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<H2>3.2 Modes</H2>

<DL class="page-index">
  <DT><A href="#overview">Overview</A></DT>
  <DT><A href="#definitions">Definitions of the Modes</A></DT>
  <DT><A href="#mode_hierarchy_diagrams">Mode Hierarchy Diagrams</A></DT>
  <DT><A href="#mode_tags">Mode Tags</A></DT>
  <DT><A href="#the_metafunction_mode">The metafunction <CODE>mode_of</CODE></A></DT>
</DL>

<HR STYLE="margin-top:1em">

<A NAME="overview"></A>
<H2>Overview</H2>

<P>
    In order for a sequence of <A HREF="concepts.html#filter_concepts">Filters</A> and <A HREF="concepts.html#device_concepts">Devices</A> to work together they must have certain properties in common. The most basic requirement is that they have the same <A HREF="traits.html#char_type">character type</A>; a collection of additional properties which Filters and Devices must share to be used for a particular purpose is called a <SPAN CLASS="term">mode</SPAN>.
</P>

<P>
    The templates <A HREF="generic_streams.html#stream_buffer"><CODE>stream_buffer</CODE></A> and <A HREF="generic_streams.html#stream"><CODE>stream</CODE></A> are each parameterized by a mode, represented by a <A HREF="#mode_tags">mode tag</A>.
</P>

<P>
    The Iostreams library supports eight modes, described in the next section (<I>see also</I> <A CLASS="caption_ref" HREF="#mode_hierarchy_diagrams">Figure 2</A>). Of these, four are most import (<I>see </I> <A CLASS="caption_ref" HREF="#mode_hierarchy_diagrams">Figure 1</A>). The two modes <A HREF="#input">input</A> and <A HREF="#output">output</A> are by far the most common. Readers new to the Iostreams library should feel free to concentrate primarily on these two modes.
</P>

<A NAME="definitions"></A>
<H2>Definitions of the Modes</H2>

<P>Modes can be categorized in several ways:
<UL>
    <LI>Whether input, output or both input and output can be performed.
    <LI>If both input and output are possible, whether there are two separate character sequences for input and output, or a single combined sequence.
    <LI>Whether the reading or writing heads are repositionable, and if so, whether there are separate heads for reading and writing or a single read/write head.
</UL>
<P>
    The concepts representing <A HREF="concepts.html#optional_behavior">optional behavior</A> are not related to mode, and so need not be shared in order for a collection of Filters and Devices to work together:
</P>

<P>Modes are summarized in the following table:</P>

<TABLE STYLE="margin-left:2em" BORDER=1 CELLPADDING=4>
<TR><TH>Mode</TH><TH>Definition/Examples</TH></TR>
    <!-- Input -->
    <TR>
        <TD VALIGN="top"><A NAME="input"></A><B>Input</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD><TD>Involves a single sequence of characters, for input</TD></TR>
                <TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::cin</CODE></TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Output -->
    <TR>
        <TD VALIGN="top"><A NAME="output"></A><B>Output</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD><TD>Involves a single sequence of characters, for output</TD></TR>
                <TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::cout</CODE></TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Bidirectional -->
    <TR>
        <TD VALIGN="top"><A NAME="bidirectional"></A><B>Bidirectional</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD>
                <TD>Involves a two separate sequences of characters, one for input and one for output</TD></TR>
                <TR><TD VALIGN="top"><B>Example:</B></TD><TD>A <CODE>std::iostream</CODE> for accessing a TCP connection</TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Input-seekable -->
    <TR>
        <TD VALIGN="top"><A NAME="input_seekable"></A><B>Input-seekable</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD>
                <TD>Involves a single sequence of characters, for input, with a repositionable reading head</TD></TR>
                <TR><TD VALIGN="top"><B>Examples:</B></TD><TD><CODE>std::ifstream</CODE>, <CODE>std::istringstream</CODE></TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Output-seekable -->
    <TR>
        <TD VALIGN="top"><A NAME="output_seekable"></A><B><PRE CLASS="plain_code"><P>Output-seekable</P></PRE></B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD>
                <TD>Involves a single sequence of characters, for output, with a repositionable writing head</TD></TR>
                <TR><TD VALIGN="top"><B>Examples:</B></TD><TD><CODE>std::ofstream</CODE>, <CODE>std::ostringstream</CODE></TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Seekable -->
    <TR>
        <TD VALIGN="top"><A NAME="seekable"></A><B>Seekable</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD>
                    <TD>Involves a single sequence of characters, for input and output, with a combined repositionable read/write head</TD></TR>
                <TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::fstream</CODE></TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Dual-seekable -->
    <TR>
        <TD VALIGN="top"><A NAME="dual_seekable"></A><B>Dual-seekable</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD>
                <TD>Involves a single sequence of characters, for input and output, with separate repositionable reading and writing heads</TD></TR>
                <TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::stringstream</CODE></TD></TR>
            </TABLE>
        </TD>
    </TR>

    <!-- Bidirectional-seekable -->
    <TR>
        <TD VALIGN="top"><A NAME="bidirectional_seekable"></A><B>Bidirectional-seekable</B></TD>
        <TD>
            <TABLE>
                <TR><TD VALIGN="top"><B>Definition:</B></TD><TD>Involves a two separate sequences of characters, one for input and one for output, each with a repositionable head</TD></TR>
                <TR><TD VALIGN="top"><B>Example:</B></TD><TD>No known examples</TD></TR>
            </TABLE>
        </TD>
    </TR>
    </TR>
</TABLE>
    

<P>
    For more on the selection of modes, see the <A href="../rationale.html">Rationale</A>.
</P>

<A ID="mode_hierchy_diagrams"></A>
<H2 ID="mode_hierarchy_diagrams">Mode Hierarchy Diagrams</H2>

<P>The following diagrams display the refinement hierarchies among modes.</P>

<TABLE STYLE="margin-left:2em" CELLSPACING=10>
<TR>
    <TD VALIGN="top">
        <P CLASS="caption"><B>Figure 1.</B> The four most important modes</P>
        <IMG SRC="../graphics/important_modes.gif">
    </TD>
    <TD WIDTH="30"></TD>
    <TD ALIGN="right" VALIGN="top">
        <TABLE>
        <TR><TD><IMG SRC="../graphics/key_important.gif"></TD><TD CLASS="caption">Most important</TD></TR>
        <TR><TD><IMG SRC="../graphics/key_input_seq.gif"></TD><TD CLASS="caption">Input sequence</TD></TR>
        <TR><TD><IMG SRC="../graphics/key_output_seq.gif"></TD><TD CLASS="caption">Output sequence</TD></TR>
        <TR><TD><IMG SRC="../graphics/key_bidirectional_seq.gif"></TD><TD CLASS="caption">Combined input-output sequence</TD></TR>
        <TR><TD><IMG SRC="../graphics/key_read_head.gif"></TD><TD CLASS="caption">Repositionable file-pointer for reading</TD></TR>
        <TR><TD><IMG SRC="../graphics/key_write_head.gif"></TD><TD CLASS="caption">Repositionable file-pointer for writing</TD></TR>
        <TR><TD><IMG SRC="../graphics/key_read_write_head.gif"></TD><TD CLASS="caption">Combined file-pointer for reading and writing</TD></TR>
        </TABLE>
    </TD>
</TR>
<TR>
    <TD COLSPAN="3">
        <P CLASS="caption"><B>Figure 2.</B> The complete hierarchy of modes</P>
        <IMG SRC="../graphics/modes.gif">
    </TD>
</TR>
</TABLE>

<A NAME="mode_tags"></A>
<H2>Mode Tags</H2>

<P>
    Each mode is represented by a <SPAN CLASS="term">mode tag</SPAN>, defined in the header <A href="../../../../boost/iostreams/traits.hpp"><CODE>&lt;boost/iostreams/traits.hpp&gt;</CODE></A>. There are eight tags for the eight modes: <CODE>input</CODE>, <CODE>output</CODE>, <CODE>bidirectional</CODE>, <CODE>input_seekable</CODE>, <CODE>output_seekable</CODE>, <CODE>seekable</CODE>, <CODE>dual_seekable</CODE> and <CODE>bidirectional_seekable</CODE>.<SUP><A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1">[1]</A></SUP> As with standard library iterator category tags, the tag corresponding to a mode is convertible to each of the tags corresponding to modes which the first mode refines.
</P>
<P>
    In addition, there is a <SPAN CLASS="term">pseudo-mode tag</SPAN> <CODE>dual_use</CODE>, which is used to define <A href="../concepts/dual_use_filter.html">DualUseFilters</A> &#8212; which can perform input or output, but not both simultaneously. This is useful to help reduce the number of different of Filter types. The <A href="../classes/regex_filter.html">Regular Expression Filters</A>, <A href="../classes/newline_filter.html">Newline Filters</A> and compression and decompression filters all have mode <CODE>dual_use</CODE> so that they may be used with either input or output streams.
</P>

<A NAME="the_metafunction_mode"></A>
<H2>The Metafunction <CODE>mode_of</CODE></H2>

<P>
    To determine the mode of a model of a Filter or Device, use the metafunction <A href="../classes/mode.html"><CODE>mode_of</CODE></A>.
   
</P>

<!-- Begin Footnotes -->

<HR>

<P>
    <SUP><A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref">[1]</A></SUP>It is traditional for tag structures to have names ending with "<CODE>_tag</CODE>". Since mode tags feature prominently in user code, this suffix was dropped to improve readability.
</P>

<!-- End Footnotes -->

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<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>
<P CLASS="copyright"> 
    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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	3	<HEAD>
	4	<TITLE>Modes</TITLE>
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	6	<LINK REL="stylesheet" href="../theme/iostreams.css">
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	9
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	26
	27	<H2>3.2 Modes</H2>
	28
	29	<DL class="page-index">
	30	<DT><A href="#overview">Overview</A></DT>
	31	<DT><A href="#definitions">Definitions of the Modes</A></DT>
	32	<DT><A href="#mode_hierarchy_diagrams">Mode Hierarchy Diagrams</A></DT>
	33	<DT><A href="#mode_tags">Mode Tags</A></DT>
	34	<DT><A href="#the_metafunction_mode">The metafunction <CODE>mode_of</CODE></A></DT>
	35	</DL>
	36
	37	<HR STYLE="margin-top:1em">
	38
	39	<A NAME="overview"></A>
	40	<H2>Overview</H2>
	41
	42	<P>
	43	In order for a sequence of <A HREF="concepts.html#filter_concepts">Filters</A> and <A HREF="concepts.html#device_concepts">Devices</A> to work together they must have certain properties in common. The most basic requirement is that they have the same <A HREF="traits.html#char_type">character type</A>; a collection of additional properties which Filters and Devices must share to be used for a particular purpose is called a <SPAN CLASS="term">mode</SPAN>.
	44	</P>
	45
	46	<P>
	47	The templates <A HREF="generic_streams.html#stream_buffer"><CODE>stream_buffer</CODE></A> and <A HREF="generic_streams.html#stream"><CODE>stream</CODE></A> are each parameterized by a mode, represented by a <A HREF="#mode_tags">mode tag</A>.
	48	</P>
	49
	50	<P>
	51	The Iostreams library supports eight modes, described in the next section (<I>see also</I> <A CLASS="caption_ref" HREF="#mode_hierarchy_diagrams">Figure 2</A>). Of these, four are most import (<I>see </I> <A CLASS="caption_ref" HREF="#mode_hierarchy_diagrams">Figure 1</A>). The two modes <A HREF="#input">input</A> and <A HREF="#output">output</A> are by far the most common. Readers new to the Iostreams library should feel free to concentrate primarily on these two modes.
	52	</P>
	53
	54	<A NAME="definitions"></A>
	55	<H2>Definitions of the Modes</H2>
	56
	57	<P>Modes can be categorized in several ways:
	58	<UL>
	59	<LI>Whether input, output or both input and output can be performed.
	60	<LI>If both input and output are possible, whether there are two separate character sequences for input and output, or a single combined sequence.
	61	<LI>Whether the reading or writing heads are repositionable, and if so, whether there are separate heads for reading and writing or a single read/write head.
	62	</UL>
	63	<P>
	64	The concepts representing <A HREF="concepts.html#optional_behavior">optional behavior</A> are not related to mode, and so need not be shared in order for a collection of Filters and Devices to work together:
65	</P>
66
67	<P>Modes are summarized in the following table:</P>
68
69	<TABLE STYLE="margin-left:2em" BORDER=1 CELLPADDING=4>
70	<TR><TH>Mode</TH><TH>Definition/Examples</TH></TR>
71	<!-- Input -->
72	<TR>
73	<TD VALIGN="top"><A NAME="input"></A><B>Input</B></TD>
74	<TD>
75	<TABLE>
76	<TR><TD VALIGN="top"><B>Definition:</B></TD><TD>Involves a single sequence of characters, for input</TD></TR>
77	<TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::cin</CODE></TD></TR>
78	</TABLE>
79	</TD>
80	</TR>
81
82	<!-- Output -->
83	<TR>
84	<TD VALIGN="top"><A NAME="output"></A><B>Output</B></TD>
85	<TD>
86	<TABLE>
87	<TR><TD VALIGN="top"><B>Definition:</B></TD><TD>Involves a single sequence of characters, for output</TD></TR>
88	<TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::cout</CODE></TD></TR>
89	</TABLE>
90	</TD>
91	</TR>
92
93	<!-- Bidirectional -->
94	<TR>
95	<TD VALIGN="top"><A NAME="bidirectional"></A><B>Bidirectional</B></TD>
96	<TD>
97	<TABLE>
98	<TR><TD VALIGN="top"><B>Definition:</B></TD>
99	<TD>Involves a two separate sequences of characters, one for input and one for output</TD></TR>
100	<TR><TD VALIGN="top"><B>Example:</B></TD><TD>A <CODE>std::iostream</CODE> for accessing a TCP connection</TD></TR>
101	</TABLE>
102	</TD>
103	</TR>
104
105	<!-- Input-seekable -->
106	<TR>
107	<TD VALIGN="top"><A NAME="input_seekable"></A><B>Input-seekable</B></TD>
108	<TD>
109	<TABLE>
110	<TR><TD VALIGN="top"><B>Definition:</B></TD>
111	<TD>Involves a single sequence of characters, for input, with a repositionable reading head</TD></TR>
112	<TR><TD VALIGN="top"><B>Examples:</B></TD><TD><CODE>std::ifstream</CODE>, <CODE>std::istringstream</CODE></TD></TR>
113	</TABLE>
114	</TD>
115	</TR>
116
117	<!-- Output-seekable -->
118	<TR>
119	<TD VALIGN="top"><A NAME="output_seekable"></A><B><PRE CLASS="plain_code"><P>Output-seekable</P></PRE></B></TD>
120	<TD>
121	<TABLE>
122	<TR><TD VALIGN="top"><B>Definition:</B></TD>
123	<TD>Involves a single sequence of characters, for output, with a repositionable writing head</TD></TR>
124	<TR><TD VALIGN="top"><B>Examples:</B></TD><TD><CODE>std::ofstream</CODE>, <CODE>std::ostringstream</CODE></TD></TR>
125	</TABLE>
126	</TD>
127	</TR>
128
129	<!-- Seekable -->
130	<TR>
131	<TD VALIGN="top"><A NAME="seekable"></A><B>Seekable</B></TD>
132	<TD>
133	<TABLE>
134	<TR><TD VALIGN="top"><B>Definition:</B></TD>
135	<TD>Involves a single sequence of characters, for input and output, with a combined repositionable read/write head</TD></TR>
136	<TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::fstream</CODE></TD></TR>
137	</TABLE>
138	</TD>
139	</TR>
140
141	<!-- Dual-seekable -->
142	<TR>
143	<TD VALIGN="top"><A NAME="dual_seekable"></A><B>Dual-seekable</B></TD>
144	<TD>
145	<TABLE>
146	<TR><TD VALIGN="top"><B>Definition:</B></TD>
147	<TD>Involves a single sequence of characters, for input and output, with separate repositionable reading and writing heads</TD></TR>
148	<TR><TD VALIGN="top"><B>Example:</B></TD><TD><CODE>std::stringstream</CODE></TD></TR>
149	</TABLE>
150	</TD>
151	</TR>
152
153	<!-- Bidirectional-seekable -->
154	<TR>
155	<TD VALIGN="top"><A NAME="bidirectional_seekable"></A><B>Bidirectional-seekable</B></TD>
156	<TD>
157	<TABLE>
158	<TR><TD VALIGN="top"><B>Definition:</B></TD><TD>Involves a two separate sequences of characters, one for input and one for output, each with a repositionable head</TD></TR>
159	<TR><TD VALIGN="top"><B>Example:</B></TD><TD>No known examples</TD></TR>
160	</TABLE>
161	</TD>
162	</TR>
163	</TR>
164	</TABLE>
165
166
167	<P>
168	For more on the selection of modes, see the <A href="../rationale.html">Rationale</A>.
169	</P>
170
171	<A ID="mode_hierchy_diagrams"></A>
172	<H2 ID="mode_hierarchy_diagrams">Mode Hierarchy Diagrams</H2>
173
174	<P>The following diagrams display the refinement hierarchies among modes.</P>
175
176	<TABLE STYLE="margin-left:2em" CELLSPACING=10>
177	<TR>
178	<TD VALIGN="top">
179	<P CLASS="caption"><B>Figure 1.</B> The four most important modes</P>
180	<IMG SRC="../graphics/important_modes.gif">
181	</TD>
182	<TD WIDTH="30"></TD>
183	<TD ALIGN="right" VALIGN="top">
184	<TABLE>
185	<TR><TD><IMG SRC="../graphics/key_important.gif"></TD><TD CLASS="caption">Most important</TD></TR>
186	<TR><TD><IMG SRC="../graphics/key_input_seq.gif"></TD><TD CLASS="caption">Input sequence</TD></TR>
187	<TR><TD><IMG SRC="../graphics/key_output_seq.gif"></TD><TD CLASS="caption">Output sequence</TD></TR>
188	<TR><TD><IMG SRC="../graphics/key_bidirectional_seq.gif"></TD><TD CLASS="caption">Combined input-output sequence</TD></TR>
189	<TR><TD><IMG SRC="../graphics/key_read_head.gif"></TD><TD CLASS="caption">Repositionable file-pointer for reading</TD></TR>
190	<TR><TD><IMG SRC="../graphics/key_write_head.gif"></TD><TD CLASS="caption">Repositionable file-pointer for writing</TD></TR>
191	<TR><TD><IMG SRC="../graphics/key_read_write_head.gif"></TD><TD CLASS="caption">Combined file-pointer for reading and writing</TD></TR>
192	</TABLE>
193	</TD>
194	</TR>
195	<TR>
196	<TD COLSPAN="3">
197	<P CLASS="caption"><B>Figure 2.</B> The complete hierarchy of modes</P>
198	<IMG SRC="../graphics/modes.gif">
199	</TD>
200	</TR>
201	</TABLE>
202
203	<A NAME="mode_tags"></A>
204	<H2>Mode Tags</H2>
205
206	<P>
207	Each mode is represented by a <SPAN CLASS="term">mode tag</SPAN>, defined in the header <A href="../../../../boost/iostreams/traits.hpp"><CODE><boost/iostreams/traits.hpp></CODE></A>. There are eight tags for the eight modes: <CODE>input</CODE>, <CODE>output</CODE>, <CODE>bidirectional</CODE>, <CODE>input_seekable</CODE>, <CODE>output_seekable</CODE>, <CODE>seekable</CODE>, <CODE>dual_seekable</CODE> and <CODE>bidirectional_seekable</CODE>.<SUP><A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1">[1]</A></SUP> As with standard library iterator category tags, the tag corresponding to a mode is convertible to each of the tags corresponding to modes which the first mode refines.
208	</P>
209	<P>
210	In addition, there is a <SPAN CLASS="term">pseudo-mode tag</SPAN> <CODE>dual_use</CODE>, which is used to define <A href="../concepts/dual_use_filter.html">DualUseFilters</A> — which can perform input or output, but not both simultaneously. This is useful to help reduce the number of different of Filter types. The <A href="../classes/regex_filter.html">Regular Expression Filters</A>, <A href="../classes/newline_filter.html">Newline Filters</A> and compression and decompression filters all have mode <CODE>dual_use</CODE> so that they may be used with either input or output streams.
211	</P>
212
213	<A NAME="the_metafunction_mode"></A>
214	<H2>The Metafunction <CODE>mode_of</CODE></H2>
215
216	<P>
217	To determine the mode of a model of a Filter or Device, use the metafunction <A href="../classes/mode.html"><CODE>mode_of</CODE></A>.
218
219	</P>
220
221	<!-- Begin Footnotes -->
222
223	<HR>
224
225	<P>
226	<SUP><A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref">[1]</A></SUP>It is traditional for tag structures to have names ending with "<CODE>_tag</CODE>". Since mode tags feature prominently in user code, this suffix was dropped to improve readability.
227	</P>
228
229	<!-- End Footnotes -->
230
231	<!-- Begin Footer -->
232
233	<HR>
234
235	<P CLASS="copyright">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="http://www.coderage.com/turkanis/" target="_top">Jonathan Turkanis</a></P>
236	<P CLASS="copyright">
237	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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