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<body class="article toc2 toc-left">
<div id="header">
<h1>Boost.System: Extensible Error Reporting</h1>
<div class="details">
<span id="author" class="author">Beman Dawes, Christopher Kohlhoff, Peter Dimov</span><br>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#introduction">Introduction</a></li>
<li><a href="#changes">Release History</a>
<ul class="sectlevel2">
<li><a href="#changes_in_boost_1_69">Changes in Boost 1.69</a></li>
<li><a href="#changes_in_boost_1_68">Changes in Boost 1.68</a></li>
<li><a href="#changes_in_boost_1_65">Changes in Boost 1.65</a></li>
</ul>
</li>
<li><a href="#rationale">Design Rationale</a></li>
<li><a href="#reference">Reference</a>
<ul class="sectlevel2">
<li><a href="#ref_use_of_c11_and_c14_features">Use of C&#43;&#43;11 and C&#43;&#43;14 Features</a></li>
<li><a href="#ref_macros">Macros</a></li>
<li><a href="#ref_deprecated_names">Deprecated Names</a></li>
<li><a href="#ref_boostsystemerror_code_hpp">&lt;boost/system/error_code.hpp&gt;</a>
<ul class="sectlevel3">
<li><a href="#ref_synopsis">Synopsis</a></li>
<li><a href="#ref_class_error_category">Class error_category</a></li>
<li><a href="#ref_predefined_categories">Predefined Categories</a></li>
<li><a href="#ref_class_error_code">Class error_code</a></li>
<li><a href="#ref_class_error_condition">Class error_condition</a></li>
<li><a href="#ref_nonmember_functions">Nonmember functions</a></li>
</ul>
</li>
<li><a href="#ref_boostsystemsystem_error_hpp">&lt;boost/system/system_error.hpp&gt;</a>
<ul class="sectlevel3">
<li><a href="#ref_class_system_error">Class system_error</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#history">History</a></li>
<li><a href="#acknowledgments">Acknowledgments</a></li>
<li><a href="#copyright">Copyright and License</a></li>
</ul>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="introduction">Introduction</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Error conditions originating from the operating system or other low-level
application program interfaces (API&#8217;s) are typically reported via an integer
representing an error code. When these low-level API calls are wrapped in
portable code, such as in a portable library, some users want to deal with the
error codes in portable ways. Other users need to get at the system specific
error codes, so they can deal with system specific needs. The Boost System
library provides simple, light-weight <code>error_code</code> objects that encapsulate
system-specific error code values, yet also provide access to more abstract
and portable error conditions via <code>error_condition</code> objects.</p>
</div>
<div class="paragraph">
<p>Because <code>error_code</code> objects can represent errors from sources other than the
operating system, including user-defined sources, each <code>error_code</code> and
<code>error_condition</code> has an associated <code>error_category</code>.</p>
</div>
<div class="paragraph">
<p>An exception class, <code>system_error</code>, is provided. Derived from
<code>std::runtime_error</code>, it captures the underlying <code>error_code</code> for the problem
causing the exception so that this important information is not lost.</p>
</div>
<div class="paragraph">
<p>While exceptions are the preferred C&#43;&#43; default error code reporting
mechanism, users of libraries dependent on low-level API&#8217;s often need overloads
reporting error conditions via error code arguments and/or return values rather
than via throwing exceptions. Otherwise, when errors are not exceptional
occurrences and must be dealt with as they arise, programs become littered with
try/catch blocks, unreadable, and inefficient. The Boost System library
supports both error reporting by exception and by error code.</p>
</div>
<div class="paragraph">
<p>In addition to portable errors codes and conditions supported by the
<code>error_code.hpp</code> header, system-specific headers support the Cygwin, Linux,
and Windows platforms. These headers are effectively no-ops if included for
platforms other than their intended target.</p>
</div>
<div class="paragraph">
<p>Boost.System is part of the C&#43;&#43;11 Standard Library.
A number of changes, particularly to names, were made by the C++ committee
during standardization. The Boost implementation has been tracking those changes.
See <a href="#ref_deprecated_names">Deprecated Names</a> for synonyms provided to prevent
breakage of existing user code.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="changes">Release History</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="changes_in_boost_1_69">Changes in Boost 1.69</h3>
<div class="ulist">
<ul>
<li>
<p>Boost.System is now header-only. A stub library is still built for
compatibility, but linking to it is no longer necessary.</p>
</li>
<li>
<p>Even more functions have been marked <code>constexpr</code>.</p>
</li>
<li>
<p>The destructor of <code>error_category</code> is now protected and no longer
virtual. This is a <em>potentially breaking change</em> but its impact
is expected to be limited.</p>
</li>
<li>
<p><code>error_category</code> now has a constructor that accepts a 64 bit identifier,
enabling distinct category objects to compare equal.</p>
</li>
<li>
<p>The constructors of <code>error_category</code> are now protected.</p>
</li>
<li>
<p>A non-allocating, nonthrowing overload of <code>message</code> has been added.</p>
</li>
<li>
<p>A virtual function <code>failed</code> has been added, allowing categories for
which success is not synonymous with 0.</p>
</li>
<li>
<p>The deprecated <code>boost::system::throws</code> object has been removed.</p>
</li>
<li>
<p><code>boost::throws()</code> is now deprecated and its use is discouraged.</p>
</li>
<li>
<p>The constructor of <code>system_error</code> taking a single <code>error_code</code> argument
is now explicit.</p>
</li>
<li>
<p><code>system_error::code()</code> now returns by value.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="changes_in_boost_1_68">Changes in Boost 1.68</h3>
<div class="paragraph">
<p>On a C&#43;&#43;14 compiler, many Boost.System functions and member functions
are now <code>constexpr</code>, and <code>error_code</code> and <code>error_condition</code> are literal
classes.</p>
</div>
<div class="paragraph">
<p>In addition to enabling use in constant expressions (and <code>constexpr</code>
functions), this significantly improves the quality of the generated code.</p>
</div>
<div class="paragraph">
<p>As a result of this change, however, now using Boost.System from C&#43;&#43;14
or C&#43;&#43;17 code requires that the library be also built with C&#43;&#43;14 or
above. This is the default on GCC 6 and newer, but not on GCC 5 or Clang.
One can build Boost for C&#43;&#43;14 by passing the <code>cxxstd=14</code> option to <code>b2</code>.</p>
</div>
<div class="paragraph">
<p>(Previous versions allowed code built against any C&#43;&#43; standard to link
with Boost.System built against any C&#43;&#43; standard. In 1.68, code using
any C&#43;&#43; standard can link with Boost.System built with C&#43;&#43;14 or above,
but if Boost.System is built with C&#43;&#43;11 or below, only code also built
with C&#43;&#43;11 and below can link to it successfully.)</p>
</div>
</div>
<div class="sect2">
<h3 id="changes_in_boost_1_65">Changes in Boost 1.65</h3>
<div class="paragraph">
<p>On a C&#43;&#43;11 compiler, Boost.System now provides implicit conversions
from <code>boost::system::error_category</code>, <code>error_code</code>, and <code>error_condition</code>
to their standard equivalents from <code>&lt;system_error&gt;</code>.</p>
</div>
<div class="paragraph">
<p>This allows libraries to expose a C&#43;&#43;11 interface and report errors
via <code>std::error_code</code> even when using Boost.System, directly or through a
dependency such as Boost.ASIO.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="rationale">Design Rationale</h2>
<div class="sectionbody">
<div class="paragraph">
<p><code>error_code</code> and <code>error_condition</code> are designed as value types so
they can be copied without slicing and do not require heap allocation, but
still have polymorphic behavior based on the error category. This is achieved
by abstract base class <code>error_category</code> supplying the polymorphic behavior,
and <code>error_code</code> and <code>error_condition</code> containing a pointer to an object of a
type derived from <code>error_category</code>.</p>
</div>
<div class="paragraph">
<p>Many of the detailed design decisions were driven by the requirements that
users to be able to add additional error categories, and that it be no more
difficult to write portable code than system-specific code.</p>
</div>
<div class="paragraph">
<p>The <code>operator&lt;&lt;</code> overload for <code>error_code</code> eliminates a misleading conversion to
<code>bool</code> in code like <code>cout &lt;&lt; ec</code>, where <code>ec</code> is of type <code>error_code</code>. It is also
useful in its own right.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="reference">Reference</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="ref_use_of_c11_and_c14_features">Use of C&#43;&#43;11 and C&#43;&#43;14 Features</h3>
<div class="paragraph">
<p>The library is documented to use several C&#43;&#43;11 and C&#43;&#43;14 features,
including <code>noexcept</code>, explicit conversion operators and <code>constexpr</code>. The
actual implementation uses C&#43;&#43;11 and C&#43;&#43;14 features only when they are
available, and otherwise falls back on C&#43;&#43;03 features.</p>
</div>
</div>
<div class="sect2">
<h3 id="ref_macros">Macros</h3>
<div class="paragraph">
<p>When <code>BOOST_SYSTEM_ENABLE_DEPRECATED</code> is defined, the library provides
deprecated features for compatibility. These features are bound to eventually
disappear.</p>
</div>
<div class="paragraph">
<p>When <code>BOOST_SYSTEM_USE_UTF8</code> is defined, on Windows the library returns
UTF-8 messages using code page <code>CP_UTF8</code> instead of the default <code>CP_ACP</code>.
This macro has no effect on POSIX.</p>
</div>
</div>
<div class="sect2">
<h3 id="ref_deprecated_names">Deprecated Names</h3>
<div class="paragraph">
<p>In the process of adding Boost.System to the C&#43;&#43;11 standard library, the
C&#43;&#43; committee changed some names. To ease transition, Boost.System deprecates
the old names, but will provide them when the macro <code>BOOST_SYSTEM_ENABLE_DEPRECATED</code> is defined.</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Old usage, now deprecated</th>
<th class="tableblock halign-left valign-top">Replacement</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>get_generic_category()</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>generic_category()</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>get_system_category()</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>system_category()</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>namespace posix</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>namespace errc</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>namespace posix_error</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>namespace errc</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>get_posix_category()</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>generic_category()</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>posix_category</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>generic_category()</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>errno_ecat</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>generic_category()</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>native_ecat</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>system_category()</code></p></td>
</tr>
</tbody>
</table>
</div>
<div class="sect2">
<h3 id="ref_boostsystemerror_code_hpp">&lt;boost/system/error_code.hpp&gt;</h3>
<div class="sect3">
<h4 id="ref_synopsis">Synopsis</h4>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>namespace boost {
  namespace system {

    class error_category;

    constexpr const error_category &amp; system_category() noexcept;
    constexpr const error_category &amp; generic_category() noexcept;

    class error_code;
    class error_condition;

    // "Concept" helpers

    template&lt;class T&gt;
      struct is_error_code_enum { static const bool value = false; };

    template&lt;class T&gt;
      struct is_error_condition_enum { static const bool value = false; };

    // generic error conditions

    namespace errc {
      enum errc_t
      {
        success = 0,
        address_family_not_supported,   //EAFNOSUPPORT
        address_in_use,                 //EADDRINUSE
        address_not_available,          //EADDRNOTAVAIL
        already_connected,              //EISCONN
        argument_list_too_long,         //E2BIG
        argument_out_of_domain,         //EDOM
        bad_address,                    //EFAULT
        bad_file_descriptor,            //EBADF
        bad_message,                    //EBADMSG
        broken_pipe,                    //EPIPE
        connection_aborted,             //ECONNABORTED
        connection_already_in_progress, //EALREADY
        connection_refused,             //ECONNREFUSED
        connection_reset,               //ECONNRESET
        cross_device_link,              //EXDEV
        destination_address_required,   //EDESTADDRREQ
        device_or_resource_busy,        //EBUSY
        directory_not_empty,            //ENOTEMPTY
        executable_format_error,        //ENOEXEC
        file_exists,                    //EEXIST
        file_too_large,                 //EFBIG
        filename_too_long,              //ENAMETOOLONG
        function_not_supported,         //ENOSYS
        host_unreachable,               //EHOSTUNREACH
        identifier_removed,             //EIDRM
        illegal_byte_sequence,          //EILSEQ
        inappropriate_io_control_operation, //ENOTTY
        interrupted,                    //EINTR
        invalid_argument,               //EINVAL
        invalid_seek,                   //ESPIPE
        io_error,                       //EIO
        is_a_directory,                 //EISDIR
        message_size,                   //EMSGSIZE
        network_down,                   //ENETDOWN
        network_reset,                  //ENETRESET
        network_unreachable,            //ENETUNREACH
        no_buffer_space,                //ENOBUFS
        no_child_process,               //ECHILD
        no_link,                        //ENOLINK
        no_lock_available,              //ENOLCK
        no_message_available,           //ENODATA
        no_message,                     //ENOMSG
        no_protocol_option,             //ENOPROTOOPT
        no_space_on_device,             //ENOSPC
        no_stream_resources,            //ENOSR
        no_such_device_or_address,      //ENXIO
        no_such_device,                 //ENODEV
        no_such_file_or_directory,      //ENOENT
        no_such_process,                //ESRCH
        not_a_directory,                //ENOTDIR
        not_a_socket,                   //ENOTSOCK
        not_a_stream,                   //ENOSTR
        not_connected,                  //ENOTCONN
        not_enough_memory,              //ENOMEM
        not_supported,                  //ENOTSUP
        operation_canceled,             //ECANCELED
        operation_in_progress,          //EINPROGRESS
        operation_not_permitted,        //EPERM
        operation_not_supported,        //EOPNOTSUPP
        operation_would_block,          //EWOULDBLOCK
        owner_dead,                     //EOWNERDEAD
        permission_denied,              //EACCES
        protocol_error,                 //EPROTO
        protocol_not_supported,         //EPROTONOSUPPORT
        read_only_file_system,          //EROFS
        resource_deadlock_would_occur,  //EDEADLK
        resource_unavailable_try_again, //EAGAIN
        result_out_of_range,            //ERANGE
        state_not_recoverable,          //ENOTRECOVERABLE
        stream_timeout,                 //ETIME
        text_file_busy,                 //ETXTBSY
        timed_out,                      //ETIMEDOUT
        too_many_files_open_in_system,  //ENFILE
        too_many_files_open,            //EMFILE
        too_many_links,                 //EMLINK
        too_many_synbolic_link_levels,  //ELOOP
        value_too_large,                //EOVERFLOW
        wrong_protocol_type             //EPROTOTYPE
      };

    } // namespace errc

    template&lt;&gt; struct is_error_condition_enum&lt;errc::errc_t&gt;
      { static const bool value = true; };

    // non-member functions

    constexpr bool operator==( const error_code &amp; lhs,
      const error_code &amp; rhs ) noexcept;
    bool operator==( const error_code &amp; code,
      const error_condition &amp; condition ) noexcept;
    bool operator==( const error_condition &amp; condition,
      const error_code &amp; code ) noexcept;
    constexpr bool operator==( const error_condition &amp; lhs,
      const error_condition &amp; rhs ) noexcept;

    constexpr bool operator!=( const error_code &amp; lhs,
      const error_code &amp; rhs ) noexcept;
    bool operator!=( const error_code &amp; code,
      const error_condition &amp; condition ) noexcept;
    bool operator!=( const error_condition &amp; condition,
      const error_code &amp; code ) noexcept;
    constexpr bool operator!=( const error_condition &amp; lhs,
      const error_condition &amp; rhs ) noexcept;

    constexpr bool operator&lt;( const error_code &amp; lhs,
      const error_code &amp; rhs ) noexcept;
    constexpr bool operator&lt;( const error_condition &amp; lhs,
      const error_condition &amp; rhs ) noexcept;

    constexpr error_code make_error_code( errc::errc_t e ) noexcept;
    constexpr error_condition make_error_condition( errc::errc_t e ) noexcept;

    template &lt;class charT, class traits&gt;
      std::basic_ostream&lt;charT, traits&gt;&amp;
        operator&lt;&lt;( basic_ostream&lt;charT, traits&gt;&amp; os, const error_code &amp; ec );

    std::size_t hash_value( const error_code &amp; ec );

  } // namespace system
} // namespace boost</code></pre>
</div>
</div>
<div class="paragraph">
<p>The value of each <code>errc_t</code> constant is the same as the value of the <code>&lt;cerrno&gt;</code>
macro shown in the above synopsis.</p>
</div>
<div class="paragraph">
<p>Users may specialize <code>is_error_code_enum</code> and <code>is_error_condition_enum</code>
templates to indicate that a type is eligible for class <code>error_code</code> and
<code>error_condition</code> automatic conversions respectively.</p>
</div>
</div>
<div class="sect3">
<h4 id="ref_class_error_category">Class error_category</h4>
<div class="paragraph">
<p>The class <code>error_category</code> defines the base class for types used
to identify the source and encoding of a particular category of error code.</p>
</div>
<div class="paragraph">
<p>Classes may be derived from <code>error_category</code> to support categories of
errors in addition to those defined in Boost.System.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>namespace boost {
  namespace system {

    class error_category
    {
    public: // noncopyable

      error_category( error_category const &amp; ) = delete;
      error_category&amp; operator=( error_category const &amp; ) = delete;

    protected:

      ~error_category() = default;

      constexpr error_category() noexcept;
      explicit constexpr error_category( unsigned long long id ) noexcept;

    public:

      virtual const char * name() const noexcept = 0;

      virtual error_condition default_error_condition( int ev ) const noexcept;

      virtual bool equivalent( int code, const error_condition &amp; condition )
        const noexcept;
      virtual bool equivalent( const error_code &amp; code, int condition )
        const noexcept;

      virtual std::string message( int ev ) const = 0;
      virtual char const * message( int ev, char * buffer, std::size_t len )
        const noexcept;

      virtual bool failed( int ev ) const noexcept;

      constexpr bool operator==( const error_category &amp; rhs ) const noexcept;
      constexpr bool operator!=( const error_category &amp; rhs ) const noexcept;
      constexpr bool operator&lt; ( const error_category &amp; rhs ) const noexcept;

      operator std::error_category const &amp; () const;

    private:

      unsigned long long id_; // exposition only

    };
  }
}</code></pre>
</div>
</div>
<div class="sect4">
<h5 id="ref_constructors">Constructors</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_category() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Effects: </dt>
<dd>
<p>Initializes <code>id_</code> to 0.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>Since equivalence for categories that do not have an identifier is
based on comparing object addresses, a user-defined derived category of type
<code>C</code> that uses this constructor should ensure that only one object of type <code>C</code>
exists in the program.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>explicit constexpr error_category( unsigned long long id ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Effects: </dt>
<dd>
<p>Initializes <code>id_</code> to <code>id</code>.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>User-defined derived categories that use this constructor are considered
equivalent when their identifiers match. Therefore, those categories may have more
than one instance existing in a program, but to minimize the possibility of
collision, their identifiers must be randomly chosen (at the time the category
is implemented, not at runtime). One way of generating a 64 bit random identifier
is <a href="https://www.random.org/cgi-bin/randbyte?nbytes=8&amp;format=h" class="bare">https://www.random.org/cgi-bin/randbyte?nbytes=8&amp;format=h</a>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_virtuals">Virtuals</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual const char * name() const noexcept = 0;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>In derived classes, a character literal naming the error category.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual error_condition default_error_condition( int ev ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<div class="ulist">
<ul>
<li>
<p>In derived classes, an error condition corresponding to <code>ev</code>.
The returned error condition will typically come from the generic category.</p>
</li>
<li>
<p>In the default implementation, <code>error_condition( ev, *this )</code>.</p>
</li>
</ul>
</div>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual bool equivalent( int code, const error_condition &amp; condition )
  const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<div class="ulist">
<ul>
<li>
<p>In derived classes, <code>true</code> when <code>error_code( code, *this )</code> is equivalent to <code>condition</code>.</p>
</li>
<li>
<p>In the default implementation, <code>default_error_condition( code ) == condition</code>.</p>
</li>
</ul>
</div>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual bool equivalent( const error_code &amp; code, int condition )
  const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<div class="ulist">
<ul>
<li>
<p>In derived classes, <code>true</code> when <code>code</code> is equivalent to <code>error_condition( condition, *this )</code>.</p>
</li>
<li>
<p>In the default implementation, <code>*this == code.category() &amp;&amp; code.value() == condition</code>.</p>
</li>
</ul>
</div>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual std::string message( int ev ) const = 0;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>In derived classes, a string that describes the error denoted by <code>ev</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual char const * message( int ev, char * buffer, std::size_t len )
  const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Effects: </dt>
<dd>
<div class="ulist">
<ul>
<li>
<p>Derived classes should either</p>
<div class="ulist">
<ul>
<li>
<p>return a pointer to a character literal describing the error denoted by <code>ev</code>, or</p>
</li>
<li>
<p>copy a string describing the error into <code>buffer</code>, truncating it to <code>len-1</code>
characters and storing a null terminator, and return <code>buffer</code>. If <code>len</code> is 0,
nothing is copied, but the function still returns <code>buffer</code>. Note that
when <code>len</code> is 0, <code>buffer</code> may be <code>nullptr</code>.</p>
</li>
</ul>
</div>
</li>
<li>
<p>The default implementation calls <code>message(ev)</code> and copies the result into
<code>buffer</code>, truncating it to <code>len-1</code> characters and storing a null terminator.
If <code>len</code> is 0, copies nothing. Returns <code>buffer</code>. If <code>message(ev)</code> throws an
exception, the string <code>"Message text unavailable"</code> is used.</p>
</li>
</ul>
</div>
</dd>
<dt class="hdlist1">Example: </dt>
<dd>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>const char* my_category::message(int ev, char* buffer, size_t len) const noexcept
{
  switch(ev)
  {
  case 0: return "no error";
  case 1: return "voltage out of range";
  case 2: return "impedance mismatch";
  case 31:
  case 32:
  case 33:
    std::snprintf(buffer, len, "component %d failure", ev-30);
    return buffer;
  default:
    std::snprintf(buffer, len, "unknown error %d", ev);
    return buffer;
  }
}</code></pre>
</div>
</div>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>virtual bool failed( int ev ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<div class="ulist">
<ul>
<li>
<p>In derived classes, <code>true</code> when <code>ev</code> represents a failure.</p>
</li>
<li>
<p>In the default implementation, <code>ev != 0</code>.</p>
</li>
</ul>
</div>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>All calls to this function with the same <code>ev</code> must return the same value.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_nonvirtuals">Nonvirtuals</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool operator==( const error_category &amp; rhs ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>rhs.id_ == 0? this == &amp;rhs: id_ == rhs.id_</code>.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>Two category objects are considered equivalent when they have matching
nonzero identifiers, or are the same object.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool operator!=( const error_category &amp; rhs ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>!( *this == rhs )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool operator&lt; ( const error_category &amp; rhs ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<div class="ulist">
<ul>
<li>
<p>If <code>id_ &lt; rhs.id_</code>, <code>true</code>;</p>
</li>
<li>
<p>Otherwise, if <code>id_ &gt; rhs.id_</code>, <code>false</code>;</p>
</li>
<li>
<p>Otherwise, if <code>rhs.id_ != 0</code>, <code>false</code>;</p>
</li>
<li>
<p>Otherwise, <code>std::less&lt;error_category const *&gt;()( this, &amp;rhs )</code>.</p>
</li>
</ul>
</div>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>operator std::error_category const &amp; () const;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>A reference to an <code>std::error_category</code> object corresponding
to <code>*this</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect3">
<h4 id="ref_predefined_categories">Predefined Categories</h4>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr const error_category &amp; system_category() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>A reference to an <code>error_category</code> object identifying errors
originating from the operating system.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr const error_category &amp; generic_category() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>A reference to an <code>error_category</code> object identifying portable
error conditions.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect3">
<h4 id="ref_class_error_code">Class error_code</h4>
<div class="paragraph">
<p>The class <code>error_code</code> describes an object used to hold error code
values, such as those originating from the operating system or other
low-level application program interfaces. It&#8217;s an adjunct to error reporting
by exception.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>namespace boost {
  namespace system {

    class error_code {
    public:

      // constructors:

      constexpr error_code() noexcept;
      constexpr error_code( int val, const error_category &amp; cat ) noexcept;

      template &lt;class ErrorCodeEnum&gt;
        constexpr error_code( ErrorCodeEnum e ) noexcept;

      // modifiers:

      constexpr void assign( int val, const error_category &amp; cat ) noexcept;

      template&lt;typename ErrorCodeEnum&gt;
        constexpr error_code &amp; operator=( ErrorCodeEnum e ) noexcept;

      constexpr void clear() noexcept;

      // observers:

      constexpr int value() const noexcept;
      constexpr const error_category &amp; category() const noexcept;

      error_condition default_error_condition() const noexcept;

      std::string message() const;
      char const * message( char * buffer, std::size_t len ) const noexcept;

      constexpr bool failed() const noexcept;
      constexpr explicit operator bool() const noexcept;

      operator std::error_code() const;

    private: // exposition only

      int val_;
      const error_category * cat_;

    };
  }
}</code></pre>
</div>
</div>
<div class="sect4">
<h5 id="ref_constructors_2">Constructors</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_code() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == 0</code>; <code>*cat_ == system_category()</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_code( int val, const error_category &amp; cat ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == val</code>; <code>cat_ == &amp;cat</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>template &lt;class ErrorCodeEnum&gt;
  constexpr error_code( ErrorCodeEnum e ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>*this == make_error_code( e )</code>.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>This constructor is only enabled when <code>is_error_code_enum&lt;ErrorCodeEnum&gt;::value</code> is <code>true</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_modifiers">Modifiers</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr void assign( int val, const error_category &amp; cat ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == val</code>; <code>cat_ == &amp;cat</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>template&lt;typename ErrorCodeEnum&gt;
  constexpr error_code &amp; operator=( ErrorCodeEnum e ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>*this == make_error_code( e )</code>.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>This operator is only enabled when <code>is_error_code_enum&lt;ErrorCodeEnum&gt;::value</code> is <code>true</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr void clear() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == 0</code>; <code>*cat_ == system_category()</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_observers">Observers</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr int value() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>val_</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr const error_category &amp; category() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>*cat_</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>error_condition default_error_condition() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;default_error_condition( val_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>std::string message() const;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;message( val_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>char const * message( char * buffer, std::size_t len ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;message( val_, buffer, len )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool failed() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;failed( val_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr explicit operator bool() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>val_ != 0</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>operator std::error_code() const;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>std::error_code( val_, *cat_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect3">
<h4 id="ref_class_error_condition">Class error_condition</h4>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>namespace boost {
  namespace system {

    class error_condition {
    public:

      // constructors:

      constexpr error_condition() noexcept;
      constexpr error_condition( int val, const error_category &amp; cat ) noexcept;

      template &lt;class ErrorConditionEnum&gt;
        constexpr error_condition( ErrorConditionEnum e ) noexcept;

      // modifiers:

      constexpr void assign( int val, const error_category &amp; cat ) noexcept;

      template&lt;typename ErrorConditionEnum&gt;
        constexpr error_condition &amp; operator=( ErrorConditionEnum e ) noexcept;

      constexpr void clear() noexcept;

      // observers:

      constexpr int value() const noexcept;
      constexpr const error_category &amp; category() const noexcept;

      std::string message() const;
      char const * message( char * buffer, std::size_t len ) const noexcept;

      constexpr bool failed() const noexcept;
      constexpr explicit operator bool() const noexcept;

      operator std::error_condition() const;

    private: // exposition only

      int val_;
      const error_category * cat_;

    };
  }
}</code></pre>
</div>
</div>
<div class="sect4">
<h5 id="ref_constructors_3">Constructors</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_condition() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == 0</code>; <code>*cat_ == generic_category()</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_condition( int val, const error_category &amp; cat ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == val</code>; <code>cat_ == &amp;cat</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>template &lt;class ErrorConditionEnum&gt;
  constexpr error_condition( ErrorConditionEnum e ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>*this == make_error_condition( e )</code>.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>This constructor is only enabled when <code>is_error_condition_enum&lt;ErrorConditionEnum&gt;::value</code> is <code>true</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_modifiers_2">Modifiers</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr void assign( int val, const error_category &amp; cat ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == val</code>; <code>cat_ == &amp;cat</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>template &lt;class ErrorConditionEnum&gt;
  constexpr error_condition &amp; operator=( ErrorConditionEnum e ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>*this == make_error_condition( e )</code>.</p>
</dd>
<dt class="hdlist1">Remarks: </dt>
<dd>
<p>This operator is only enabled when <code>is_error_condition_enum&lt;ErrorConditionEnum&gt;::value</code> is <code>true</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr void clear() noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>val_ == 0</code>; <code>*cat_ == generic_category()</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_observers_2">Observers</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr int value() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>val_</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr const error_category &amp; category() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>*cat_</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>std::string message() const;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;message( val_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>char const * message( char * buffer, std::size_t len ) const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;message( val_, buffer, len )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool failed() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>cat_-&gt;failed( val_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr explicit operator bool() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>val_ != 0</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>operator std::error_condition() const;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>std::error_condition( val_, *cat_ )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect3">
<h4 id="ref_nonmember_functions">Nonmember functions</h4>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool operator==( const error_code &amp; lhs,
  const error_code &amp; rhs ) noexcept;
constexpr bool operator==( const error_condition &amp; lhs,
  const error_condition &amp; rhs ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>lhs.value() == rhs.value() &amp;&amp; lhs.category() == rhs.category()</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>bool operator==( const error_code &amp; code,
  const error_condition &amp; condition ) noexcept;
bool operator==( const error_condition &amp; condition,
  const error_code &amp; code ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool operator!=( const error_code &amp; lhs,
  const error_code &amp; rhs ) noexcept;
constexpr bool operator!=( const error_condition &amp; lhs,
  const error_condition &amp; rhs ) noexcept;
bool operator!=( const error_code &amp; code,
  const error_condition &amp; condition ) noexcept;
bool operator!=( const error_condition &amp; condition,
  const error_code &amp; code ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>!( lhs == rhs )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr bool operator&lt;( const error_code &amp; lhs,
  const error_code &amp; rhs ) noexcept;
constexpr bool operator&lt;( const error_condition &amp; lhs,
  const error_condition &amp; rhs ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>lhs.category() &lt; rhs.category() || ( lhs.category() == rhs.category() &amp;&amp; lhs.value() &lt; rhs.value() )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_code make_error_code( errc::errc_t e ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>error_code( e, generic_category() )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>constexpr error_condition make_error_condition( errc::errc_t e ) noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>error_condition( e, generic_category() )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>template &lt;class charT, class traits&gt;
  std::basic_ostream&lt;charT, traits&gt;&amp;
    operator&lt;&lt;( basic_ostream&lt;charT, traits&gt;&amp; os, const error_code &amp; ec );</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Effects: </dt>
<dd>
<p><code>os &lt;&lt; ec.category().name() &lt;&lt; ':' &lt;&lt; ec.value()</code>.</p>
</dd>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>os</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>std::size_t hash_value( const error_code &amp; ec );</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>A hash value representing <code>ec</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect2">
<h3 id="ref_boostsystemsystem_error_hpp">&lt;boost/system/system_error.hpp&gt;</h3>
<div class="sect3">
<h4 id="ref_class_system_error">Class system_error</h4>
<div class="paragraph">
<p>The class <code>system_error</code> describes an exception object used to
report errors that have an associated <code>error_code</code>. Such errors
typically originate from operating system or other low-level
application program interfaces.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>namespace boost
{
  namespace system
  {
    class system_error: public std::runtime_error
    {
    public:

      explicit system_error( error_code ec );
      system_error( error_code ec, const char * what_arg );
      system_error( error_code ec, const std::string &amp; what_arg );

      system_error( int ev, const error_category &amp; ecat );
      system_error( int ev, const error_category &amp; ecat,
        const char * what_arg );
      system_error( int ev, const error_category &amp; ecat,
        const std::string &amp; what_arg );

      error_code code() const noexcept;
      const char * what() const noexcept;
    };
  }
}</code></pre>
</div>
</div>
<div class="sect4">
<h5 id="ref_constructors_4">Constructors</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>explicit system_error( error_code ec );
system_error( error_code ec, const char * what_arg );
system_error( error_code ec, const std::string &amp; what_arg );</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>code() == ec</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>system_error( int ev, const error_category &amp; ecat,
  const char * what_arg );
system_error( int ev, const error_category &amp; ecat,
  const std::string &amp; what_arg );
system_error( int ev, const error_category &amp; ecat );</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Ensures: </dt>
<dd>
<p><code>code() == error_code( ev, ecat )</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
<div class="sect4">
<h5 id="ref_observers_3">Observers</h5>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>error_code code() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p><code>ec</code> or <code>error_code( ev, ecat )</code>, from the constructor, as appropriate.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code>const char * what() const noexcept;</code></pre>
</div>
</div>
<div class="ulist none">
<ul class="none">
<li>
<p></p>
<div class="dlist">
<dl>
<dt class="hdlist1">Returns: </dt>
<dd>
<p>A null-terminated character string incorporating the arguments supplied
in the constructor, typically of the form <code>what_arg + ": " + code.message()</code>.</p>
</dd>
</dl>
</div>
</li>
</ul>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="history">History</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1975.html">N1975</a>,
Filesystem Library Proposal for TR2, accepted for Library Technical
Report 2 (TR2) at the Berlin meeting, included additional components to
supplement the Standard Library&#8217;s Diagnostics clause. Since then, these error
reporting components have received wider public scrutiny and enhancements have
been made to the design. The enhanced version has been used by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2054.pdf">N2054</a>,
Networking  Library Proposal for TR2, demonstrating that these error reporting
components are useful beyond the original Filesystem Library.</p>
</div>
<div class="paragraph">
<p>The original proposal viewed error categories as a binary choice between
<code>errno</code> (i.e. POSIX-style) and the native operating system&#8217;s error
codes. The proposed components now allow as many additional error categories as
are needed by either implementations or by users. The need to support additional
error categories, for example, occurs in some networking library implementations
because they are built on top of the POSIX <code>getaddrinfo</code> API that
uses error codes not based on <code>errno</code>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="acknowledgments">Acknowledgments</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Christopher Kohlhoff and Peter Dimov made important contributions to the
design. Comments and suggestions were also received from Pavel Vozenilek,
Gennaro Prota, Dave Abrahams, Jeff Garland, Iain Hanson, Oliver Kowalke, and
Oleg Abrosimov. Christopher Kohlhoff suggested several improvements to the
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2066.html">N2066</a>
paper. Johan Nilsson&#8217;s comments led to several of the refinements in N2066.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="copyright">Copyright and License</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This documentation is</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Copyright 2003-2017 Beman Dawes</p>
</li>
<li>
<p>Copyright 2018 Peter Dimov</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>and is distributed under the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Version 1.0</a>.</p>
</div>
</div>
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