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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 &#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 a Closable Filter or Device, <CODE>close</CODE> delegates to a member function <CODE>close</CODE>; for other Devices, it does nothing. 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:
36</P>
37
38<H2>Examples</H2>
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<P>For Filters controlling a single sequence, the signature of <CODE>close</CODE> is:</P>
48<PRE CLASS="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Device&gt;
49    <SPAN CLASS="keyword">void</SPAN> close(Device&amp;) { <SPAN CLASS="omitted"> ... </SPAN> }</PRE>
50<P>
51    For Filters and Devices controlling separate input and output sequences, the above signatures should be modified by adding a <CODE>std::ios_base::openmode</CODE> parameter at the end of the parameter list. 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.
52</P>
53   
54
55<H2>Refinement of</H2>
56
57<A NAME="types"></A>
58<H2>Associated Types</H2>
59
60<TABLE CELLPADDING="5" BORDER="1">
61    <TR><TD>Character type</TD><TD>The type of the characters in the associated sequence</TD></TR>
62    <TR><TD>Category</TD><TD>A category tag convertible to <A HREF="../guide/traits.html#category_tags"><CODE>closable_tag</CODE></A></TD></TR>
63</TABLE>
64
65<H2>Notation</H2>
66
67<TABLE CELLPADDING="2">
68    <TR><TD><CODE>C</CODE></TD><TD>-</TD><TD>A type which is a model of Closable</TD></TR>
69    <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>
70    <TR><TD><CODE>c</CODE></TD><TD>-</TD><TD>Object of type <CODE>C</CODE></TD></TR>
71    <TR><TD><CODE>d</CODE></TD><TD>-</TD><TD>Object of type <CODE>D</CODE></TD></TR>
72    <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>
73</TABLE>
74
75<H2>Valid Expressions / Semantics</H2>
76
77<P>Same as <A HREF="filter.html">Filter</A> or <A HREF="device.html">Device</A>, with the following additional requirements:</P>
78
79<TABLE CELLPADDING="5" BORDER="1">
80    <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Precondition</TH><TH>Semantics</TH><TH>Postcondition</TH></TR>
81    <TR>
82        <TD>
83            <PRE CLASS="plain_code"><CODE><A HREF="../functions/close.html#close_device">boost::iostreams::close</A>(c, w)</CODE></PRE>
84        </TD>
85        <TD><CODE>void</CODE></TD>
86        <TD>Category convertible to <A HREF="../guide/traits.html#category_tags"><CODE>device_tag</CODE></A></TD>
87        <TD>
88            If <CODE>w</CODE> is <CODE>std::ios_base::out</CODE>, writes zero or more characters to the output sequence
89        </TD>
90        <TD><CODE>c</CODE> is ready to begin processing a new character sequence</TD>
91    </TR>
92    <TR>
93        <TD>
94            <PRE CLASS="plain_code"><CODE><A HREF="../functions/close.html#close_filter">boost::iostreams::close</A>(c, d, w)</CODE></PRE>
95        </TD>
96        <TD><CODE>void</CODE></TD>
97        <TD>Category convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A></TD>
98        <TD>
99            <P>
100                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>
101            </P>
102        </TD>
103        <TD><CODE>c</CODE> is ready to begin processing a new character sequence</TD>
104    </TR>
105</TABLE>
106
107<H2>Exceptions</H2>
108
109<P>
110    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.
111</P>
112
113<H2>Models</H2>
114
115<P>Many of the Filters and Devices provided by the Iostreams library are Closable, but this is an implementation detail.</P>
116
117<!-- Begin Footer -->
118
119<HR>
120<P CLASS="copyright">Revised
121<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
12220 May, 2004
123<!--webbot bot="Timestamp" endspan i-checksum="38504" -->
124</P>
125
126<P CLASS="copyright">&copy; Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P>
127<P CLASS="copyright"> 
128    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>)
129</P>
130
131<!-- End Footer -->
132
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