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| 20 | <font size="7">System Library</font> | |
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| 25 | <table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" bgcolor="#D7EEFF" width="100%"> | |
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| 27 | <td><a href="../../../index.htm">Boost Home</a> <a href="index.html"> | |
| 28 | Library Home</a> Tutorial <a href="reference.html"> | |
| 29 | Reference</a></td> | |
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| 33 | <table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" align="right"> | |
| 34 | <tr> | |
| 35 | <td width="100%" bgcolor="#D7EEFF" align="center"> | |
| 36 | <i><b>Contents</b></i></td> | |
| 37 | </tr> | |
| 38 | <tr> | |
| 39 | <td width="100%" bgcolor="#E8F5FF"> | |
| 40 | <a href="#Introduction">Introduction</a><br> | |
| 41 | <a href="#Release_History">Release History</a><br> | |
| 42 | <a href="#Design_Rationale">Design Rationale</a><br> | |
| 43 | <a href="#History">History</a><br> | |
| 44 | <a href="#Acknowledgements">Acknowledgements</a> | |
| 45 | </td> | |
| 46 | </tr> | |
| 47 | <tr> | |
| 48 | <td width="100%" bgcolor="#D7EEFF" align="center"> | |
| 49 | <b><i>Headers</i></b></td> | |
| 50 | </tr> | |
| 51 | <tr> | |
| 52 | <td width="100%" bgcolor="#E8F5FF"> | |
| 53 | <a href="reference.html#Header-error_code"><boost/system/error_code.hpp></a><br> | |
| 54 | <a href="reference.html#Header-system_error"> | |
| 55 | <boost/system/system_error.hpp></a><br> | |
| 56 | <a href="../../../boost/system/cygwin_error.hpp"> | |
| 57 | <boost/system/cygwin_error.hpp></a><br> | |
| 58 | <a href="../../../boost/system/linux_error.hpp"> | |
| 59 | <boost/system/linux_error.hpp></a><br> | |
| 60 | <a href="../../../boost/system/windows_error.hpp"> | |
| 61 | <boost/system/windows_error.hpp></a></td> | |
| 62 | </tr> | |
| 63 | </table> | |
| 64 | ||
| 65 | <h2><a name="Introduction">Introduction</a></h2> | |
| 66 | ||
| 67 | <p>Error conditions originating from the operating system or other low-level | |
| 68 | application program interfaces (API's) are typically reported via an integer | |
| 69 | representing an error code. When these low-level API calls are wrapped in | |
| 70 | portable code, such as in a portable library, some users want to deal with the | |
| 71 | error codes in portable ways. Other users need to get at the system specific | |
| 72 | error codes, so they can deal with system specific needs. The Boost System | |
| 73 | library provides simple, light-weight <a href="reference.html#Class-error_code"> | |
| 74 | error_code</a> objects that encapsulate system-specific error code values, yet | |
| 75 | also provide access to more abstract and portable error conditions via | |
| 76 | <a href="reference.html#Class-error_condition">error_condition</a> objects. | |
| 77 | Because error_code objects can represent errors from sources other than the | |
| 78 | operating system, including user-defined sources, each error_code and | |
| 79 | error_condition has an associated <a href="reference.html#Class-error_category"> | |
| 80 | error_category</a>.</p> | |
| 81 | ||
| 82 | <p>An exception class, <a href="reference.html#Class-system_error"> | |
| 83 | system_error</a>, is provided. Derived from std::runtime_error, it captures the | |
| 84 | underlying error_code for the problem causing the exception so that this | |
| 85 | important information is not lost.</p> | |
| 86 | <p>While exceptions are the preferred C++ default error code reporting | |
| 87 | mechanism, users of libraries dependent on low-level API's often need overloads | |
| 88 | reporting error conditions via error code arguments and/or return values rather | |
| 89 | than via throwing exceptions. Otherwise, when errors are not exceptional | |
| 90 | occurrences and must be dealt with as they arise, programs become littered with | |
| 91 | try/catch blocks, unreadable, and very inefficient. The Boost System library | |
| 92 | supports both error reporting by exception and by error code.</p> | |
| 93 | <p>In addition to portable errors codes and conditions supported by the <code> | |
| 94 | error_code.hpp</code> header, system-specific headers support the Cygwin, Linux, | |
| 95 | and Windows platforms. These headers are effectively no-ops if included for | |
| 96 | platforms other than their intended target.</p> | |
| 97 | <table border="1" cellpadding="10" cellspacing="1" style="border-collapse: collapse" bordercolor="#111111"> | |
| 98 | <tr> | |
| 99 | <td>The Boost System Library is part of the C++11 Standard Library. | |
| 100 | A number of changes, particularly to names, were made by the C++ committee | |
| 101 | during standardization. The Boost implementation is tracking those changes. | |
| 102 | See <a href="reference.html#Deprecated-names">Deprecated names</a> for | |
| 103 | synonyms provided to prevent breakage of existing user code. See | |
| 104 | <a href="reference.html#Breaking-changes">Breaking changes</a> for changes | |
| 105 | that unavoidably break existing user code. All breaking changes are noisy | |
| 106 | and will cause compile-time errors.</td> | |
| 107 | </tr> | |
| 108 | </table> | |
| 109 | <h2><a name="Release_History">Release History</a></h2> | |
| 110 | <p><code>system-2014-06-02</code>: First modular Boost release. Minor | |
| 111 | maintenance changes.</p> | |
| 112 | <h2><a name="Design_Rationale">Design Rationale</a></h2> | |
| 113 | <p>Class <code>error_code</code> and <code>error_condition</code> are designed as a value types so | |
| 114 | they can be copied | |
| 115 | without slicing and do not requiring heap allocation, but still have polymorphic | |
| 116 | behavior based on the error category. This is achieved by abstract base class | |
| 117 | <code>error_category</code> supplying the polymorphic behavior, and <code> | |
| 118 | error_code</code> and <code>error_condition</code> containing a pointer to an object of a type derived from <code> | |
| 119 | error_category</code>.</p> | |
| 120 | <p>Many of the detailed design decisions were driven by the requirements that | |
| 121 | users to be able to add additional error categories, and that it be no more | |
| 122 | difficult to write portable code than system-specific code.</p> | |
| 123 | <p>The <code>operator<<</code> overload for <code>error_code</code> eliminates a | |
| 124 | misleading conversion to bool in code like <code>cout << ec</code>, where <code> | |
| 125 | ec</code> is of type <code>error_code</code>. It is also useful in its own | |
| 126 | right.</p> | |
| 127 | <h2><a name="History">History</a></h2> | |
| 128 | <p><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1975.html"> | |
| 129 | N1975</a>, Filesystem Library Proposal for TR2, accepted for Library Technical | |
| 130 | Report 2 (TR2) at the Berlin meeting, included additional components to | |
| 131 | supplement the Standard Library's Diagnostics clause. Since then, these error | |
| 132 | reporting components have received wider public scrutiny and enhancements have | |
| 133 | been made to the design. The enhanced version has been used by N2054, Networking | |
| 134 | Library Proposal for TR2, demonstrating that these error reporting components | |
| 135 | are useful beyond the original Filesystem Library.</p> | |
| 136 | <p>The original proposal viewed error categories as a binary choice between | |
| 137 | <code>errno</code> (i.e. POSIX-style) and the native operating system's error | |
| 138 | codes. The proposed components now allow as many additional error categories as | |
| 139 | are needed by either implementations or by users. The need to support additional | |
| 140 | error categories, for example, occurs in some networking library implementations | |
| 141 | because they are built on top of the POSIX <code>getaddrinfo</code> API that | |
| 142 | uses error codes not based on <code>errno</code>.</p> | |
| 143 | <h2><a name="Acknowledgements">Acknowledgements</a></h2> | |
| 144 | <p>Christopher Kohlhoff and Peter Dimov made important contributions to the | |
| 145 | design. Comments and suggestions were also received from Pavel Vozenilek, | |
| 146 | Gennaro Prota, Dave Abrahams, Jeff Garland, Iain Hanson, Oliver Kowalke, and | |
| 147 | Oleg Abrosimov. Christopher Kohlhoff suggested several improvements to the N2066 | |
| 148 | paper. Johan Nilsson's comments led to several of the refinements in N2066 .</p> | |
| 149 | <hr> | |
| 150 | ||
| 151 | <p>Revised | |
| 152 | <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%B %d, %Y" startspan -->June 02, 2014<!--webbot bot="Timestamp" endspan i-checksum="13984" --> </font> | |
| 153 | </p> | |
| 154 | ||
| 155 |