ceph/src/boost/libs/iostreams/doc/concepts/sink.html

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  12     <H1 CLASS="title">Sink</H1>
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  16
  17 <H2>Definition</H2>
  18
  19 <P>
  20     A Sink is a <A HREF="device.html">Device</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#output">output</A>.
  21 </P>
  22
  23 <H2>Description</H2>
  24
  25 <P>A Sink provides write-access to a sequence of characters of a given type. In general, a Sink may expose this sequence in two ways:</P>
  26 <OL>
  27     <LI STYLE="list-style-type:lower-roman">
  28         by defining a member function <CODE>write</CODE>, invoked indirectly by the Iostreams library through the function <A HREF="../functions/write.html"><CODE>boost::iostreams::write</CODE></A>;
  29     </LI>
  30     <LI STYLE="list-style-type:lower-roman">
  31         by defining a member function <CODE>output_sequence</CODE> returning a pair of pointers delimiting the sequence in its entirety.
  32     </LI>
  33 </OL>
  34
  35 <P>As a special case, Boost.Iostreams treats standard output streams as Sinks. (For details, see <A HREF="../functions/write.html"><CODE>write</CODE></A>.)</P>
  36
  37 <P>The mode of a Sink is <A HREF="../guide/modes.html#output"><CODE>output</CODE></A> or one of its refinements.</P>
  38
  39 <H2>Note</H2>
  40
  41 <P>To be usable with the streams and stream buffers provided by the Boost Iostreams library, Sinks must model <A HREF="blocking.html">Blocking</A>.
  42
  43 <H2>Example</H2>
  44
  45 <P>A model of Sink can be defined as follows:</P>
  46
  47 <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> Sink {
  48     <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN>      char_type;
  49     <SPAN CLASS="keyword">typedef</SPAN> sink_tag  category;
  50     std::streamsize write(<SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n)
  51     {
  52         <SPAN CLASS="comment">// Write up to n characters from the buffer
  53         // s to the output sequence, returning the
  54         // number of characters written</SPAN>
  55     }
  56 };</PRE>
  57
  58 <P>
  59     Here <CODE>sink_tag</CODE> is a <A HREF="../guide/traits.html#category_tags">category tag</A> identifying the type as a model of Sink. Typically a Sink can be defined by deriving from the helper classes <A HREF="../classes/device.html#synopsis"><CODE>sink</CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wsink</CODE></A> and defining a member function <CODE>write</CODE>.
  60
  61 <H2>Refinement of</H2>
  62
  63 <P><A HREF="device.html">Device</A>.</P>
  64
  65 <A NAME="types"></A>
  66 <H2>Associated Types</H2>
  67
  68 <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P>
  69
  70 <TABLE CELLPADDING="5" BORDER="1">
  71     <TR><TD>Category</TD><TD>A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>device_tag</CODE></A> and to <A HREF="../guide/modes.html#mode_tags"><CODE>output</CODE></A></TD></TR>
  72 </TABLE>
  73
  74 <A NAME="notation"></A>
  75 <H2>Notation</H2>
  76
  77 <TABLE CELLPADDING="2">
  78     <TR><TD><CODE>S</CODE></TD><TD>- A type which is a model of Sink</TD></TR>
  79     <TR><TD><CODE>Ch</CODE></TD><TD>- The character type</TD></TR>
  80     <TR><TD><CODE>snk</CODE></TD><TD>- Object of type <CODE>S</CODE></TD></TR>
  81     <TR><TD><CODE>s</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></SPAN></TD></TR>
  82     <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR>
  83     <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
  84 </TABLE>
  85
  86 <A NAME="expressions"></A>
  87 <H2>Valid Expressions / Semantics</H2>
  88
  89 <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P>
  90
  91 <TABLE CELLPADDING="5" BORDER="1">
  92     <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR>
  93     <TR>
  94         <TD>
  95             <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A>&lt;S&gt;::type</CODE></PRE>
  96         </TD>
  97         <TD><CODE>typename</CODE> of the character type</TD>
  98         <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
  99     </TR>
 100     <TR>
 101         <TD>
 102             <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A>&lt;S&gt;::type</CODE></PRE>
 103         </TD>
 104         <TD><CODE>typename</CODE> of the category</TD>
 105         <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD>
 106     </TR>
 107     <TR>
 108         <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">io::write</A>(snk, s, n)</CODE></PRE></TD>
 109         <TD><CODE>std::streamsize</CODE></TD>
 110         <TD>Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD>
 111         <TD>
 112             Writes up to <CODE>n</CODE> characters from the sequence beginning at <CODE>s</CODE> to the output sequence controlled by <CODE>dev</CODE>, returning the number of characters written
 113         </TD>
 114     </TR>
 115     <TR>
 116         <TD><PRE CLASS="plain_code"><CODE>snk.output_sequence()</CODE></PRE></TD>
 117         <TD><PRE CLASS="plain_code"><CODE>std::pair&lt;Ch*,Ch*&gt;</CODE></PRE></TD>
 118         <TD>Convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD>
 119         <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>snk</CODE></TD>
 120     </TR>
 121 </TABLE>
 122
 123 <H2>Exceptions</H2>
 124
 125 <P>
 126     Errors which occur during the execution of member functions <CODE>write</CODE> or <CODE>output_sequence</CODE>
 127     are indicated by throwing exceptions. Attempting to write past the end of the sequence is always an error.
 128 </P>
 129
 130 <P>
 131     After an exception is thrown, a Sink must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour.
 132 </P>
 133
 134 <H2>Models</H2>
 135
 136 <UL>
 137     <LI>Standard output streams and stream buffers.
 138     <LI><A HREF="../classes/array.html#array_sink"><CODE>array_sink</CODE></A>, <A HREF="../classes/file.html#file_sink"><CODE>file_sink</CODE></A>, <A HREF="../classes/file_descriptor.html#file_descriptor_sink"><CODE>file_descriptor_sink</CODE></A>, <A HREF="../classes/mapped_file.html#mapped_file_sink"><CODE>mapped_file_sink</CODE></A>.
 139 </UL>
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 145 <P CLASS="copyright">&copy; Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>&copy; Copyright 2004-2007 <a href="http://www.coderage.com/turkanis/" target="_top">Jonathan Turkanis</a></P>
 146 <P CLASS="copyright">
 147     Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)
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