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| <H2>3.2 Modes</H2> |
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| <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_hierchy_diagrams">Mode Hierchy 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> |
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| <HR STYLE="margin-top:1em"> |
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| <A NAME="overview"></A> |
| <H2>Overview</H2> |
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| <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>. |
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| <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> |
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| <P> |
| The Iostreams library supports eight modes, described in the next section (<I>see also</I> <A CLASS="caption_ref" HREF="#mode_hierchy_diagrams">Figure 2</A>). Of these, four are most import (<I>see </I> <A CLASS="caption_ref" HREF="#mode_hierchy_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> |
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| <A NAME="definitions"></A> |
| <H2>Definitions of the Modes</H2> |
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| <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> |
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| <P>Modes are summarized in the following table:</P> |
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| <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> |
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| <!-- 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> |
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| <!-- 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> |
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| <!-- 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> |
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| <!-- 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> |
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| <!-- 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> |
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| <!-- 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> |
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| <P> |
| For more on the selection of modes, see the <A href="../rationale.html">Rationale</A>. |
| </P> |
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| <A NAME="mode_hierchy_diagrams"></A> |
| <H2>Mode Hierchy Diagrams</H2> |
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| <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> |
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| <A NAME="mode_tags"></A> |
| <H2>Mode Tags</H2> |
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| <P> |
| 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. |
| </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> — 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> |
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| <A NAME="the_metafunction_mode"></A> |
| <H2>The Metafunction <CODE>mode_of</CODE></H2> |
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| <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>. |
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| </P> |
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| <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> |
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| <P CLASS="copyright">Revised 02 Feb 2008</P> |
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| <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> |
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| 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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