[29] | 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
---|
| 2 | <HTML> |
---|
| 3 | <HEAD> |
---|
| 4 | <TITLE>SeekableDevice</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">SeekableDevice</H1> |
---|
| 13 | <HR CLASS="banner"> |
---|
| 14 | |
---|
| 15 | <!-- End Banner --> |
---|
| 16 | |
---|
| 17 | <H2>Definition</H2> |
---|
| 18 | |
---|
| 19 | <P> |
---|
| 20 | A SeekableDevice is a <A HREF="device.html">Device</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#seekable">seekable</A>. |
---|
| 21 | </P> |
---|
| 22 | |
---|
| 23 | <H2>Description</H2> |
---|
| 24 | |
---|
| 25 | <P> |
---|
| 26 | A SeekableDevice provides read- write- and random-access to a sequence of characters of a given type, with a single repositionable read/write head. A SeekableDevice may expose this sequence in two ways: |
---|
| 27 | <OL> |
---|
| 28 | <LI STYLE="list-style-type:lower-roman"> |
---|
| 29 | by defining member functions <CODE>read</CODE>, <CODE>write</CODE> and <CODE>seek</CODE>; or |
---|
| 30 | </LI> |
---|
| 31 | <LI STYLE="list-style-type:lower-roman"> |
---|
| 32 | by defining member functions <CODE>input_sequence</CODE> and <CODE>output_sequence</CODE>, returning pairs of pointers delimiting the sequences in its entirety. The return values of these two functions must be the same.<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A> |
---|
| 33 | </LI> |
---|
| 34 | </OL> |
---|
| 35 | </P> |
---|
| 36 | <P>A SeekableDevice has mode <A HREF="../guide/modes.html#seekable"><CODE>seekable</CODE></A>.</P> |
---|
| 37 | |
---|
| 38 | <H2>Note</H2> |
---|
| 39 | |
---|
| 40 | <P>To be usable with streams and stream buffers, SeekableDevices must model <A HREF="blocking.html">Blocking</A>. |
---|
| 41 | |
---|
| 42 | <H2>Example</H2> |
---|
| 43 | |
---|
| 44 | <P>A model of SeekableDevice can be defined as follows:</P> |
---|
| 45 | |
---|
| 46 | <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> SeekableDevice { |
---|
| 47 | <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type; |
---|
| 48 | <SPAN CLASS="keyword">typedef</SPAN> seekable_device_tag category; |
---|
| 49 | std::streamsize read(<SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) |
---|
| 50 | { |
---|
| 51 | <SPAN CLASS="comment">// Read up to n characters from the input |
---|
| 52 | // sequence into the buffer s, returning |
---|
| 53 | // the number of characters read, or -1 |
---|
| 54 | // to indicate end-of-sequence.</SPAN> |
---|
| 55 | } |
---|
| 56 | <SPAN CLASS="keyword">void</SPAN> write(<SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) |
---|
| 57 | { |
---|
| 58 | <SPAN CLASS="comment">// Write up to n characters from the buffer |
---|
| 59 | // s to the output sequence, returning the |
---|
| 60 | // number of characters written</SPAN> |
---|
| 61 | } |
---|
| 62 | std::streampos seek(stream_offset off, std::ios_base::seekdir way) |
---|
| 63 | { |
---|
| 64 | <SPAN CLASS="comment">// Advances the read/write head by off characters, |
---|
| 65 | // returning the new position, where the offset is |
---|
| 66 | // calculated from: |
---|
| 67 | // - the start of the sequence if way == ios_base::beg |
---|
| 68 | // - the current position if way == ios_base::cur |
---|
| 69 | // - the end of the sequence if way == ios_base::end</SPAN> |
---|
| 70 | } |
---|
| 71 | };</PRE> |
---|
| 72 | |
---|
| 73 | <P> |
---|
| 74 | Here <CODE>seekable_device_tag</CODE> is a <A HREF="../guide/traits.html#category_tags">category tag</A> identifying the containing type as a model of SeekableDevice. When defining a new SeekableDevice, it suffices to use the tag <CODE>seekable_device_tag</CODE>. One can also derive from the helper classes <A HREF="../classes/device.html"><CODE>device<seekable></CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wdevice<seekable></CODE></A>. |
---|
| 75 | </P> |
---|
| 76 | |
---|
| 77 | <H2>Refinement of</H2> |
---|
| 78 | |
---|
| 79 | <P><A HREF="source.html">Source</A>, <A HREF="sink.html">Sink</A>.</P> |
---|
| 80 | |
---|
| 81 | <H2>Associated Types</H2> |
---|
| 82 | |
---|
| 83 | <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P> |
---|
| 84 | |
---|
| 85 | <TABLE CELLPADDING="5" BORDER="1"> |
---|
| 86 | <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>seekable</CODE></A></TD></TR> |
---|
| 87 | </TABLE> |
---|
| 88 | |
---|
| 89 | <H2>Notation</H2> |
---|
| 90 | |
---|
| 91 | <TABLE CELLPADDING="2"> |
---|
| 92 | <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of SeekableDevice</TD></TR> |
---|
| 93 | <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>D</CODE></TD></TR> |
---|
| 94 | <TR><TD><CODE>dev</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR> |
---|
| 95 | <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></SPAN></TD></TR> |
---|
| 96 | <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></SPAN></TD></TR> |
---|
| 97 | <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR> |
---|
| 98 | <TR><TD><CODE>off</CODE></TD><TD>- Object of type <A HREF="../functions/positioning.html#synopsis"><CODE>stream_offset</CODE></A></TD></TR> |
---|
| 99 | <TR><TD><CODE>way</CODE></TD><TD>- Object of type <CODE>std::ios_base::seekdir</CODE></TD></TR> |
---|
| 100 | <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR> |
---|
| 101 | </TABLE> |
---|
| 102 | |
---|
| 103 | <H2>Valid Expressions / Semantics</H2> |
---|
| 104 | |
---|
| 105 | <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P> |
---|
| 106 | |
---|
| 107 | <TABLE CELLPADDING="5" BORDER="1"> |
---|
| 108 | <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR> |
---|
| 109 | <TR> |
---|
| 110 | <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/read.html">io::read</A>(dev, s1, n)</CODE></PRE></TD> |
---|
| 111 | <TD><CODE>std::streamsize</CODE></TD> |
---|
| 112 | <TD ROWSPAN="3">Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> |
---|
| 113 | <TD> |
---|
| 114 | Reads up to <CODE>n</CODE> characters from the sequence controlled by <CODE>dev</CODE> into <CODE>s1</CODE>, returning the number of characters read, or <CODE>-1</CODE> to indicate end-of-sequence |
---|
| 115 | </TD> |
---|
| 116 | </TR> |
---|
| 117 | <TR> |
---|
| 118 | <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">io::write</A>(dev, s2, n)</CODE></PRE></TD> |
---|
| 119 | <TD><CODE>std::streamsize</CODE></TD> |
---|
| 120 | <TD> |
---|
| 121 | Writes up to <CODE>n</CODE> characters from the sequence beginning at <CODE>s2</CODE> to the sequence controlled by <CODE>dev</CODE>, returning the number of characters written |
---|
| 122 | </TD> |
---|
| 123 | </TR> |
---|
| 124 | <TR> |
---|
| 125 | <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/seek.html">io::seek</A>(dev, off, way)</CODE></PRE></TD> |
---|
| 126 | <TD><CODE>std::streampos</CODE></TD> |
---|
| 127 | <TD>Advances the read/write head by <CODE>off</CODE> characters, returning the new position, where the offset is calculated from: |
---|
| 128 | <UL STYLE="margin:0,0,0,auto"> |
---|
| 129 | <LI STYLE="list-style-type:disc;list-style-image:none">the start of the sequence if <CODE>way</CODE> is <CODE>ios_base::beg</CODE> |
---|
| 130 | <LI STYLE="list-style-type:disc;list-style-image:none">the current position if <CODE>way</CODE> is <CODE>ios_base::cur</CODE> |
---|
| 131 | <LI STYLE="list-style-type:disc;list-style-image:none">the end of the sequence if <CODE>way</CODE> is <CODE>ios_base::end</CODE> |
---|
| 132 | </UL> |
---|
| 133 | </TD> |
---|
| 134 | </TR> |
---|
| 135 | <TR> |
---|
| 136 | <TD><PRE CLASS="plain_code"><CODE>dev.input_sequence()</CODE></PRE></TD> |
---|
| 137 | <TD><PRE CLASS="plain_code"><CODE>std::pair<Ch*,Ch*></CODE></PRE></TD> |
---|
| 138 | <TD ROWSPAN="2">Convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> |
---|
| 139 | <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>dev</CODE></TD> |
---|
| 140 | </TR> |
---|
| 141 | <TR> |
---|
| 142 | <TD><PRE CLASS="plain_code"><CODE>dev.output_sequence()</CODE></PRE></TD> |
---|
| 143 | <TD><PRE CLASS="plain_code"><CODE>std::pair<Ch*,Ch*></CODE></PRE></TD> |
---|
| 144 | <TD>Returns a pair of pointers delimiting the sequence controlled by <CODE>dev</CODE></TD> |
---|
| 145 | </TR> |
---|
| 146 | </TABLE> |
---|
| 147 | |
---|
| 148 | <H2>Exceptions</H2> |
---|
| 149 | |
---|
| 150 | <P> |
---|
| 151 | Errors which occur during the execution of member functions <CODE>read</CODE>, <CODE>write</CODE>, <CODE>seek</CODE>, <CODE>input_sequence</CODE> or <CODE>output_sequence</CODE> are indicated by throwing exceptions. Reaching the end of the sequence is not an error, but attempting to write past the end of the sequence is. |
---|
| 152 | </P> |
---|
| 153 | |
---|
| 154 | <P> |
---|
| 155 | After an exception is thrown, a SeekableDevice must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. |
---|
| 156 | </P> |
---|
| 157 | |
---|
| 158 | <H2>Models</H2> |
---|
| 159 | |
---|
| 160 | <UL> |
---|
| 161 | <LI><A HREF="../classes/array.html#array"><CODE>array</CODE></A>, <A HREF="../classes/file.html#file"><CODE>file</CODE></A>, <A HREF="../classes/file_descriptor.html#file_descriptor"><CODE>file_descriptor</CODE></A>, <A HREF="../classes/mapped_file.html#mapped_file"><CODE>mapped_file</CODE></A>. |
---|
| 162 | </UL> |
---|
| 163 | |
---|
| 164 | <!-- Begin Footnotes --> |
---|
| 165 | |
---|
| 166 | <P> |
---|
| 167 | <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>A more elegant specification would require a single member function <CODE>sequence</CODE>; the present version was selected to simplify the implementation of <A HREF="../guide/generic_streams.html#stream_buffer"><CODE>stream_buffer</CODE></A>. |
---|
| 168 | </P> |
---|
| 169 | |
---|
| 170 | <!-- End Footnotes --> |
---|
| 171 | |
---|
| 172 | <!-- Begin Footer --> |
---|
| 173 | |
---|
| 174 | <HR> |
---|
| 175 | <P CLASS="copyright">Revised |
---|
| 176 | <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> |
---|
| 177 | 20 May, 2004 |
---|
| 178 | <!--webbot bot="Timestamp" endspan i-checksum="38504" --> |
---|
| 179 | </P> |
---|
| 180 | |
---|
| 181 | <P CLASS="copyright">© Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P> |
---|
| 182 | <P CLASS="copyright"> |
---|
| 183 | 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>) |
---|
| 184 | </P> |
---|
| 185 | |
---|
| 186 | <!-- End Footer --> |
---|
| 187 | |
---|
| 188 | </BODY> |
---|