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| <a href="index.html">Endian Home</a> |
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| <a href="arithmetic.html">Arithmetic Types</a> |
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| <i><b>Contents</b></i></td> |
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| <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> |
