1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<HTML> 3<HEAD> 4 <TITLE>Closable</TITLE> 5 <LINK REL="stylesheet" HREF="../../../../boost.css"> 6 <LINK REL="stylesheet" HREF="../theme/iostreams.css"> 7</HEAD> 8<BODY> 9 10<!-- Begin Banner --> 11 12 <H1 CLASS="title">Closable</H1> 13 <HR CLASS="banner"> 14 15<!-- End Banner --> 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 — <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><<SPAN CLASS="keyword">typename</SPAN> Device> 52 <SPAN CLASS="keyword">void</SPAN> close(Device&);</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><<SPAN CLASS="keyword">typename</SPAN> Device> 66 <SPAN CLASS="keyword">void</SPAN> close(Device&, 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><<SPAN CLASS="keyword">typename</SPAN> Device> 73 <SPAN CLASS="keyword">void</SPAN> close(Device&, 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> 140</UL> 141 142<!-- Begin Footer --> 143 144<HR> 145 146<P CLASS="copyright">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="https://www.boost.org/users/people/jonathan_turkanis.html" target="_top">Jonathan Turkanis</a></P> 147<P CLASS="copyright"> 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>) 149</P> 150 151<!-- End Footer --> 152 153</BODY>