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2 | <HTML> |
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3 | <HEAD> |
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4 | <TITLE>BidirectionalDevice</TITLE> |
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5 | <LINK REL="stylesheet" HREF="../../../../boost.css"> |
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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">BidirectionalDevice</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 | An BidirectionalDevice is a <A HREF="device.html">Device</A> whose <A HREF="../guide/modes.html">mode</A> refines <A HREF="../guide/modes.html#bidirectional">bidirectional</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 | An BidirectionalDevice provides read-access to a sequence of characters of a given type and write-access |
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27 | to a separate sequence of characters of the same type. An BidirectionalDevice may expose these sequences in three ways:<A CLASS="footnote_ref" NAME="note_1_ref" HREF="#note_1"><SUP>[1]</SUP></A> |
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28 | <OL> |
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29 | <LI STYLE="list-style-type:lower-roman"> |
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30 | by defining member functions <CODE>read</CODE> and <CODE>write</CODE> , invoked indirectly by the Iostreams Library through the functions <A HREF="../functions/read.html"><CODE>boost::iostreams::read</CODE></A> and <A HREF="../functions/write.html"><CODE>boost::iostreams::write</CODE></A>; |
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31 | </LI> |
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32 | <LI STYLE="list-style-type:lower-roman"> |
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33 | by overloading or specializing <A HREF="../functions/read.html"><CODE>boost::iostreams::read</CODE></A> and <A HREF="../functions/write.html"><CODE>boost::iostreams::write</CODE></A>; or |
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34 | </LI> |
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35 | <LI STYLE="list-style-type:lower-roman"> |
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36 | by defining member functions <CODE>input_sequence</CODE> and <CODE>output_sequence</CODE> returning pairs of pointers delimiting the two sequences in their entirety. |
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37 | </LI> |
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38 | </OL> |
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39 | </P> |
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40 | |
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41 | <P>The i/o mode of a BidirectionalDevice is <A HREF="../guide/modes.html#bidirectional">bidirectional</A> or <A HREF="../guide/modes.html#bidirectional_seekable">bidirectional-seekable</A>.</P> |
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42 | |
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43 | <H2>Example</H2> |
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44 | |
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45 | <P>A model of BidirectionalDevice can be defined as follows:</P> |
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46 | |
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47 | <PRE CLASS="broken_ie"><SPAN CLASS="keyword">struct</SPAN> BidirectionalDevice { |
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48 | <SPAN CLASS="keyword">typedef</SPAN> <SPAN CLASS="keyword">char</SPAN> char_type; |
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49 | <SPAN CLASS="keyword">typedef</SPAN> bidirectional_device_tag category; |
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50 | std::streamsize read(<SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) |
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51 | { |
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52 | <SPAN CLASS="comment">// Reads up to n characters from the input |
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53 | // sequence into the buffer s, returning the number |
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54 | // of characters read. Returning a value less than n |
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55 | // indicates end-of-sequence.</SPAN> |
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56 | } |
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57 | std::streamsize write(<SPAN CLASS="keyword">const</SPAN> <SPAN CLASS="keyword">char</SPAN>* s, std::streamsize n) |
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58 | { |
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59 | <SPAN CLASS="comment">// Write up to n characters from the buffer |
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60 | // s to the output sequence, returning the |
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61 | // number of characters written</SPAN> |
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62 | } |
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63 | };</PRE> |
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64 | |
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65 | <P> |
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66 | Here <CODE>category</CODE> is a tag <CODE>struct</CODE> identifying the containing type as a model of BidirectionalDevice. When defining a new BidirectionalDevice, it suffices to use the tag <CODE>bidirectional_device_tag</CODE>. One can also derive from the helper classes <A HREF="../classes/device.html"><CODE>device<bidirectional></CODE></A> or <A HREF="../classes/device.html#synopsis"><CODE>wdevice<bidirectional></CODE></A>. |
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67 | </P> |
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68 | |
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69 | <H2>Refinement of</H2> |
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70 | |
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71 | <P><A HREF="source.html">Source</A>, <A HREF="sink.html">Sink</A>.</P> |
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72 | |
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73 | <H2>Associated Types</H2> |
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74 | |
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75 | <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P> |
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76 | |
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77 | <TABLE CELLPADDING="5" BORDER="1"> |
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78 | <TR><TD>Category</TD><TD>A type convertible to <A HREF="../guide/traits.html#category_tags">device_tag</A> and to <A HREF="../guide/modes.html#mode_tags"><CODE>bidirectional</CODE></A></TD></TR> |
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79 | </TABLE> |
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80 | |
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81 | <H2>Notation</H2> |
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82 | |
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83 | <TABLE CELLPADDING="2"> |
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84 | <TR><TD><CODE>D</CODE></TD><TD>- A type which is a model of BidirectionalDevice</TD></TR> |
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85 | <TR><TD><CODE>Ch</CODE></TD><TD>- The character type of <CODE>D</CODE></TD></TR> |
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86 | <TR><TD><CODE>dev</CODE></TD><TD>- Object of type <CODE>D</CODE></TD></TR> |
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87 | <TR><TD><CODE>s1</CODE></TD><TD>- Object of type <CODE>Ch*</CODE></SPAN></TD></TR> |
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88 | <TR><TD><CODE>s2</CODE></TD><TD>- Object of type <CODE>const Ch*</CODE></SPAN></TD></TR> |
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89 | <TR><TD><CODE>n</CODE></TD><TD>- Object of type <CODE>std::streamsize</CODE></TD></TR> |
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90 | </TABLE> |
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91 | |
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92 | <H2>Valid Expressions / Semantics</H2> |
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93 | |
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94 | <P>Same as <A HREF="device.html#types">Device</A>, with the following additional requirements:</P> |
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95 | |
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96 | <TABLE CELLPADDING="5" BORDER="1"> |
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97 | <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Category Precondition</TH><TH>Semantics</TH></TR> |
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98 | <TR> |
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99 | <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/read.html">boost::iostreams::read</A>(dev, s1, n)</CODE></PRE></TD> |
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100 | <TD><CODE>std::streamsize</CODE></TD> |
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101 | <TD ROWSPAN="2">Not convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> |
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102 | <TD> |
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103 | Reads up to <CODE>n</CODE> characters from the input sequence controlled by <CODE>dev</CODE> into <CODE>s1</CODE>, returning the number of characters read; returning a value less than n indicates end-of-sequence |
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104 | </TD> |
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105 | </TR> |
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106 | <TR> |
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107 | <TD><PRE CLASS="plain_code"><CODE><A HREF="../functions/write.html">boost::iostreams::write</A>(dev, s2, n)</CODE></PRE></TD> |
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108 | <TD><CODE>std::streamsize</CODE></TD> |
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109 | <TD> |
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110 | 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 |
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111 | </TD> |
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112 | </TR> |
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113 | <TR> |
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114 | <TD><PRE CLASS="plain_code"><CODE>dev.input_sequence()</CODE></PRE></TD> |
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115 | <TD><PRE CLASS="plain_code"><CODE>std::pair<Ch*,Ch*></CODE></PRE></TD> |
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116 | <TD ROWSPAN="2">Convertible to <A HREF="direct.html"><CODE>direct_tag</CODE></A></TD> |
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117 | <TD>Returns a pair of pointers delimiting the input sequence controlled by <CODE>dev</CODE></TD> |
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118 | </TR> |
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119 | <TR> |
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120 | <TD><PRE CLASS="plain_code"><CODE>dev.output_sequence()</CODE></PRE></TD> |
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121 | <TD><PRE CLASS="plain_code"><CODE>std::pair<Ch*,Ch*></CODE></PRE></TD> |
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122 | <TD>Returns a pair of pointers delimiting the output sequence controlled by <CODE>dev</CODE></TD> |
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123 | </TR> |
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124 | </TABLE> |
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125 | |
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126 | <H2>Exceptions</H2> |
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127 | |
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128 | <P> |
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129 | Errors which occur during the execution of member functions <CODE>read</CODE>, <CODE>write</CODE>, <CODE>input_sequence</CODE> or <CODE>output_sequence</CODE> are indicated by throwing exceptions. Reaching the end of the input sequence is not an error, but attempting to write past the end of the output sequence is. |
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130 | </P> |
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131 | |
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132 | <P> |
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133 | After an exception is thrown, an BidirectionalDevice must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. |
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134 | </P> |
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135 | |
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136 | |
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137 | <H2>Models</H2> |
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138 | |
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139 | <HR> |
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140 | |
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141 | <!-- Begin Footnotes --> |
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142 | |
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143 | <P> |
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144 | <A CLASS="footnote_ref" NAME="note_1" HREF="#note_1_ref"><SUP>[1]</SUP></A>Strictly speaking, (i) and (ii) can be varied independently for input and output, so there are actually five possibilities. |
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145 | </P> |
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146 | |
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147 | <!-- End Footnotes --> |
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148 | |
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149 | <!-- Begin Footer --> |
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150 | |
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151 | <HR> |
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152 | <P CLASS="copyright">Revised |
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153 | <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> |
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154 | 20 May, 2004 |
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155 | <!--webbot bot="Timestamp" endspan i-checksum="38504" --> |
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156 | </P> |
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157 | |
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158 | <P CLASS="copyright">© Copyright <A HREF="http://www.kangaroologic.com" TARGET="_top">Jonathan Turkanis</A>, 2004</P> |
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159 | <P CLASS="copyright"> |
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160 | 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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161 | </P> |
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