/=</code></b>, <b> <code>&</code></b>, <b> <code>&=</code></b>, +<b> <code>|</code></b>, <b> <code>|=</code></b>, <b> +<code>^</code></b>, <b> <code>^=</code></b>, <b> <code><<</code></b>, <b> <code><<=</code></b>, <code> +<b>>></b></code>, and <b> +<code>>>=</code></b>. Binary relational operators are <b> <code>==</code></b>, +<b> <code>!=</code></b>, <b> +<code><</code></b>, <b> <code><=</code></b>, <b> <code>></code></b>, +and <b> <code>>=</code></b>.</p> +<p>Implicit conversion to the underlying value type is provided. An implicit +constructor converting from the underlying value type is provided. </p> +<h2><a name="Example">Example</a></h2> +<p>The <a href="../example/endian_example.cpp">endian_example.cpp</a> program writes a +binary file containing four-byte, big-endian and little-endian integers:</p> +<blockquote> + <pre>#include <iostream> +#include <cstdio> +#include <boost/endian/arithmetic.hpp> +#include <boost/static_assert.hpp> + +using namespace boost::endian; + +namespace +{ + // This is an extract from a very widely used GIS file format. + // Why the designer decided to mix big and little endians in + // the same file is not known. But this is a real-world format + // and users wishing to write low level code manipulating these + // files have to deal with the mixed endianness. + + struct header + { + big_int32_t file_code; + big_int32_t file_length; + little_int32_t version; + little_int32_t shape_type; + }; + + const char* filename = "test.dat"; +} + +int main(int, char* []) +{ + header h; + + BOOST_STATIC_ASSERT(sizeof(h) == 16U); // reality check + + h.file_code = 0x01020304; + h.file_length = sizeof(header); + h.version = 1; + h.shape_type = 0x01020304; + + // Low-level I/O such as POSIX read/write or <cstdio> + // fread/fwrite is sometimes used for binary file operations + // when ultimate efficiency is important. Such I/O is often + // performed in some C++ wrapper class, but to drive home the + // point that endian integers are often used in fairly + // low-level code that does bulk I/O operations, <cstdio> + // fopen/fwrite is used for I/O in this example. + + std::FILE* fi = std::fopen(filename, "wb"); // MUST BE BINARY + + if (!fi) + { + std::cout << "could not open " << filename << '\n'; + return 1; + } + + if (std::fwrite(&h, sizeof(header), 1, fi)!= 1) + { + std::cout << "write failure for " << filename << '\n'; + return 1; + } + + std::fclose(fi); + + std::cout << "created file " << filename << '\n'; + + return 0; +} +</pre> +</blockquote> +<p>After compiling and executing <a href="../example/endian_example.cpp">endian_example.cpp</a>, +a hex dump of <code>test.dat</code> shows:</p> +<blockquote> + <pre>01020304 00000010 01000000 04030201</pre> +</blockquote> +<p>Notice that the first two 32-bit integers are big endian while the second two +are little endian, even though the machine this was compiled and run on was +little endian.</p> +<h2><a name="Limitations">Limitations</a></h2> +<p>Requires <code><climits></code> <code>CHAR_BIT == 8</code>. If <code>CHAR_BIT</code> +is some other value, compilation will result in an <code>#error</code>. This +restriction is in place because the design, implementation, testing, and +documentation has only considered issues related to 8-bit bytes, and there have +been no real-world use cases presented for other sizes.</p> +<p>In C++03, <code>endian_arithmetic</code> does not meet the requirements for POD types +because it has constructors, private data members, and a base class. This means +that common use cases are relying on unspecified behavior in that the C++ +Standard does not guarantee memory layout for non-POD types. This has not been a +problem in practice since all known C++ compilers lay out memory as if <code> +endian</code> were a POD type. In C++11, it is possible to specify the +default constructor as trivial, and private data members and base classes no longer disqualify a type from being a POD +type. Thus under C++11, <code>endian_arithmetic</code> +will no longer be relying on unspecified behavior.</p> +<h2><a name="Feature-set">Feature set</a></h2> +<ul> + <li>Big endian| little endian | native endian byte ordering.</li> + <li>Signed | unsigned</li> + <li>Unaligned | aligned</li> + <li>1-8 byte (unaligned) | 1, 2, 4, 8 byte (aligned)</li> + <li>Choice of value type</li> +</ul> +<h2>Enums and t<a name="Types">ypedefs</a></h2> +<p>Two scoped enums are provided:</p> +<blockquote> + <pre>enum class order {big, little, native}; + +enum class align {no, yes}; </pre> +</blockquote> +<p>One class template is provided:</p> +<blockquote> + <pre>template <order Order, typename T, std::size_t n_bits, + align Align = align::no> +class endian_arithmetic; +</pre> +</blockquote> +<p>Typedefs, such as <code>big_int32_t</code>, provide convenient naming +conventions for common use cases:</p> +<blockquote> +<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" width="49%"> + <tr> + <td width="18%" align="center"><b><i>Name</i></b></td> + <td width="49%" align="center"><b><i>Alignment</i></b></td> + <td width="10%" align="center"><b><i>Endianness</i></b></td> + <td width="10%" align="center"><b><i>Sign</i></b></td> + <td width="15%" align="center"><b><i>Sizes in bits (n)</i></b></td> + </tr> + <tr> + <td width="18%"><code>big_int</code><b><i>n</i></b><code>_t</code></td> + <td width="49%" align="center"><code>no</code></td> + <td width="10%" align="center"><code>big</code></td> + <td width="10%" align="center">signed</td> + <td width="15%">8,16,24,32,40,48,56,64</td> + </tr> + <tr> + <td width="18%"><code>big_uint</code><i><b>n</b></i><code>_t</code></td> + <td width="49%" align="center"><code>no</code></td> + <td width="10%" align="center"><code>big</code></td> + <td width="10%" align="center">unsigned</td> + <td width="15%">8,16,24,32,40,48,56,64</td> + </tr> + <tr> + <td width="18%"><code>little_int</code><i><b>n</b></i><code>_t</code></td> + <td width="49%" align="center"><code>no</code></td> + <td width="10%" align="center"><code>little</code></td> + <td width="10%" align="center">signed</td> + <td width="15%">8,16,24,32,40,48,56,64</td> + </tr> + <tr> + <td width="18%"><code>little_uint</code><i><b>n</b></i><code>_t</code></td> + <td width="49%" align="center"><code>no</code></td> + <td width="10%" align="center"><code>little</code></td> + <td width="10%" align="center">unsigned</td> + <td width="15%">8,16,24,32,40,48,56,64</td> + </tr> + <tr> + <td width="18%"><code>native_int</code><i><b>n</b></i><code>_t</code></td> + <td width="49%" align="center"><code>no</code></td> + <td width="10%" align="center"><code>native</code></td> + <td width="10%" align="center">signed</td> + <td width="15%">8,16,24,32,40,48,56,64</td> + </tr> + <tr> + <td width="18%"><code>native_uint</code><i><b>n</b></i><code>_t</code></td> + <td width="49%" align="center"><code>no</code></td> + <td width="10%" align="center"><code>native</code></td> + <td width="10%" align="center">unsigned</td> + <td width="15%">8,16,24,32,40,48,56,64</td> + </tr> + <tr> + <td width="18%"><code>big_int</code><i><b>n</b></i><code>_at</code></td> + <td width="49%" align="center"><code>yes</code></td> + <td width="10%" align="center"><code>big</code></td> + <td width="10%" align="center">signed</td> + <td width="15%">8,16,32,64</td> + </tr> + <tr> + <td width="18%"><code>big_uint</code><i><b>n</b></i><code>_at</code></td> + <td width="49%" align="center"><code>yes</code></td> + <td width="10%" align="center"><code>big</code></td> + <td width="10%" align="center">unsigned</td> + <td width="15%">8,16,32,64</td> + </tr> + <tr> + <td width="18%" dir="ltr"><code>little_int</code><i><b>n</b></i><code>_at</code></td> + <td width="49%" align="center" dir="ltr"><code>yes</code></td> + <td width="10%" align="center" dir="ltr"><code>little</code></td> + <td width="10%" align="center" dir="ltr">signed</td> + <td width="15%" dir="ltr">8,16,32,64</td> + </tr> + <tr> + <td width="18%" dir="ltr"><code>little_uint</code><i><b>n</b></i><code>_at</code></td> + <td width="49%" align="center" dir="ltr"><code>yes</code></td> + <td width="10%" align="center" dir="ltr"><code>little</code></td> + <td width="10%" align="center" dir="ltr">unsigned</td> + <td width="15%" dir="ltr">8,16,32,64</td> + </tr> + </table> +</blockquote> +<p>The unaligned types do not cause compilers to insert padding bytes in classes +and structs. This is an important characteristic that can be exploited to minimize wasted space in +memory, files, and network transmissions. </p> +<p><font color="#FF0000"><b><i><span style="background-color: #FFFFFF">Warning:</span></i></b></font><span style="background-color: #FFFFFF"> +Code that uses a</span>ligned types is possibly non-portable because alignment +requirements vary between hardware architectures and because alignment may be +affected by compiler switches or pragmas. For example, alignment of an 64-bit +integer may be to a 32-bit boundary on a 32-bit machine. Furthermore, aligned types +are only available on architectures with 8, 16, 32, and 64-bit integer types.</p> +<p><i><b>Recommendation:</b></i> Prefer unaligned arithmetic types.</p> +<p><i><b>Recommendation:</b></i> Protect yourself against alignment ills. For +example:</p> +<blockquote> + <pre>static_assert(sizeof(containing_struct) == 12, "sizeof(containing_struct) is wrong"); </pre> +</blockquote> +<p><b><i>Note:</i></b> <b><i>Note:</i></b> One-byte arithmetic types +have identical layout on all platforms, so they never actually reverse endianness. They are provided to enable generic code, and +to improve code readability and searchability.</p> +<h2><a name="Class_template_endian">Class template <code>endian</code></a><code>_arithmetic</code></h2> +<p>An endian is an integer byte-holder with user-specified <a href="#endianness"> +endianness</a>, value type, size, and <a href="#alignment">alignment</a>. The +usual operations on integers are supplied.</p> +<h3><a name="Synopsis">Synopsis</a></h3> +<pre>namespace boost +{ + namespace endian + { + // C++11 features emulated if not available + + enum class <a name="order">order</a> + { + big, // big-endian + little, // little-endian + native = <b><i>implementation-defined</i></b> // same as order::big or order::little<b><i> + </i></b>}; + + enum class <a name="alignment">align</a> {no, yes}; + + template <order Order, class T, std::size_t n_bits, + align Align = align::no> + class endian_arithmetic + : public endian_buffer<Order, T, n_bits, Align> + { + public: + typedef T value_type; + + // if BOOST_ENDIAN_FORCE_PODNESS is defined && C++11 PODs are not + // available then these two constructors will not be present + <a href="#endian">endian_arithmetic</a>() noexcept = default; + <a href="#explicit-endian">endian_arithmetic</a>(T v) noexcept; + + endian_arithmetic& <a href="#operator-eq">operator=</a>(T v) noexcept; + <a href="#operator-T">operator value_type</a>() const noexcept; + value_type value() const noexcept; // for exposition; see endian_buffer + const char* <a href="#data">data</a>() const noexcept; // for exposition; see endian_buffer + + // arithmetic operations + // note that additional operations are provided by the value_type + value_type operator+(const endian& x) noexcept; + endian& operator+=(endian& x, value_type y) noexcept; + endian& operator-=(endian& x, value_type y) noexcept; + endian& operator*=(endian& x, value_type y) noexcept; + endian& operator/=(endian& x, value_type y) noexcept; + endian& operator%=(endian& x, value_type y) noexcept; + endian& operator&=(endian& x, value_type y) noexcept; + endian& operator|=(endian& x, value_type y) noexcept; + endian& operator^=(endian& x, value_type y) noexcept; + endian& operator<<=(endian& x, value_type y) noexcept; + endian& operator>>=(endian& x, value_type y noexcept; + value_type operator<<(const endian& x, value_type y) noexcept; + value_type operator>>(const endian& x, value_type y) noexcept; + endian& operator++(endian& x) noexcept; + endian& operator--(endian& x) noexcept; + endian operator++(endian& x, int) noexcept; + endian operator--(endian& x, int) noexcept; + + // Stream inserter + template <class charT, class traits> + friend std::basic_ostream<charT, traits>& + operator<<(std::basic_ostream<charT, traits>& os, const T& x); + + // Stream extractor + template <class charT, class traits> + friend std::basic_istream<charT, traits>& + operator>>(std::basic_istream<charT, traits>& is, T& x); + }; + + // typedefs + + // unaligned big endian signed integer types + typedef endian<order::big, int_least8_t, 8> big_int8_t; + typedef endian<order::big, int_least16_t, 16> big_int16_t; + typedef endian<order::big, int_least32_t, 24> big_int24_t; + typedef endian<order::big, int_least32_t, 32> big_int32_t; + typedef endian<order::big, int_least64_t, 40> big_int40_t; + typedef endian<order::big, int_least64_t, 48> big_int48_t; + typedef endian<order::big, int_least64_t, 56> big_int56_t; + typedef endian<order::big, int_least64_t, 64> big_int64_t; + + // unaligned big endian unsigned integer types + typedef endian<order::big, uint_least8_t, 8> big_uint8_t; + typedef endian<order::big, uint_least16_t, 16> big_uint16_t; + typedef endian<order::big, uint_least32_t, 24> big_uint24_t; + typedef endian<order::big, uint_least32_t, 32> big_uint32_t; + typedef endian<order::big, uint_least64_t, 40> big_uint40_t; + typedef endian<order::big, uint_least64_t, 48> big_uint48_t; + typedef endian<order::big, uint_least64_t, 56> big_uint56_t; + typedef endian<order::big, uint_least64_t, 64> big_uint64_t; + + // unaligned little endian signed integer types + typedef endian<order::little, int_least8_t, 8> little_int8_t; + typedef endian<order::little, int_least16_t, 16> little_int16_t; + typedef endian<order::little, int_least32_t, 24> little_int24_t; + typedef endian<order::little, int_least32_t, 32> little_int32_t; + typedef endian<order::little, int_least64_t, 40> little_int40_t; + typedef endian<order::little, int_least64_t, 48> little_int48_t; + typedef endian<order::little, int_least64_t, 56> little_int56_t; + typedef endian<order::little, int_least64_t, 64> little_int64_t; + + // unaligned little endian unsigned integer types + typedef endian<order::little, uint_least8_t, 8> little_uint8_t; + typedef endian<order::little, uint_least16_t, 16> little_uint16_t; + typedef endian<order::little, uint_least32_t, 24> little_uint24_t; + typedef endian<order::little, uint_least32_t, 32> little_uint32_t; + typedef endian<order::little, uint_least64_t, 40> little_uint40_t; + typedef endian<order::little, uint_least64_t, 48> little_uint48_t; + typedef endian<order::little, uint_least64_t, 56> little_uint56_t; + typedef endian<order::little, uint_least64_t, 64> little_uint64_t; + + // unaligned native endian signed integer types + typedef <b><i>implementation-defined</i></b>_int8_t native_int8_t; + typedef <b><i>implementation-defined</i></b>_int16_t native_int16_t; + typedef <b><i>implementation-defined</i></b>_int24_t native_int24_t; + typedef <b><i>implementation-defined</i></b>_int32_t native_int32_t; + typedef <b><i>implementation-defined</i></b>_int40_t native_int40_t; + typedef <b><i>implementation-defined</i></b>_int48_t native_int48_t; + typedef <b><i>implementation-defined</i></b>_int56_t native_int56_t; + typedef <b><i>implementation-defined</i></b>_int64_t native_int64_t; + + // unaligned native endian unsigned integer types + typedef <b><i>implementation-defined</i></b>_uint8_t native_uint8_t; + typedef <b><i>implementation-defined</i></b>_uint16_t native_uint16_t; + typedef <b><i>implementation-defined</i></b>_uint24_t native_uint24_t; + typedef <b><i>implementation-defined</i></b>_uint32_t native_uint32_t; + typedef <b><i>implementation-defined</i></b>_uint40_t native_uint40_t; + typedef <b><i>implementation-defined</i></b>_uint48_t native_uint48_t; + typedef <b><i>implementation-defined</i></b>_uint56_t native_uint56_t; + typedef <b><i>implementation-defined</i></b>_uint64_t native_uint64_t; + + // aligned big endian signed integer types + typedef endian<order::big, int8_t, 8, align::yes> big_int8_at; + typedef endian<order::big, int16_t, 16, align::yes> big_int16_at; + typedef endian<order::big, int32_t, 32, align::yes> big_int32_at; + typedef endian<order::big, int64_t, 64, align::yes> big_int64_at; + + // aligned big endian unsigned integer types + typedef endian<order::big, uint8_t, 8, align::yes> big_uint8_at; + typedef endian<order::big, uint16_t, 16, align::yes> big_uint16_at; + typedef endian<order::big, uint32_t, 32, align::yes> big_uint32_at; + typedef endian<order::big, uint64_t, 64, align::yes> big_uint64_at; + + // aligned little endian signed integer types + typedef endian<order::little, int8_t, 8, align::yes> little_int8_at; + typedef endian<order::little, int16_t, 16, align::yes> little_int16_at; + typedef endian<order::little, int32_t, 32, align::yes> little_int32_at; + typedef endian<order::little, int64_t, 64, align::yes> little_int64_at; + + // aligned little endian unsigned integer types + typedef endian<order::little, uint8_t, 8, align::yes> little_uint8_at; + typedef endian<order::little, uint16_t, 16, align::yes> little_uint16_at; + typedef endian<order::little, uint32_t, 32, align::yes> little_uint32_at; + typedef endian<order::little, uint64_t, 64, align::yes> little_uint64_at; + + // aligned native endian typedefs are not provided because + // <cstdint> types are superior for that use case + + } // namespace endian +} // namespace boost</pre> +<p>The <i><b><code>implementation-defined</code></b></i> text above is either +<code>big</code> or <code>little</code> according to the endianness of the +platform.</p> +<h3><a name="Members">Members</a></h3> +<div dir="ltr"> + <pre><code><a name="endian">endian</a>() = default; // C++03: endian(){}</code></pre> +</div> +<blockquote> +<p><i>Effects:</i> Constructs an uninitialized object of type <code>endian_arithmetic<E, T, n_bits, A></code>.</p> +</blockquote> +<pre><code><a name="explicit-endian">endian</a>(T v);</code></pre> +<blockquote> +<p><i>Effects:</i> Constructs an object of type <code>endian_arithmetic<E, T, n_bits, A></code>.</p> +<p><i>Postcondition:</i> <code>x == v,</code> where <code>x</code> is the +constructed object.</p> +</blockquote> +<pre><code>endian& <a name="operator-eq">operator=</a>(T v);</code></pre> +<blockquote> + <p><i>Postcondition:</i> <code>x == v,</code> where <code>x</code> is the + constructed object.</p> + <p><i>Returns:</i> <code>*this</code>.</p> +</blockquote> +<pre><code><a name="operator-T">operator T</a>() const;</code></pre> +<blockquote> +<p><i>Returns:</i> The current value stored in <code>*this</code>, converted to +<code>value_type</code>.</p> +</blockquote> +<pre><code>const char* <a name="data">data</a>() const;</code></pre> +<blockquote> +<p><i>Returns:</i> A pointer to the first byte of the endian binary value stored +in <code>*this</code>.</p> +</blockquote> +<h3>Other operators</h3> +<p>Other operators on endian objects are forwarded to the equivalent +operator on <code>value_type</code>.</p> +<h3><a name="Stream-inserter">Stream inserter</a></h3> +<pre>template <class charT, class traits> +friend std::basic_ostream<charT, traits>& + operator<<(std::basic_ostream<charT, traits>& os, const T& x); +</pre> +<blockquote> +<p><i>Returns:</i> <code>os << +x</code>.</p> +</blockquote> +<h3><a name="Stream-extractor">Stream extractor</a></h3> +<pre>template <class charT, class traits> +friend std::basic_istream<charT, traits>& + operator>>(std::basic_istream<charT, traits>& is, T& x); +</pre> +<blockquote> +<p><i>Effects: </i>As if:</p> + <blockquote> + <pre>T i; +if (is >> i) + x = i; +</pre> + </blockquote> + <p><i>Returns: </i><code>is</code><i>.</i></p> +</blockquote> +<h2><a name="FAQ">FAQ</a></h2> + +<p>See the <a href="index.html#FAQ">Endian home page</a> FAQ for a library-wide +FAQ.</p> + +<p><b>Why not just use Boost.Serialization?</b> Serialization involves a +conversion for every object involved in I/O. Endian integers require no +conversion or copying. They are already in the desired format for binary I/O. +Thus they can be read or written in bulk.</p> +<p><b>Are endian types PODs?</b> Yes for C++11. No for C++03, although several +<a href="#Compilation">macros</a> are available to force PODness in all cases.</p> +<p><b>What are the implications of endian integer types not being PODs with C++03 +compilers?</b> They +can't be used in unions. Also, compilers aren't required to align or lay +out storage in portable ways, although this potential problem hasn't prevented +use of Boost.Endian with +real compilers.</p> +<p><b>What good is <i>native </i>endianness?</b> It provides alignment and +size guarantees not available from the built-in types. It eases generic +programming.</p> +<p><b>Why bother with the aligned endian types?</b> Aligned integer operations +may be faster (as much as 10 to 20 times faster) if the endianness and alignment of +the type matches the endianness and alignment requirements of the machine. The code, +however, will be somewhat less portable than with the unaligned types.</p> +<p><b>Why provide the arithmetic operations?</b> Providing a full set of operations reduces program +clutter and makes code both easier to write and to read. Consider +incrementing a variable in a record. It is very convenient to write:</p> +<pre wrap> ++record.foo;</pre> +<p wrap>Rather than:</p> +<pre wrap> int temp(record.foo); + ++temp; + record.foo = temp;</pre> +<h2><a name="Design">Design</a> considerations for Boost.Endian types</h2> +<ul> + <li>Must be suitable for I/O - in other words, must be memcpyable.</li> + <li>Must provide exactly the size and internal byte ordering specified.</li> + <li>Must work correctly when the internal integer representation has more bits + that the sum of the bits in the external byte representation. Sign extension + must work correctly when the internal integer representation type has more + bits than the sum of the bits in the external bytes. For example, using + a 64-bit integer internally to represent 40-bit (5 byte) numbers must work for + both positive and negative values.</li> + <li>Must work correctly (including using the same defined external + representation) regardless of whether a compiler treats char as signed or + unsigned.</li> + <li>Unaligned types must not cause compilers to insert padding bytes.</li> + <li>The implementation should supply optimizations with great care. Experience has shown that optimizations of endian + integers often become pessimizations when changing + machines or compilers. Pessimizations can also happen when changing compiler switches, + compiler versions, or CPU models of the same architecture.</li> +</ul> +<h2><a name="Experience">Experience</a></h2> +<p>Classes with similar functionality have been independently developed by +several Boost programmers and used very successful in high-value, high-use +applications for many years. These independently developed endian libraries +often evolved from C libraries that were also widely used. Endian types have proven widely useful across a wide +range of computer architectures and applications.</p> +<h2><a name="Motivating-use-cases">Motivating use cases</a></h2> +<p>Neil Mayhew writes: "I can also provide a meaningful use-case for this +library: reading TrueType font files from disk and processing the contents. The +data format has fixed endianness (big) and has unaligned values in various +places. Using Boost.Endian simplifies and cleans the code wonderfully."</p> +<h2><a name="C++0x">C++11</a></h2> +<p>The availability of the C++11 +<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2346.htm"> +Defaulted Functions</a> feature is detected automatically, and will be used if +present to ensure that objects of <code>class endian_arithmetic</code> are trivial, and +thus PODs.</p> +<h2><a name="Compilation">Compilation</a></h2> +<p>Boost.Endian is implemented entirely within headers, with no need to link to +any Boost object libraries.</p> +<p>Several macros allow user control over features:</p> +<ul> + <li>BOOST_ENDIAN_NO_CTORS causes <code>class endian_arithmetic</code> to have no + constructors. The intended use is for compiling user code that must be + portable between compilers regardless of C++11 + <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2346.htm"> + Defaulted Functions</a> support. Use of constructors will always fail, <br> + </li> + <li>BOOST_ENDIAN_FORCE_PODNESS causes BOOST_ENDIAN_NO_CTORS to be defined if + the compiler does not support C++11 + <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2346.htm"> + Defaulted Functions</a>. This is ensures that objects of <code>class endian_arithmetic</code> + are PODs, and so can be used in C++03 unions. + In C++11, <code>class endian_arithmetic</code> objects are PODs, even though they have + constructors, so can always be used in unions.</li> +</ul> +<h2><a name="Acknowledgements">Acknowledgements</a></h2> +<p>Original design developed by Darin Adler based on classes developed by Mark +Borgerding. Four original class templates combined into a single <code>endian_arithmetic</code> +class template by Beman Dawes, who put the library together, provided +documentation, added the typedefs, and also added the <code>unrolled_byte_loops</code> +sign partial specialization to correctly extend the sign when cover integer size +differs from endian representation size.</p> +<hr> +<p>Last revised: +<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->25 March, 2015<!--webbot bot="Timestamp" endspan i-checksum="28920" --></p> +<p>© Copyright Beman Dawes, 2006-2009, 2013</p> +<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> + +</body> + +</html>
\ No newline at end of file |