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

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  12     <H1 CLASS="title">Closable</H1>
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  16
  17 <H2>Description</H2>
  18
  19 <P>
  20     A Closable Filter or Device receives notifications before a stream or stream buffer which contains it is closed.
  21 </P>
  22
  23 <H4>Motivation</H4>
  24
  25 <P>
  26     The C++ standard library does not recognize the notion of an arbitrary stream or stream buffer being <I>open</I> or <I>closed</I>. For the Iostreams library, however, the notion of opening and closing streams and stream buffers is important. Some Filters and Devices have special behavior reserved for the end of a stream &#8212; <I>e.g.</I>, an
  27     <A HREF="output_filter.html">OutputFilter</A> might conclude each sequence of data with a newline character.
  28     Other Filters or Devices maintain state during a sequence of i/o operations which must be
  29     reset before the Filter or Device can be reused. Such Filters and Devices need to be notified before a stream is closed, and should model Closable.
  30 </P>
  31
  32 <H4>Closure Notifications</H4>
  33
  34 <P>
  35     The Iostreams library sends closure notifications by invoking the function <A HREF="../functions/close.html"><CODE>close</CODE></A>. For most Closable Filter or Device, <CODE>close</CODE> delegates to a member function <CODE>close</CODE>; for non-Closable Devices, <CODE>close</CODE> calls <A HREF="../functions/flush.html"><CODE>flush</CODE></A>. The <A HREF="../functions/close.html#when">details</A> regarding when and how <CODE>close</CODE> is invoked are complicated. However, defining a Filter or Device which receives closure notifications is easy, as illustrated in the following sections.
  36 </P>
  37
  38 <H4>Single-Sequence Devices</H4>
  39
  40 <P>
  41     If a Device controls a single sequence of characters, it can be made Closable simply by specifying a <A HREF="../guide/traits.html#category_tags">category tag</A> which is convertible to <CODE>closable_tag</CODE> and by adding a member function <CODE>close</CODE>, like so:
  42 </P>
  43 <PRE CLASS="broken_ie">    <SPAN CLASS="keyword">void</SPAN> close() { <SPAN CLASS="comment"> /* process closure notification */ </SPAN> }</PRE>
  44 <P>
  45     Filters and Devices which derive from the <A HREF="../guide/concepts.html#convenience">convenience templates and <CODE>typedef</CODE>s</A> have category tags convertible to <CODE>closable_tag</CODE> provided automatically.
  46 </P>
  47
  48 <H4>Single-Sequence Filters</H4>
  49
  50 <P>For Filters controlling a single sequence, the signature of <CODE>close</CODE> is:</P>
  51 <PRE CLASS="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Device&gt;
  52     <SPAN CLASS="keyword">void</SPAN> close(Device&amp;);</PRE>
  53
  54 <H4>Two-Sequence Devices</H4>
  55
  56 <P>
  57     If a Device controls separate input and output sequences, the signature of <CODE>close</CODE> is:</P>
  58     <PRE CLASS="broken_ie">    <SPAN CLASS="keyword">void</SPAN> close(std::ios_base::openmode);</PRE>
  59 <P>This function will be called twice: first with argument <CODE>std::ios_base::in</CODE>, to signal the closing of the input sequence, and later with argument <CODE>std::ios_base::out</CODE>, to signal the closing of the output sequence. </P>
  60
  61 <H4>Two-Sequence Filters</H4>
  62
  63 <P>
  64     If a Filter controls separate input and output sequences, the signature of <CODE>close</CODE> is:</P>
  65 <PRE CLASS="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Device&gt;
  66     <SPAN CLASS="keyword">void</SPAN> close(Device&amp;, std::ios_base::openmode);</PRE>
  67 <P>This function will be called twice: first with argument <CODE>std::ios_base::in</CODE>, to signal the closing of the input sequence, and later with argument <CODE>std::ios_base::out</CODE>, to signal the closing of the output sequence.</P>
  68
  69 <H4>Dual-Use Filters</H4>
  70
  71 <P>The signature of  <CODE>close</CODE> for <A HREF="dual_use_filter.html">DualUserFilters</A> is the same as that for two-sequence Filters:</P>
  72 <PRE CLASS="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Device&gt;
  73     <SPAN CLASS="keyword">void</SPAN> close(Device&amp;, std::ios_base::openmode);</PRE>
  74 <P>This function will be called only once, depending on whether the filter is being used for input or output.</P>
  75
  76 <H2>Refinement of</H2>
  77
  78 <A NAME="types"></A>
  79 <H2>Associated Types</H2>
  80
  81 <TABLE CELLPADDING="5" BORDER="1">
  82     <TR><TD>Character type</TD><TD>The type of the characters in the associated sequence</TD></TR>
  83     <TR><TD>Category</TD><TD>A category tag convertible to <A HREF="../guide/traits.html#category_tags"><CODE>closable_tag</CODE></A></TD></TR>
  84 </TABLE>
  85
  86 <H2>Notation</H2>
  87
  88 <TABLE CELLPADDING="2">
  89     <TR><TD><CODE>C</CODE></TD><TD>-</TD><TD>A type which is a model of Closable</TD></TR>
  90     <TR><TD><CODE>D</CODE></TD><TD>-</TD><TD>A type which is a model of <A HREF="blocking.html">Blocking</A>, with the same character type as <CODE>C</CODE> and with mode refining the mode of <CODE>C</CODE></TD></TR>
  91     <TR><TD><CODE>c</CODE></TD><TD>-</TD><TD>Object of type <CODE>C</CODE></TD></TR>
  92     <TR><TD><CODE>d</CODE></TD><TD>-</TD><TD>Object of type <CODE>D</CODE></TD></TR>
  93     <TR><TD><CODE>w</CODE></TD><TD>-</TD><TD>Object of type <CODE>std::ios_base::openmode</CODE>, equal to <CODE>std::ios_base::in</CODE> or <CODE>std::ios_base::out</CODE></TD></TR>
  94 </TABLE>
  95
  96 <H2>Valid Expressions / Semantics</H2>
  97
  98 <P>Same as <A HREF="filter.html">Filter</A> or <A HREF="device.html">Device</A>, with the following additional requirements:</P>
  99
 100 <TABLE CELLPADDING="5" BORDER="1">
 101     <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Precondition</TH><TH>Semantics</TH><TH>Postcondition</TH></TR>
 102     <TR>
 103         <TD>
 104             <PRE CLASS="plain_code"><CODE><A HREF="../functions/close.html#close_device">boost::iostreams::close</A>(c, w)</CODE></PRE>
 105         </TD>
 106         <TD><CODE>void</CODE></TD>
 107         <TD>Category convertible to <A HREF="../guide/traits.html#category_tags"><CODE>device_tag</CODE></A></TD>
 108         <TD>
 109             If <CODE>w</CODE> is <CODE>std::ios_base::out</CODE>, writes zero or more characters to the output sequence
 110         </TD>
 111         <TD><CODE>c</CODE> is ready to begin processing a new character sequence</TD>
 112     </TR>
 113     <TR>
 114         <TD>
 115             <PRE CLASS="plain_code"><CODE><A HREF="../functions/close.html#close_filter">boost::iostreams::close</A>(c, d, w)</CODE></PRE>
 116         </TD>
 117         <TD><CODE>void</CODE></TD>
 118         <TD>Category convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A></TD>
 119         <TD>
 120             <P>
 121                 If <CODE>w</CODE> is <CODE>std::ios_base::out</CODE>, writes zero or more characters to <CODE>d</CODE> using <A HREF="../functions/put.html"><CODE>boost::iostreams::put</CODE></A> and <A HREF="../functions/write.html"><CODE>boost::iostreams::write</CODE></A>
 122             </P>
 123         </TD>
 124         <TD><CODE>c</CODE> is ready to begin processing a new character sequence</TD>
 125     </TR>
 126 </TABLE>
 127
 128 <H2>Exceptions</H2>
 129
 130 <P>
 131     Errors which occur during the execution of <A CLASS="code" HREF="../functions/close.html"><CODE>close</CODE></A> are be indicated by throwing exceptions. Such exceptions are caught and ignored if they occur during the execution of stream or stream buffer destructor.
 132 </P>
 133
 134 <H2>Models</H2>
 135
 136 <UL>
 137     <LI><A HREF="../guide/generic_streams.html">Generic Streams and Stream Buffers</A>.
 138     <LI><A HREF="../guide/filtering_streams.html">Filtering Streams and Stream Buffers</A>.
 139     <LI>Standard file streams and stream buffers.</A>
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 146 <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>
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 148     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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