| [29] | 1 | <!doctype HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
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| 2 | <html> |
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| 3 | <!-- |
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| 4 | (C) Copyright 2002-4 Robert Ramey - http://www.rrsd.com . |
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| 5 | Use, modification and distribution is subject to the Boost Software |
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| 6 | License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
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| 7 | http://www.boost.org/LICENSE_1_0.txt) |
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| 8 | --> |
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| 9 | <head> |
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| 10 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
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| 11 | <link rel="stylesheet" type="text/css" href="../../../boost.css"> |
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| 12 | <link rel="stylesheet" type="text/css" href="style.css"> |
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| 13 | <title>Serialization - Dataflow Iterators</title> |
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| 14 | </head> |
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| 15 | <body link="#0000ff" vlink="#800080"> |
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| 16 | <table border="0" cellpadding="7" cellspacing="0" width="100%" summary="header"> |
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| 17 | <tr> |
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| 18 | <td valign="top" width="300"> |
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| 19 | <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3> |
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| 20 | </td> |
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| 21 | <td valign="top"> |
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| 22 | <h1 align="center">Serialization</h1> |
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| 23 | <h2 align="center">Dataflow Iterators</h2> |
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| 24 | </td> |
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| 25 | </tr> |
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| 26 | </table> |
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| 27 | <hr> |
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| 28 | <h3>Motivation</h3> |
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| 29 | Consider the problem of translating an arbitrary length sequence of 8 bit bytes |
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| 30 | to base64 text. Such a process can be summarized as: |
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| 31 | <p> |
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| 32 | source => 8 bit bytes => 6 bit integers => encode to base64 characters => insert line breaks => destination |
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| 33 | <p> |
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| 34 | We would prefer the solution that is: |
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| 35 | <ul> |
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| 36 | <li>Decomposable. so we can code, test, verify and use each (simple) stage of the conversion |
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| 37 | independently. |
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| 38 | <li>Composable. so we can used this composite as a new component somewhere else. |
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| 39 | <li>Efficient, so we're not required to re-implement it again. |
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| 40 | <li>Scalable, so that it works well for short and arbitrarily long sequences. |
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| 41 | </ul> |
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| 42 | The approach that comes closest to meeting these requirements is that described |
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| 43 | and implemented with <a href="../../iterator/doc/index.html">Iterator Adaptors</a>. |
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| 44 | The fundamental feature of an Iterator Adaptor template that makes in interesting to |
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| 45 | us is that it takes as a parameter a base iterator from which it derives its |
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| 46 | input. This suggests that something like the following might be possible. |
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| 47 | <pre><code> |
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| 48 | typedef |
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| 49 | insert_linebreaks< // insert line breaks every 72 characters |
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| 50 | base64_from_binary< // convert binary values ot base64 characters |
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| 51 | transform_width< // retrieve 6 bit integers from a sequence of 8 bit bytes |
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| 52 | const char *, |
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| 53 | 6, |
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| 54 | 8 |
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| 55 | > |
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| 56 | > |
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| 57 | ,72 |
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| 58 | > |
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| 59 | base64_text; // compose all the above operations in to a new iterator |
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| 60 | |
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| 61 | std::copy( |
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| 62 | base64_text(address), |
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| 63 | base64_text(address + count), |
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| 64 | ostream_iterator<CharType>(os) |
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| 65 | ); |
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| 66 | </code></pre> |
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| 67 | Indeed, this seems to be exactly the kind of problem that iterator adaptors are |
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| 68 | intended to address. The Iterator Adaptor library already includes |
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| 69 | modules which can be configured to implement some of the operations above. For example, |
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| 70 | included is <a target="transform_iterator" href="../../iterator/doc/transform_iterator.html"> |
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| 71 | transform_iterator</a>, which can be used to implement 6 bit integer => base64 code. |
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| 72 | |
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| 73 | <h3>Dataflow Iterators</h3> |
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| 74 | Unfortunately, not all iterators which inherit from Iterator Adaptors are guarenteed |
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| 75 | to meet the composability goals stated above. To accomplish this purpose, they have |
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| 76 | to be written with some additional considerations in mind. |
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| 77 | |
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| 78 | We define a Dataflow Iterator as an class inherited from <code style="white-space: normal">iterator_adaptor</code> which |
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| 79 | fulfills as a small set of additional requirements. |
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| 80 | |
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| 81 | <h4>Templated Constructors</h4> |
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| 82 | <p> |
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| 83 | Templated constructor have the form: |
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| 84 | <pre><code> |
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| 85 | template<class T> |
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| 86 | dataflow_iterator(T start) : |
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| 87 | iterator_adaptor(Base(start)) |
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| 88 | {} |
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| 89 | </code></pre> |
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| 90 | When these constructors are applied to our example of above, the following code is generated: |
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| 91 | <pre><code> |
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| 92 | std::copy( |
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| 93 | insert_linebreaks( |
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| 94 | base64_from_binary( |
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| 95 | transform_width( |
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| 96 | address |
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| 97 | ), |
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| 98 | ) |
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| 99 | ), |
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| 100 | insert_linebreaks( |
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| 101 | base64_from_binary( |
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| 102 | transform_width( |
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| 103 | address + count |
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| 104 | ) |
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| 105 | ) |
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| 106 | ) |
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| 107 | ostream_iterator<char>(os) |
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| 108 | ); |
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| 109 | </code></pre> |
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| 110 | The recursive application of this template is what automatically generates the |
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| 111 | constructor <code style="white-space: normal">base64_text(const char *)</code> in our example above. The original |
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| 112 | Iterator Adaptors include a <code style="white-space: normal">make_xxx_iterator</code> to fulfill this function. |
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| 113 | However, I believe these are unwieldy to use compared to the above solution usiing |
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| 114 | Templated constructors. |
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| 115 | <p> |
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| 116 | Unfortunately, some systems which fail to properly support partial function template |
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| 117 | ordering cannot support the concept of a templated constructor as implemented above. |
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| 118 | A special"wrapper" macro has been created to work around this problem. With this "wrapper" |
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| 119 | the above example is modified to: |
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| 120 | <pre><code> |
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| 121 | std::copy( |
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| 122 | base64_text(BOOST_MAKE_PFTO_WRAPPER(address)), |
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| 123 | base64_text(BOOST_MAKE_PFTO_WRAPPER(address + count)), |
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| 124 | ostream_iterator<char>(os) |
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| 125 | ); |
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| 126 | </code></pre> |
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| 127 | This macro is defined in <a target="pfto" href="../../../boost/pfto.hpp"><boost/pfto.hpp></a>. |
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| 128 | For more information about this topic, check the source. |
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| 129 | |
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| 130 | <h4>Dereferencing</h4> |
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| 131 | Dereferencing some iterators can cause problems. For example, a natural |
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| 132 | way to write a <code style="white-space: normal">remove_whitespace</code> iterator is to increment past the initial |
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| 133 | whitespaces when the iterator is constructed. This will fail if the iterator passed to the |
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| 134 | constructor "points" to the end of a string. The |
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| 135 | <a target="filter_iterator" href="../../iterator/doc/filter_iterator.html"> |
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| 136 | <code style="white-space: normal">filter_iterator</code></a> is implemented |
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| 137 | in this way so it can't be used in our context. So, for implementation of this iterator, |
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| 138 | space removal is deferred until the iterator actually is dereferenced. |
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| 139 | |
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| 140 | <h4>Comparison</h4> |
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| 141 | The default implementation of iterator equality of <code style="white-space: normal">iterator_adaptor</code> just |
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| 142 | invokes the equality operator on the base iterators. Generally this is satisfactory. |
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| 143 | However, this implies that other operations (E. G. dereference) do not prematurely |
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| 144 | increment the base iterator. Avoiding this can be surprisingly tricky in some cases. |
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| 145 | (E.G. transform_width) |
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| 146 | |
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| 147 | <p> |
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| 148 | Iterators which fulfill the above requirements should be composable and the above sample |
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| 149 | code should implement our binary to base64 conversion. |
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| 150 | |
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| 151 | <h3>Iterators Included in the Library</h3> |
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| 152 | Dataflow iterators for the serialization library are all defined in the hamespace |
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| 153 | <code style="white-space: normal">boost::archive::iterators</code> included here are: |
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| 154 | <dl class="index"> |
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| 155 | <dt><a target="base64_from_binary" href="../../../boost/archive/iterators/base64_from_binary.hpp"> |
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| 156 | base64_from_binary</a></dt> |
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| 157 | <dd>transforms a sequence of integers to base64 text</dd> |
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| 158 | |
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| 159 | <dt><a target="base64_from_binary" href="../../../boost/archive/iterators/binary_from_base64.hpp"> |
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| 160 | binary_from_base64</a></dt> |
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| 161 | <dd>transforms a sequence of base64 characters to a sequence of integers</dd> |
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| 162 | |
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| 163 | <dt><a target="insert_linebreaks" href="../../../boost/archive/iterators/insert_linebreaks.hpp"> |
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| 164 | insert_linebreaks</a></dt> |
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| 165 | <dd>given a sequence, creates a sequence with newline characters inserted</dd> |
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| 166 | |
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| 167 | <dt><a target="mb_from_wchar" href="../../../boost/archive/iterators/mb_from_wchar.hpp"> |
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| 168 | mb_from_wchar</a></dt> |
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| 169 | <dd>transforms a sequence of wide characters to a sequence of multi-byte characters</dd> |
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| 170 | |
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| 171 | <dt><a target="remove_whitespace" href="../../../boost/archive/iterators/remove_whitespace.hpp"> |
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| 172 | remove_whitespace</a></dt> |
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| 173 | <dd>given a sequence of characters, returns a sequence with the white characters |
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| 174 | removed. This is a derivation from the <code style="white-space: normal">boost::filter_iterator</code></dd> |
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| 175 | |
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| 176 | <dt><a target="transform_width" href="../../../boost/archive/iterators/transform_width.hpp"> |
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| 177 | transform_width</a></dt> |
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| 178 | <dd>transforms a sequence of x bit elements into a sequence of y bit elements. This |
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| 179 | is a key component in iterators which translate to and from base64 text.</dd> |
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| 180 | |
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| 181 | <dt><a target="wchar_from_mb" href="../../../boost/archive/iterators/wchar_from_mb.hpp"> |
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| 182 | wchar_from_mb</a></dt> |
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| 183 | <dd>transform a sequence of multi-byte characters in the current locale to wide characters.</dd> |
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| 184 | |
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| 185 | <dt><a target="xml_escape" href="../../../boost/archive/iterators/xml_escape.hpp"> |
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| 186 | xml_escape</a></dt> |
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| 187 | <dd>escapes xml meta-characters from xml text</dd> |
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| 188 | |
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| 189 | <dt><a target="xml_unescape" href="../../../boost/archive/iterators/xml_unescape.hpp"> |
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| 190 | xml_unescape</a></dt> |
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| 191 | <dd>unescapes xml escape sequences to create a sequence of normal text<dd> |
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| 192 | </dl> |
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| 193 | <p> |
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| 194 | The standard stream iterators don't quite work for us. On systems which implement <code style="white-space: normal">wchar_t</code> |
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| 195 | as unsigned short integers (E.G. VC 6) they didn't function as I expected. I also made some |
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| 196 | adjustments to be consistent with our concept of Dataflow Iterators. Like the rest of our |
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| 197 | iterators, they are found in the namespace <code style="white-space: normal">boost::archive::interators</code> to avoid |
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| 198 | conflict the standard library version. |
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| 199 | <dl class = "index"> |
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| 200 | <dt><a target="istream_iterator" href="../../../boost/archive/iterators/istream_iterator.hpp"> |
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| 201 | istream_iterator</a></dt> |
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| 202 | <dt><a target="ostream_iterator" href="../../../boost/archive/iterators/ostream_iterator.hpp"> |
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| 203 | ostream_iterator</a></dt> |
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| 204 | </dl> |
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| 205 | |
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| 206 | <hr> |
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| 207 | <p><i>© Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004. |
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| 208 | Distributed under the Boost Software License, Version 1.0. (See |
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| 209 | accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
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| 210 | </i></p> |
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| 211 | </body> |
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| 212 | </html> |
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