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1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
2<HTML>
3<HEAD>
4    <TITLE>Sink</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">Sink</H1>
13    <HR CLASS="banner">
14
15<!-- End Banner -->
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    <SPAN CLASS="keyword">void</SPAN> 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/read.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>
140
141<!-- Begin Footer -->
142
143<HR>
144<P CLASS="copyright">Revised
145<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
14620 May, 2004
147<!--webbot bot="Timestamp" endspan i-checksum="38504" -->
148</P>
149
150<P CLASS="copyright">&copy; Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P>
151<P CLASS="copyright"> 
152    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>)
153</P>
154
155<!-- End Footer -->
156
157</BODY>
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