[29] | 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
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| 2 | <HTML> |
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| 3 | <HEAD> |
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| 4 | <TITLE>SeekableFilter</TITLE> |
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| 5 | <LINK REL="stylesheet" HREF="../../../../boost.css"> |
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| 6 | <LINK REL="stylesheet" HREF="../theme/iostreams.css"> |
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| 7 | </HEAD> |
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| 8 | <BODY> |
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| 9 | |
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| 10 | <!-- Begin Banner --> |
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| 11 | |
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| 12 | <H1 CLASS="title">SeekableFilter</H1> |
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| 13 | <HR CLASS="banner"> |
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| 14 | |
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| 15 | <!-- End Banner --> |
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| 16 | |
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| 17 | <H2>Definition</H2> |
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| 18 | |
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| 19 | <P> |
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| 20 | A SeekableFilter is a <A HREF="filter.html">Filter</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#seekable">seekable</A>. |
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| 21 | </P> |
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| 22 | |
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| 23 | <H2>Description</H2> |
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| 24 | |
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| 25 | <P> |
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| 26 | A SeekableFilter operates on the character sequence controlled by a <A HREF="seekable_device.html">SeekableDevice</A>, providing access to a filtered sequence having the same character type. It may expose the filtered sequence in two ways: |
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| 27 | <OL> |
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| 28 | <LI STYLE="list-style-type:lower-roman"> |
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| 29 | by defining member function <CODE>get</CODE>, <CODE>put</CODE> and <CODE>seek</CODE>. |
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| 30 | </LI> |
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| 31 | <LI STYLE="list-style-type:lower-roman"> |
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| 32 | by defining member functions <CODE>read</CODE>, <CODE>write</CODE> and <CODE>seek</CODE>. |
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| 33 | </LI> |
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| 34 | </OL> |
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| 35 | The second alternative is provided for enhanced performance. SeekableFilters implementing this alternative are referred to as <I>Multi-Character</I>. (<I>See</I> <A HREF="multi_character.html">Multi-Character Filter</A>.) |
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| 36 | </P> |
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| 37 | |
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| 38 | <H2>Refinement of</H2> |
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| 39 | |
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| 40 | <P><A HREF="input_filter.html">InputFilter</A>, <A HREF="output_filter.html">OutputFilter</A>.</P> |
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| 41 | |
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| 42 | <H2>Associated Types</H2> |
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| 43 | |
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| 44 | <TABLE CELLPADDING="5" BORDER="1"> |
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| 45 | <TR><TD>Character type</TD><TD>The type of the characters in the filtered sequences</TD></TR> |
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| 46 | <TR> |
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| 47 | <TD>Category</TD> |
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| 48 | <TD> |
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| 49 | A type convertible to <A HREF="../guide/traits.html#category_tags"><CODE>filter_tag</CODE></A> and to <A HREF="../guide/modes.html#seekable"><CODE>seekable</CODE></A> |
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| 50 | </TD> |
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| 51 | </TR> |
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| 52 | <TR> |
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| 53 | <TD>Mode</TD> |
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| 54 | <TD> |
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| 55 | The unique <I>most-derived</I> <A HREF="../guide/modes.html#mode_tags">mode tag</A> to which Category is convertible |
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| 56 | </TD> |
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| 57 | </TR> |
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| 58 | </TABLE> |
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| 59 | |
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| 60 | <H2>Notation</H2> |
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| 61 | |
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| 62 | <TABLE CELLPADDING="2"> |
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| 63 | <TR><TD><CODE>F</CODE></TD><TD>- A type which is a model of SeekableFilter</TD></TR> |
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| 64 | <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of <A HREF="device.html">Device</A>, with the same character type as <CODE>F</CODE> and with mode convertible to the mode of <CODE>F</CODE></TD></TR> |
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| 65 | <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>F</CODE></TD></TR> |
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| 66 | <TR><TD><CODE>Tr</CODE></A></TD><TD>- <A HREF="../classes/char_traits.html"><CODE>boost::iostreams::char_traits<Ch></CODE></A></TD></TR> |
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| 67 | <TR><TD><CODE>f</CODE></TD><TD>- Object of type <CODE>F</CODE></TD></TR> |
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| 68 | <TR><TD><CODE>d</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR> |
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| 69 | <TR><TD><CODE>c</CODE></TD><TD>- Object of type <CODE>Ch</CODE></TD></TR> |
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| 70 | <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></TD></TR> |
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| 71 | <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></TD></TR> |
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| 72 | <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR> |
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| 73 | <TR><TD><CODE>off</CODE></TD><TD>- Object of type <A HREF="../functions/positioning.html#synopsis"><CODE>stream_offset</CODE></A></TD></TR> |
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| 74 | <TR><TD><CODE>way</CODE></TD><TD>- Object of type <CODE>std::ios_base::seekdir</CODE></TD></TR> |
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| 75 | <TR><TD><CODE>which</CODE></TD><TD>- Object of type <CODE>std::ios_base::openmode</CODE></TD></TR> |
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| 76 | <TR><TD><CODE>io</CODE></TD><TD>- Alias for namespace <CODE>boost::iostreams</CODE></TD></TR> |
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| 77 | </TABLE> |
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| 78 | |
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| 79 | <H2>Valid Expressions / Semantics</H2> |
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| 80 | |
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| 81 | <TABLE CELLPADDING="5" BORDER="1"> |
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| 82 | <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR> |
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| 83 | <TR> |
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| 84 | <TD> |
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| 85 | <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#char_type_of_ref">char_type_of</A><F>::type</CODE></PRE> |
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| 86 | </TD> |
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| 87 | <TD><CODE>typename</CODE> of the character type</TD> |
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| 88 | <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD> |
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| 89 | </TR> |
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| 90 | <TR> |
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| 91 | <TD> |
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| 92 | <PRE CLASS="plain_code"><CODE>typename <A HREF="../guide/traits.html#category_ref">category_of</A><F>::type</CODE></PRE> |
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| 93 | </TD> |
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| 94 | <TD><CODE>typename</CODE> of the category</TD> |
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| 95 | <TD ALIGN="center">-</TD><TD ALIGN="center">-</TD> |
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| 96 | </TR> |
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| 97 | <TR> |
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| 98 | <TD><PRE CLASS="plain_code"><CODE>f.get(d)</CODE></PRE></TD> |
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| 99 | <TD><CODE>Tr::int_type</CODE></TD> |
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| 100 | <TD ROWSPAN="2"> |
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| 101 | Not convertible to <A HREF="../guide/traits.html#category_tags"><CODE>multi_char_tag</CODE></A> |
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| 102 | </TD> |
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| 103 | <TD> |
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| 104 | Returns the next character in the sequence controlled by <CODE>f</CODE>, or <CODE>Tr::eof()</CODE> if the end of the sequence has been reached. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>. |
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| 105 | |
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| 106 | <!-- reads up to <CODE>n</CODE> characters from the sequence controlled by <CODE>dev</CODE> into <CODE>s</CODE>, returning the number of characters read; returning a value less than n indicates end-of-sequence --> |
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| 107 | </TD> |
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| 108 | </TR> |
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| 109 | <TR> |
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| 110 | <TD><PRE CLASS="plain_code"><CODE>f.put(d, c)</CODE></PRE></TD> |
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| 111 | <TD><CODE>bool</CODE></TD> |
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| 112 | <TD> |
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| 113 | Writes the character <CODE>c</CODE> to the sequence controlled by <CODE>f</CODE>. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/write.html"><CODE>io::write</CODE></A>. |
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| 114 | </TD> |
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| 115 | </TR> |
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| 116 | <TR> |
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| 117 | <TD><PRE CLASS="plain_code"><CODE>f.read(d, s1, n)</CODE></PRE></TD> |
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| 118 | <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD> |
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| 119 | <TD ROWSPAN="2"> |
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| 120 | Convertible to <A HREF="../guide/traits.html#category_tags"><CODE>multi_char_tag</CODE></A> |
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| 121 | </TD> |
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| 122 | <TD> |
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| 123 | Reads up to <CODE></CODE> characters from the sequence controlled by <CODE>f</CODE> into the buffer <CODE>s1</CODE>, returning the number of characters read. Returning a value less than <CODE>n</CODE> indicates end-of-sequence. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::read</CODE></A> and <A HREF="../functions/putback.html"><CODE>io::putback</CODE></A>. |
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| 124 | </TD> |
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| 125 | </TR> |
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| 126 | <TR> |
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| 127 | <TD><PRE CLASS="plain_code"><CODE>f.write(d, s2, n)</CODE></PRE></TD> |
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| 128 | <TD><PRE CLASS="plain_code"><CODE>std::streamsize</CODE></PRE></TD> |
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| 129 | <TD> |
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| 130 | Writes n characters from the buffer <CODE>s2</CODE> to the sequence controlled by <CODE>f</CODE>. The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/write.html"><CODE>io::write</CODE></A>. |
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| 131 | </TD> |
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| 132 | </TR> |
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| 133 | <TR> |
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| 134 | <TD><PRE CLASS="plain_code"><CODE>f.seek(d, off, way)</CODE></PRE></TD> |
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| 135 | <TD><CODE>std::streampos</CODE></TD> |
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| 136 | <TD ALIGN="center">-</TD> |
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| 137 | <TD> |
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| 138 | <P CLASS="concept"> |
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| 139 | Advances the read/write head by <CODE>off</CODE> characters, returning the new position, where the offset is calculated from: |
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| 140 | </P> |
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| 141 | <UL STYLE="margin:0,0,0,auto"> |
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| 142 | <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> |
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| 143 | <LI STYLE="list-style-type:disc;list-style-image:none">the current position if <CODE>way</CODE> is <CODE>ios_base::cur</CODE> |
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| 144 | <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> |
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| 145 | </UL> |
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| 146 | <P CLASS="concept"> |
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| 147 | The sequence controlled by <CODE>d</CODE> may be accessed using <A HREF="../functions/read.html"><CODE>io::seek</CODE></A> and using <A HREF="../functions/putback.html"><CODE>io::write</CODE></A> if the <A HREF="../guide/modes.html">mode</A> of <CODE>F</CODE> is convertible to <A HREF="../guide/modes.html#output"><CODE>output</CODE></A>. |
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| 148 | </P> |
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| 149 | </TD> |
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| 150 | </TR> |
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| 151 | </TABLE> |
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| 152 | |
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| 153 | <H2>Exceptions</H2> |
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| 154 | |
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| 155 | <P> |
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| 156 | Errors which occur during the execution of <CODE>get</CODE>, <CODE>put</CODE>, <CODE>read</CODE>, <CODE>write</CODE> or <CODE>seek</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. |
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| 157 | </P> |
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| 158 | |
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| 159 | <P> |
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| 160 | After an exception is thrown, a SeekableFilter must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. Furthermore, unless it is <A HREF="closable.html">Closable</A>, it must be ready to begin processing a new character sequence. |
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| 161 | </P> |
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| 162 | |
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| 163 | <H2>Models</H2> |
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| 164 | |
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| 165 | <!-- Begin Footer --> |
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| 166 | |
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| 167 | <HR> |
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| 168 | <P CLASS="copyright">Revised |
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| 169 | <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> |
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| 170 | 20 May, 2004 |
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| 171 | <!--webbot bot="Timestamp" endspan i-checksum="38504" --> |
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| 172 | </P> |
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| 173 | |
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| 174 | <P CLASS="copyright">© Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P> |
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| 175 | <P CLASS="copyright"> |
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| 176 | 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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| 177 | </P> |
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| 178 | |
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| 179 | <!-- End Footer --> |
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| 180 | |
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| 181 | </BODY> |
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