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<html>
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<head>
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<title>Boost System Library</title>
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<link rel="stylesheet" type="text/css" href="../../../doc/src/minimal.css">
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<body>
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<font size="7">System Library</font>
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Boost Home
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Library Home Tutorial
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Reference
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Contents
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Introduction
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Release History
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Design Rationale
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History
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Acknowledgements
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Headers
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<boost/system/error_code.hpp>
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<boost/system/system_error.hpp>
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<boost/system/cygwin_error.hpp>
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<boost/system/linux_error.hpp>
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<boost/system/windows_error.hpp>
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Error conditions originating from the operating system or other low-level
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application program interfaces (API's) are typically reported via an integer
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representing an error code. When these low-level API calls are wrapped in
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portable code, such as in a portable library, some users want to deal with the
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error codes in portable ways. Other users need to get at the system specific
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error codes, so they can deal with system specific needs. The Boost System
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library provides simple, light-weight
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error_code objects that encapsulate system-specific error code values, yet
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also provide access to more abstract and portable error conditions via
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error_condition objects.
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Because error_code objects can represent errors from sources other than the
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operating system, including user-defined sources, each error_code and
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error_condition has an associated
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error_category.
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An exception class,
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system_error, is provided. Derived from std::runtime_error, it captures the
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underlying error_code for the problem causing the exception so that this
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important information is not lost.
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While exceptions are the preferred C++ default error code reporting
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mechanism, users of libraries dependent on low-level API's often need overloads
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reporting error conditions via error code arguments and/or return values rather
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than via throwing exceptions. Otherwise, when errors are not exceptional
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occurrences and must be dealt with as they arise, programs become littered with
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try/catch blocks, unreadable, and very inefficient. The Boost System library
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supports both error reporting by exception and by error code.
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In addition to portable errors codes and conditions supported by the
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error_code.hpp header, system-specific headers support the Cygwin, Linux,
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and Windows platforms. These headers are effectively no-ops if included for
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platforms other than their intended target.
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The Boost System Library is part of the C++11 Standard Library.
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A number of changes, particularly to names, were made by the C++ committee
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during standardization. The Boost implementation is tracking those changes.
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See Deprecated names for
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synonyms provided to prevent breakage of existing user code. See
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Breaking changes for changes
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that unavoidably break existing user code. All breaking changes are noisy
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and will cause compile-time errors.
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system-2014-06-02: First modular Boost release. Minor
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maintenance changes.
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Class error_code and error_condition are designed as a value types so
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they can be copied
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without slicing and do not requiring heap allocation, but still have polymorphic
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behavior based on the error category. This is achieved by abstract base class
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error_category supplying the polymorphic behavior, and
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error_code and error_condition containing a pointer to an object of a type derived from
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error_category.
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Many of the detailed design decisions were driven by the requirements that
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users to be able to add additional error categories, and that it be no more
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difficult to write portable code than system-specific code.
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The operator<< overload for error_code eliminates a
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misleading conversion to bool in code like cout << ec, where
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ec is of type error_code. It is also useful in its own
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right.
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N1975, Filesystem Library Proposal for TR2, accepted for Library Technical
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Report 2 (TR2) at the Berlin meeting, included additional components to
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supplement the Standard Library's Diagnostics clause. Since then, these error
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reporting components have received wider public scrutiny and enhancements have
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been made to the design. The enhanced version has been used by N2054, Networking
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Library Proposal for TR2, demonstrating that these error reporting components
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are useful beyond the original Filesystem Library.
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The original proposal viewed error categories as a binary choice between
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errno (i.e. POSIX-style) and the native operating system's error
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codes. The proposed components now allow as many additional error categories as
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are needed by either implementations or by users. The need to support additional
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error categories, for example, occurs in some networking library implementations
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because they are built on top of the POSIX getaddrinfo API that
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uses error codes not based on errno.
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Christopher Kohlhoff and Peter Dimov made important contributions to the
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design. Comments and suggestions were also received from Pavel Vozenilek,
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Gennaro Prota, Dave Abrahams, Jeff Garland, Iain Hanson, Oliver Kowalke, and
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Oleg Abrosimov. Christopher Kohlhoff suggested several improvements to the N2066
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paper. Johan Nilsson's comments led to several of the refinements in N2066 .
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Revised
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June 02, 2014 </font>
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© Copyright Beman Dawes, 1999
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Distributed under the Boost Software License, Version 1.0.
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(See file LICENSE_1_0.txt
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or www.boost.org/LICENSE_1_0.txt)
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</body>
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</html>
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