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      <i><b>Contents</b></i></td>
  </tr>
  <tr>
    <td width="100%" bgcolor="#E8F5FF">
      <a href="#Introduction">Introduction</a><br>
      <a href="#C++11">C++11</a><br>
      <a href="#Macros">Macros</a><br>
      <a href="#Deprecated-names">Deprecated names</a><br>
      <a href="#Breaking-changes">Breaking changes</a><br>
      <a href="#Header-error_code">Header &lt;boost/system/error_code.hpp&gt;</a><br>
      <a href="#Class-error_category">Class <code>error_category</code></a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_category-synopsis">Class <code>error_category</code> synopsis</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_category-virtual-members">Class <code>error_category</code> virtual members</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_category-non-virtual-members">Class <code>error_category</code> non-virtual members</a><br>
&nbsp;&nbsp; <a href="#Class-error_category-non-member-functions">Class <code>error_category</code> 
      non-member functions</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_category-predefined-objects">Class <code>error_category</code> predefined objects</a><br>
      <a href="#Class-error_code">Class <code>error_code</code></a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_code-synopsis">Class <code>error_code</code> synopsis</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_code-constructors">Class <code>error_code</code> constructors</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_code-modifiers">Class <code>error_code</code> modifiers</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_code-observers">Class <code>error_code</code> observers</a><br>
      <a href="#Class-error_condition">Class <code>error_condition</code></a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_condition-synopsis">Class <code>error_condition</code> synopsis</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_condition-constructors">Class <code>error_condition</code> constructors</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_condition-modifiers">Class <code>error_condition</code> modifiers</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-error_condition-observers">Class <code>error_condition</code> observers</a><br>
      <a href="#Function-boost-throws">Function <code>boost::throws()</code></a><br>
      <a href="#Non-member-functions">Non-member functions</a><br>
      <a href="#Header-system_error">Header &lt;boost/system/system_error.hpp&gt;</a><br>
      &nbsp;&nbsp;&nbsp;<a href="#Class-system_error">Class <code>system_error</code></a><br>
    </td>
  </tr>
</table>

<h2><a name="Introduction">Introduction</a></h2>

<p>This reference documentation describes components that&nbsp;programs may use to report error conditions originating from the operating 
system or other low-level application program interfaces.</p>
<p>Boost.System library components never change the value of <code>
errno</code>.</p>
<h2><a name="C++11">C++11</a></h2>
<p>The library is documented to use several C++11 features, including <code>
noexcept</code> and explicit conversion operators. The actual implementation 
uses C++11 features only when they are available, and otherwise falls back on 
C++03 features.</p>
<h2><a name="Macros">Macros</a></h2>
<p>Users may define the following macros if desired. Sensible defaults are
provided, so users may ignore these macros if they prefer.</p>
<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" height="368">
  <tr>
    <td height="16"><b><i>Macro Name</i></b></td>
    <td height="16"><b><i>Default</i></b></td>
    <td height="16"><b><i>Effect if defined</i></b></td>
  </tr>
  <tr>
    <td valign="top" height="64"><code>BOOST_ERROR_CODE_HEADER_ONLY</code></td>
    <td valign="top" height="64">Not defined.</td>
    <td valign="top" height="64">The implementation is header-only, and the 
    Boost.System library is not built. Overrides other link and library macros.</td>
  </tr>
  <tr>
    <td valign="top" height="64"><code>BOOST_SYSTEM_DYN_LINK</code></td>
    <td valign="top" height="64">Defined if <code>BOOST_ALL_DYN_LINK</code> is defined, 
    otherwise not defined.</td>
    <td valign="top" height="64">Boost.System library is dynamically linked. If not defined, 
    static linking is assumed.</td>
  </tr>
  <tr>
    <td valign="top" height="47"><code>BOOST_SYSTEM_NO_LIB</code></td>
    <td valign="top" height="47">Defined if <code>BOOST_ALL_NO_LIB</code> is defined, 
    otherwise not defined.</td>
    <td valign="top" height="47">Boost.System library does not use the Boost auto-link 
    facility.</td>
  </tr>
  <tr>
    <td valign="top" height="32"><code>BOOST_SYSTEM_ENABLE_DEPRECATED</code></td>
    <td valign="top" height="32">Not defined.</td>
    <td valign="top" height="32">Deprecated features are provided for compatibility.</td>
  </tr>
  </table>
<h2><a name="Deprecated-names">Deprecated names</a></h2>
<p>In the process of adding Boost.System to the C++11 standard library, the
C++ 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>
<table border="1" cellpadding="5" cellspacing="1" style="border-collapse: collapse" bordercolor="#111111">
  <tr>
    <td><b><i>Old usage, now deprecated</i></b></td>
    <td><b><i>Replacement</i></b></td>
  </tr>
  <tr>
    <td><code>get_generic_category()</code></td>
    <td><code>generic_category()</code></td>
  </tr>
  <tr>
    <td><code>get_system_category()</code></td>
    <td><code>system_category()</code></td>
  </tr>
  <tr>
    <td><code>namespace posix</code></td>
    <td><code>namespace errc</code></td>
  </tr>
  <tr>
    <td><code>namespace posix_error</code></td>
    <td><code>namespace errc</code></td>
  </tr>
  <tr>
    <td><code>enum posix_errno</code></td>
    <td><code>enum errc_t</code></td>
  </tr>
  <tr>
    <td><code>get_posix_category()</code></td>
    <td><code>generic_category()</code></td>
  </tr>
  <tr>
    <td><code>posix_category</code></td>
    <td><code>generic_category()</code></td>
  </tr>
  <tr>
    <td><code>errno_ecat</code></td>
    <td><code>generic_category()</code></td>
  </tr>
  <tr>
    <td><code>native_ecat</code></td>
    <td><code>system_category()</code></td>
  </tr>
</table>
<h2><a name="Breaking-changes">Breaking changes</a></h2>
<p>Two static consts are replaced by functions. These are breaking changes best 
fixed by globally adding () to these names to turn them into function calls.</p>
<table border="1" cellpadding="5" cellspacing="1" style="border-collapse: collapse" bordercolor="#111111">
  <tr>
    <td><b><i>Old usage, now broken</i></b></td>
    <td><b><i>Replacement</i></b></td>
  </tr>
  <tr>
    <td><code>generic_category</code></td>
    <td><code>generic_category()</code></td>
  </tr>
  <tr>
    <td><code>system_category</code></td>
    <td><code>system_category()</code></td>
  </tr>
  </table>
<p>User-defined BOOST_POSIX_API and BOOST_WINDOWS_API macros are no longer supported.</p>
<h2><a name="Header-error_code">Header &lt;boost/system/error_code.hpp&gt;</a></h2>
<h3>&lt;boost/system/error_code.hpp&gt; synopsis</h3>
<blockquote>
<pre>namespace boost
{
  namespace system
  {
    class <a href="#Class-error_category">error_category</a>;
    const error_category &amp;  <a href="#system_category">system_category</a>() noexcept;
    const error_category &amp;  <a href="#generic_category">generic_category</a>() noexcept;

    class <a href="#Class-error_code">error_code</a>;
    class <a href="#Class-error_condition">error_condition</a>;

    //  "Concept" helpers
    template< class T="" >
      struct is_error_code_enum { static const bool value = false; };

    template< class T="" >
      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<> struct is_error_condition_enum<posix::posix_errno>&lt;errc::errc_t&gt;
      { static const bool value = true; };

    //  <a href="#Non-member-functions">non-member functions</a>

    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;
    bool operator==( const error_condition &amp; lhs, const error_condition &amp; rhs ) noexcept;

    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;
    bool operator!=( const error_condition &amp; lhs, const error_condition &amp; rhs ) noexcept;

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

    error_code make_error_code( errc::errc_t e ) noexcept;
    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 );

    size_t hash_value( const error_code &amp; ec );

  } // namespace system

  system::error_code &amp; throws();
    
} // namespace boost</pre>
</blockquote>
<p>The value of each<code> errc_t</code> constant shall be the same as the 
value of the <code>&lt;cerrno&gt;</code> macro shown in the above synopsis.</p>
<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>

<pre>const error_category &amp; <a name="system_category">system_category</a>();</pre>
<blockquote>
  <p><i>Returns:</i> A reference to a <code>error_category</code> object 
  identifying errors originating from the operating system.</p>
</blockquote>
<pre>const error_category &amp; <a name="generic_category">generic_category</a>();</pre>
<blockquote>
  <p><i>Returns:</i> A reference to a <code>error_category</code> object 
  identifying portable error conditions.</p>
</blockquote>

<h2><a name="Class-error_category">Class <code>error_category</code></a></h2>
<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>
<p><i>[Note:</i> Classes may be derived from <code>error_category</code> 
to support additional categories of errors. <i>--end note]</i></p>
<p>The class <code>error_category</code> serves as a base class for types used 
to identify the source and encoding of a particular category of error code. 
Classes may be derived from <code>error_category</code> to support categories of 
errors in addition to those defined in the Boost System library. Such classes 
shall behave as specified in this subclause. [<i> Note:</i> <code>error_category</code> 
objects are passed by reference, and two such objects are equal if they have the 
same address. This means that applications using custom <code>error_category</code> 
types should create a single object of each such type. <i>—end note</i> ]</p>
<h3><a name="Class-error_category-synopsis">Class <code>error_category</code> synopsis</a></h3>
<blockquote>
<pre>namespace boost
{
  namespace system
  {
    class error_category : public noncopyable
    {
    public:
      virtual ~error_category();

      virtual const char *     name() const noexcept = 0;
      virtual string           message( int ev ) const = 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;

      bool operator==( const error_category &amp; rhs ) const noexcept;
      bool operator!=( const error_category &amp; rhs ) const noexcept;
      bool operator&lt; ( const error_category &amp; rhs ) const noexcept;
    };
  }
}</pre>
</blockquote>
<h3><a name="Class-error_category-virtual-members">Class <code>error_category</code> virtual members</a></h3>
<p>Classes derived from <code>error_category</code> shall behave as specified in 
this subclause.</p>
<pre>virtual const char * name() const noexcept =0;</pre>
<blockquote>
  <p><i>Returns: </i>a string naming the error category.</p>
</blockquote>
<pre>virtual string message( int ev ) const noexcept =0;</pre>
<blockquote>
  <p><i>Returns:</i> A string that describes the error denoted by
  <code>ev</code>.</p>
</blockquote>
<pre>virtual error_condition default_error_condition( int ev ) const noexcept;</pre>
<blockquote>
  <p><i>Returns:</i>&nbsp; <code>error_condition( ev, *this )</code>.</p>
  <blockquote>
    <p dir="ltr">&nbsp;[<i>--Note:</i> Derived classes will typically convert <code>ev</code> 
    to some portable <code>error_category</code>, such as <code>generic_category()</code>, 
    and return it as an <code>error_condition</code> for that category. <i>--end 
    note</i>]</p>
  </blockquote>
  </blockquote>
<pre>virtual bool equivalent( int code, const error_condition &amp; condition ) const noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>default_error_condition( code ) == condition</code>.</p>
  </blockquote>
<pre>virtual bool equivalent( const error_code &amp; code, int condition ) const noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>*this == code.category() &amp;&amp; code.value() == condition</code>.</p>
  </blockquote>
<h3><a name="Class-error_category-non-virtual-members">Class <code>error_category</code> non-virtual members</a></h3>
<pre>bool operator==( const error_category &amp; rhs ) const noexcept;</pre>
<blockquote>
<p><i>Returns:</i> <code>this == &amp;rhs</code>.</p>
</blockquote>
<pre>bool operator!=( const error_category &amp; rhs ) const noexcept;</pre>
<blockquote>
<p><i>Returns:</i> <code>this != &amp;rhs</code>.</p>
</blockquote>

<pre>bool operator&lt;( const error_category &amp; rhs ) const noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>std::less&lt;const error_category*&gt;()( this, &amp;rhs&nbsp; 
  noexcept)</code>.</p>
  <blockquote>
    <p><i>[Note:</i> <code>std::less</code> provides a total ordering for 
    pointers. <i>--end note]</i></p>
  </blockquote>
</blockquote>
<h2><a name="Class-error_code">Class <code>
error_code</code></a></h2>
<p>The class <code>error_code</code> describes an object used to hold error code 
values, such as those originating from the operating<br>
system or other low-level application program interfaces. <i>[ Note: </i>Class
<code>error_code</code> is an adjunct to error reporting by<br>
exception. <i>—end note ]</i></p>
<h3><a name="Class-error_code-synopsis">Class <code>
error_code</code> synopsis</a></h3>
<blockquote>
<pre>namespace boost
{
  namespace system
  {
    class error_code {
    public:

      // constructors:
      error_code() noexcept;
      error_code( val, const error_category &amp; cat ) noexcept;
      template &lt;class <code>ErrorCodeEnum</code>&gt;
        error_code(<code> ErrorCodeEnum</code> e ) noexcept;

      // modifiers:
      void assign( int val, const error_category &amp; cat ) noexcept;
      template&lt;typename <code>ErrorCodeEnum</code>&gt;
        error_code &amp; operator=( <code>ErrorCodeEnum</code> val ) noexcept;
      void clear() noexcept;

      // observers:
      int                    value() const noexcept;
      cont error_category &amp;  category() const noexcept;
      error_condition        default_error_condition() const noexcept;
      string                 message() const;
      operator unspecified-bool-type() const;

    private:
      int val_;                     // <i>exposition only</i>
      const error_category * cat_;  // <i>exposition only</i>
    };
  }
}</pre>
</blockquote>
<h3><a name="Class-error_code-constructors">Class <code>
error_code</code> constructors</a></h3>
<pre>error_code() noexcept;</pre>
<blockquote>
  <p><i>Effects: </i>Constructs an object of type <code>error_code</code>.</p>
  <p><i>Postconditions:</i> <code>val_ == 0 &amp;&amp; cat_ == &amp;system_category()</code>.</p>
</blockquote>
<pre>error_code( int val, const error_category &amp; cat ) noexcept;</pre>
<blockquote>
  <p><i>Effects: </i>Constructs an object of type <code>error_code</code>.</p>
  <p><i>Postconditions:</i> <code>val_ == val &amp;&amp; cat_ == &amp;cat</code>.</p>
</blockquote>
<pre>template &lt;class <code>ErrorCodeEnum</code>&gt;
  error_code(<code> ErrorCodeEnum</code> val ) noexcept;</pre>
<blockquote>
  <p><i>Effects: </i>Constructs an object of type <code>error_code</code>.</p>
  <p><i>Postconditions:</i> <code>*this == make_error_code( val )</code>.</p>
  <p><i>Remarks:</i> This constructor shall not participate in overload 
  resolution unless <code>is_error_code_enum&lt;ErrorCodeEnum&gt;::value</code> is
  <code>true</code>.</p>
</blockquote>
<h3><a name="Class-error_code-modifiers">Class <code>
error_code</code> modifiers</a></h3>
<pre>void assign( int val, const error_category &amp; cat ) noexcept;</pre>
<blockquote>
  <p><i>Postconditions:</i> <code>val_ == val &amp;&amp; cat_ == &amp;cat</code>.</p>
</blockquote>
<pre>template&lt;typename <code>ErrorCodeEnum</code>&gt;
  error_code &amp; operator=( <code>ErrorCodeEnum</code> val ) noexcept;</pre>
<blockquote>
  <p><i>Postconditions:</i> <code>*this == make_error_code( val )</code>.</p>
  <p><i>Remarks:</i> This operator shall not participate in overload resolution 
  unless <code>is_error_code_enum&lt;ErrorCodeEnum&gt;::value</code> is <code>true</code>.</p>
</blockquote>
<pre><code>void clear() noexcept;</code></pre>
<blockquote>
  <p><i>postcondition:</i> <code>value() == 0 &amp;&amp; category() == 
  system_category()</code></p>
</blockquote>
  <h3><a name="Class-error_code-observers">Class <code>
error_code</code> observers</a></h3>
  <pre><code>int value() const noexcept;</code></pre>
<blockquote>
<p><i>Returns:</i> <code>val_</code>.</p>
  </blockquote>
<pre><code>const error_category &amp; category() const noexcept;</code></pre>
<blockquote>
<p><i>Returns:</i> <code>*cat_</code>.</p>
  </blockquote>
  <pre>error_condition default_error_condition() const noexcept;</pre>
<blockquote>
<p><i>Returns:</i>&nbsp; <code>category().default_error_condition( value())</code>.</p>
</blockquote>
  <pre><code>string message() const;</code></pre>
<blockquote>
<p><i>Returns:</i>&nbsp; <code>category().message( value())</code>.</p>
  <p><i>Throws:</i> Nothing.</p>
</blockquote>
<pre>operator unspecified-bool-type() const;</pre>
<blockquote>
<p><i>Returns:</i> if <code>value() != 0</code>, returns a value that will evaluate
<code>true</code> in a boolean context; otherwise, returns a value that will 
evaluate <code>false</code> in a boolean context. The value type returned shall 
not be convertible to <code>int</code>.</p>
<p><i>Throws:</i> nothing.</p>
<p><i>[Note: </i>This conversion can be used in contexts where a <code>bool</code> 
is expected ( e.g., an <code>if</code> condition ); however, implicit conversions 
( e.g., to <code>int</code>) that can occur with <code>bool</code> are not 
allowed, eliminating some sources of user error. One possible implementation 
choice for this type is pointer-to-member. <i>--end note ]</i></p>
</blockquote>
<h2><a name="Class-error_condition">Class <code>error_condition</code></a></h2>
<p>The class <code>error_condition</code> describes an object used to hold 
values identifying error conditions. <i>[ Note: </i><code>error_condition</code> 
values are portable abstractions, while <code>error_code</code> values are 
implementation specific. <i>--end note ]</i></p>
<h3><a name="Class-error_condition-synopsis">Class <code>error_condition</code> synopsis</a></h3>
<blockquote>
<pre>namespace boost
{
  namespace system
  {
    class error_condition
    {
    public:

      // constructors:
      error_condition() noexcept;
      error_condition( int val, const error_category &amp; cat ) noexcept;
      template &lt;class ErrorConditionEnum&gt;
        error_condition( errorConditionEnum val ) noexcept;

      // modifiers:
      void assign( int val, const error_category &amp; cat ) noexcept;
      template&lt;typename ErrorConditionEnum&gt;
        error_condition &amp; operator=( ErrorConditionEnum val ) noexcept;
      void clear() noexcept;

      // observers:
      int value() const noexcept;
      const error_category &amp; category() const noexcept;
      string message() const;
      operator unspecified-bool-type () const noexcept;

    private:
      int val_;                      // <i>exposition only</i>
      const error_category * cat_;   // <i>exposition only</i>
    };
  }
}</pre>
</blockquote>
<h3><a name="Class-error_condition-constructors">Class <code>error_condition</code> 
constructors</a></h3>
<pre>error_condition() noexcept; </pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of type <code>error_condition</code>.</p>
  <p><i>Postconditions:</i> <code>val_ == 0 and cat_ == &amp;generic_category()</code>.</p>
</blockquote>
<pre>error_condition( int val, const error_category &amp; cat ) noexcept;</pre>
<blockquote>
  <p><i>Effects: </i>Constructs an object of type error_condition.</p>
  <p><i>Postconditions:</i> <code>val_ == val and cat_ == &amp;cat</code>.</p>
</blockquote>
<pre>template &lt;class ErrorConditionEnum&gt;
  error_condition( ErrorConditionEnum e ) noexcept;</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of type <code>error_condition</code>.</p>
  <p><i>Postconditions:</i> <code>*this == make_error_condition(e)</code>.</p>
  <p><i>Remarks:</i> This constructor shall not participate in overload 
  resolution unless <code>is_error_condition_enum&lt;ErrorConditionEnum&gt;::value</code> 
  is <code>true</code>.</p>
</blockquote>
<h3><a name="Class-error_condition-modifiers">Class <code>error_condition</code> 
modifiers</a></h3>
<pre>void assign( int val, const error_category &amp; cat ) noexcept; </pre>
<blockquote>
  <p><i>Postconditions:</i> <code>val_ == val and cat_ == &amp;cat</code>. </p>
</blockquote>
<pre>template&lt;typename ErrorConditionEnum&gt;
  error_condition &amp; operator=( ErrorConditionEnum e ) noexcept;</pre>
<blockquote>
  <p><i>Postconditions:</i> <code>*this == make_error_condition( e )</code>.</p>
  <p><i>Returns:</i> <code>*this</code>.</p>
  <p><i>Remarks:</i> This operator shall not participate in overload resolution 
  unless <code>is_error_condition_enum&lt;ErrorConditionEnum&gt;::value</code> is
  <code>true</code>.</p>
</blockquote>
<pre>void clear() noexcept;</pre>
<blockquote>
  <p><i>Postcondition:</i> <code>value() == 0 &amp;&amp; category() == generic_category()</code></p>
</blockquote>
<h3><a name="Class-error_condition-observers">Class <code>error_condition</code> 
observers</a></h3>
<pre>int value() const noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>val_</code>.</p>
</blockquote>
<pre>const error_category &amp; category() const noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>*cat_</code>.</p>
</blockquote>
<pre>string message() const;</pre>
<blockquote>
  <p><i>Returns:</i> <code>category().message( value() )</code>.</p>
</blockquote>
<pre>operator unspecified-bool-type () const;</pre>
<blockquote>
  <p><i>Returns: </i>If <code>value() != 0</code>, returns a value that will 
  evaluate <code>true</code> in a boolean context; otherwise, returns a value 
  that will evaluate <code>false</code>. The return type shall not be 
  convertible to <code>int</code>. </p>
  <p><i>Throws:</i> Nothing.</p>
  <p><i>&nbsp;[ Note:</i> This conversion can be used in contexts where a <code>bool</code> 
  is expected ( e.g., an if condition ); however, implicit conversions ( e.g., to
  <code>int</code>) that can occur with <code>bool</code> are not allowed, 
  eliminating some sources of user error. One possible implementation choice for 
  this type is pointer to member. <i>--end note</i> <i>]</i></p>
</blockquote>
<h2><a name="Non-member-functions">Non-member functions</a></h2>
  <pre>bool operator==( const error_code &amp; lhs, const error_code &amp; rhs ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>lhs.category() == rhs.category() &amp;&amp; lhs.value() == 
  rhs.value()</code>.</p>
</blockquote>
<pre>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;</pre>
<blockquote>
  <p><i>Returns:</i> <code>code.category().equivalent( code.value(), condition )<br>
  || condition.category().equivalent( code, condition.value() )</code>.</p>
</blockquote>
<pre>bool operator==( const error_condition &amp; lhs, const error_condition &amp; rhs ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>lhs.category() == rhs.category() &amp;&amp; lhs.value() == 
  rhs.value()</code>.</p>
</blockquote>
<pre>bool operator!=( const error_code &amp; lhs, const error_code &amp; rhs ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>!(lhs == rhs )</code>.</p>
</blockquote>
<pre>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;</pre>
<blockquote>
  <p><i>Returns:</i><code> !( code ==&nbsp; condition )</code>.</p>
</blockquote>
<pre>bool operator!=( const error_condition &amp; lhs, const error_condition &amp; rhs ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>!(lhs == rhs )</code>.</p>
</blockquote>
<pre>bool operator&lt;( const error_code &amp; lhs, const error_code &amp; rhs ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>lhs.category() &lt; rhs.category()<br>
  &nbsp; || (lhs.category() == rhs.category() &amp;&amp; lhs.value() &lt; rhs.value())</code>.</p>
</blockquote>
<pre>bool operator&lt;( const error_condition &amp; lhs, const error_condition &amp; rhs ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>lhs.category() &lt; rhs.category()<br>
  &nbsp; || (lhs.category() == rhs.category() &amp;&amp; lhs.value() &lt; rhs.value())</code>.</p>
</blockquote>
<pre>error_code make_error_code( errc::errc_t e ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>error_code( e, generic_category())</code>.</p>
</blockquote>
<pre>error_condition make_error_condition( errc::errc_t e ) noexcept;</pre>
<blockquote>
  <p><i>Returns:</i> <code>error_condition( static_cast&lt;int&gt;( e ), generic_category())</code>.</p>
</blockquote>

<pre>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 );</pre>
<blockquote>
<p><i>Effects:</i> <code>os &lt;&lt; ec.category().name() &lt;&lt; ':' &lt;&lt; ec.value()</code>.</p>
<p><i>Returns:</i> <code>os</code>.</p>
</blockquote>
<pre>size_t <a name="hash_value">hash_value</a>( const error_code &amp; ec );</pre>
<blockquote>
  <p><i>Returns: </i>&nbsp;A hash value representing <code>ec</code>.</p>
</blockquote>

<h2><a name="Function-boost-throws">Function <code>boost::throws()</code></a></h2>

  <pre>system::error_code&amp; throws();</pre>
<blockquote>
<p><i>Returns:</i> A <code>system::error_code</code> reference 
for use in some user-defined function signature as a &quot;throw on error&quot; tag.</p>
<p><i>Remark:</i> The only valid use for the returned reference is to test its 
address for equality or inequality to the address of the reference returned by 
another call to throws(). The returned reference itself has been poisoned so 
that an attempt to dereference it will fail.</p>
<p>[<i>Example: </i>Example of a function that uses the <code>throws()</code> idiom:</p>
  <blockquote>
  <pre>int divide(int dividend, int divisor,
           boost::system::error_code&amp; ec = boost::throws())
{
  if (divisor == 0)                             // is there an error?
  {
    if (&amp;ec == &amp;boost::throws())                // throw on error
      throw &quot;oops!&quot;;                            // whatever exception you prefer
    ec = error_code(EDOM, generic_category());  // report error via error_code
    return 0;
  }

  if (&amp;ec != &amp;boost::throws())                  // error reporting via error_code
    ec.clear();
  return dividend / divisor;
}</pre>
  </blockquote>
  <p>&mdash; <i>end Example</i>]</p>
</blockquote>

<h2><a name="Header-system_error">Header &lt;boost/system/system_error.hpp&gt;</a></h2>

<h3><a name="Class-system_error">Class <code>
system_error</code></a></h3>
<p>The class <code>system_error</code> describes an exception object used to 
report errors that have an associated <code><a href="#Class-error_code">error_code</a></code>. Such errors typically originate from operating system or other low-level 
application program interfaces.</p>
<blockquote>
<pre>namespace boost
{
  namespace system
  {
    class system_error : public std::runtime_error
    {
    public:
      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,
                    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);

      const error_code &amp; code() const throw();
      const char *       what() const throw();
    };
  }
}</pre>
</blockquote>
<pre>system_error( error_code ec );</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of class <code>system_error</code>.</p>
  <p><i>Postcondition:</i> <code>code() == ec <br>
&nbsp; &amp;&amp; std::strcmp( this-&gt;runtime_error::what(), &quot;&quot; ) == 0</code></p>
</blockquote>
<pre>system_error( error_code ec, const char * what_arg );</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of class <code>system_error</code>.</p>
  <p><i>Postcondition:</i> <code>code() == ec <br>
&nbsp; &amp;&amp; std::strcmp( this-&gt;runtime_error::what(), what_arg ) == 0</code></p>
</blockquote>
<pre>system_error( error_code ec, const std::string &amp; what_arg );</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of class <code>system_error</code>.</p>
  <p><i>Postcondition:</i> <code>code() == ec <br>
&nbsp; &amp;&amp; std::strcmp( this-&gt;runtime_error::what(), what_arg.c_str() ) == 0</code></p>
</blockquote>
<pre>system_error( int ev, const error_category &amp; ecat,
             const char * what_arg );</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of class <code>system_error</code>.</p>
  <p><i>Postcondition:</i> <code>code() == error_code( ev, ecat )<br>
&nbsp; &amp;&amp; std::strcmp( this-&gt;runtime_error::what(), what_arg ) == 0</code></p>
</blockquote>
<pre>system_error( int ev, const error_category &amp; ecat,
             const std::string &amp; what_arg );</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of class <code>system_error</code>.</p>
  <p><i>Postcondition:</i> <code>code() == error_code( ev, ecat )<br>
&nbsp; &amp;&amp; std::strcmp( this-&gt;runtime_error::what(), what_arg.c_str() ) == 0</code></p>
</blockquote>
<pre>system_error( int ev, const error_category &amp; ecat );</pre>
<blockquote>
  <p><i>Effects:</i> Constructs an object of class <code>system_error</code>.</p>
  <p><i>Postcondition:</i> <code>code() == error_code( ev, ecat )<br>
&nbsp; &amp;&amp; std::strcmp( this-&gt;runtime_error::what(), &quot;&quot; ) == 0</code></p>
</blockquote>
<pre>const error_code &amp; code() const;</pre>
<blockquote>
  <p><i>Returns:</i> <code>ec</code> or <code>error_code( ev, ecat )</code>, from 
  the constructor, as appropriate.</p>
</blockquote>
<pre>const char * what() const;</pre>
<blockquote>
  <p><i>Returns: </i>A string incorporating <code>
  this-&gt;runtime_error::what()</code> and <code>
  code.message()</code>.</p>
</blockquote>
<hr>

<p>Revised 
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%B %d, %Y" startspan -->September 07, 2017<!--webbot bot="Timestamp" endspan i-checksum="38006" --> </font>
</p>

<p>© Copyright Beman Dawes, 2006, 2007, 2008, 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>

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