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25<div class="section" lang="en">
26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="string_algo.design"></a>Design Topics</h2></div></div></div>
28<div class="toc"><dl>
29<dt><span class="section"><a href="design.html#string_algo.string">String Representation</a></span></dt>
30<dt><span class="section"><a href="design.html#string_algo.sequence_traits">Sequence Traits</a></span></dt>
31<dt><span class="section"><a href="design.html#string_algo.find">Find Algorithms</a></span></dt>
32<dt><span class="section"><a href="design.html#string_algo.replace">Replace Algorithms</a></span></dt>
33<dt><span class="section"><a href="design.html#string_algo.split">Find Iterators &amp; Split Algorithms</a></span></dt>
34<dt><span class="section"><a href="design.html#string_algo.exception">Exception Safety</a></span></dt>
35</dl></div>
36<div class="section" lang="en">
37<div class="titlepage"><div><div><h3 class="title">
38<a name="string_algo.string"></a>String Representation</h3></div></div></div>
39<p>
40            As the name suggest, this library works mainly with strings. However, in the context of this library,
41            a string is not restricted to any particular implementation (like <code class="computeroutput">std::basic_string</code>),
42            rather it is a concept. This allows the algorithms in this library to be reused for any string type,
43            that satisfies the given requirements.
44        </p>
45<p>
46            <span class="bold"><strong>Definition:</strong></span> A string is a
47            <a href="../../../libs/range/doc/range.html" target="_top">range</a> of characters accessible in sequential
48            ordered fashion. Character is any value type with "cheap" copying and assignment.               
49        </p>
50<p>
51            First requirement of string-type is that it must accessible using
52            <a href="../../../libs/range/index.html" target="_top">Boost.Range</a>. This facility allows to access
53            the elements inside the string in a uniform iterator-based fashion.
54            This is sufficient for our library
55        </p>
56<p>           
57            Second requirement defines the way in which the characters are stored in the string. Algorithms in
58            this library work with an assumption that copying a character is cheaper then allocating extra
59            storage to cache results. This is a natural assumption for common character types. Algorithms will
60            work even if this requirement is not satisfied, however at the cost of performance degradation.
61        </p>
62<p>
63        </p>
64<p>
65            In addition some algorithms have additional requirements on the string-type. Particularly, it is required
66            that an algorithm can create a new string of the given type. In this case, it is required that
67            the type satisfies the sequence (Std §23.1.1) requirements.
68        </p>
69<p>
70            In the reference and also in the code, requirement on the string type is designated by the name of
71            template argument. <code class="computeroutput">RangeT</code> means that the basic range requirements must hold.
72            <code class="computeroutput">SequenceT</code> designates extended sequence requirements.
73        </p>
74</div>
75<div class="section" lang="en">
76<div class="titlepage"><div><div><h3 class="title">
77<a name="string_algo.sequence_traits"></a>Sequence Traits</h3></div></div></div>
78<p>
79            The major difference between <code class="computeroutput">std::list</code> and <code class="computeroutput">std::vector</code> is not in the interfaces
80            they provide, but rather in the inner details of the class and the way how it performs
81            various operations. The problem is that it is not possible to infer this difference from the
82            definitions of classes without some special mechanism.
83            However, some algorithms can run significantly faster with the knowledge of the properties
84            of a particular container.
85        </p>
86<p>
87            Sequence traits allow one to specify additional properties of a sequence container (see Std.§32.2).
88            These properties are then used by algorithms to select optimized handling for some operations.
89            The sequence traits are declared in the header
90            <code class="computeroutput"><a href="reference.html#header.boost.algorithm.string.sequence_traits.hpp" title="Header &lt;boost/algorithm/string/sequence_traits.hpp&gt;">boost/algorithm/string/sequence_traits.hpp</a></code>.
91        </p>
92<p>
93            In the table C denotes a container and c is an object of C.
94        </p>
95<div class="table">
96<a name="id1641718"></a><p class="title"><b>Table 14.12. Sequence Traits</b></p>
97<table class="table" summary="Sequence Traits">
98<colgroup>
99<col>
100<col>
101</colgroup>
102<thead><tr>
103<th align="left">Trait</th>
104<th align="left">Description</th>
105</tr></thead>
106<tbody>
107<tr>
108<td align="left">
109<code class="computeroutput"><a href="../boost/algorithm/has_native_replace.html" title="Class template has_native_replace">has_native_replace&lt;C&gt;</a></code>::value</td>
110<td align="left">Specifies that the sequence has std::string like replace method</td>
111</tr>
112<tr>
113<td align="left">
114<code class="computeroutput"><a href="../boost/algorithm/has_stable_iterators.html" title="Class template has_stable_iterators">has_stable_iterators&lt;C&gt;</a></code>::value</td>
115<td align="left">
116                            Specifies that the sequence has stable iterators. It means,
117                            that operations like <code class="computeroutput">insert</code>/<code class="computeroutput">erase</code>/<code class="computeroutput">replace</code> 
118                            do not invalidate iterators.
119                        </td>
120</tr>
121<tr>
122<td align="left">
123<code class="computeroutput"><a href="../boost/algorithm/has_const_time_insert.html" title="Class template has_const_time_insert">has_const_time_insert&lt;C&gt;</a></code>::value</td>
124<td align="left">
125                            Specifies that the insert method of the sequence has
126                            constant time complexity.
127                        </td>
128</tr>
129<tr>
130<td align="left">
131<code class="computeroutput"><a href="../boost/algorithm/has_const_time_erase.html" title="Class template has_const_time_erase">has_const_time_erase&lt;C&gt;</a></code>::value</td>
132<td align="left">
133                            Specifies that the erase method of the sequence has constant time complexity
134                        </td>
135</tr>
136</tbody>
137</table>
138</div>
139<p>
140            Current implementation contains specializations for std::list&lt;T&gt; and
141            std::basic_string&lt;T&gt; from the standard library and SGI's std::rope&lt;T&gt; and std::slist&lt;T&gt;.
142        </p>
143</div>
144<div class="section" lang="en">
145<div class="titlepage"><div><div><h3 class="title">
146<a name="string_algo.find"></a>Find Algorithms</h3></div></div></div>
147<p>
148            Find algorithms have similar functionality to <code class="computeroutput">std::search()</code> algorithm. They provide a different
149            interface which is more suitable for common string operations.
150            Instead of returning just the start of matching subsequence they return a range which is necessary
151            when the length of the matching subsequence is not known beforehand.
152            This feature also allows a partitioning of  the input sequence into three
153            parts: a prefix, a substring and a suffix.
154        </p>
155<p>
156            Another difference is an addition of various searching methods besides find_first, including find_regex.
157        </p>
158<p>
159            It the library, find algorithms are implemented in terms of
160            <a href="concept.html#string_algo.finder_concept" title="Finder Concept">Finders</a>. Finders are used also by other facilities
161            (replace,split).
162            For convenience, there are also function wrappers for these finders to simplify find operations.
163        </p>
164<p>
165            Currently the library contains only naive implementation of find algorithms with complexity
166            O(n * m) where n is the size of the input sequence and m is the size of the search sequence.
167            There are algorithms with complexity O(n), but for smaller sequence a constant overhead is
168            rather big. For small m &lt;&lt; n (m by magnitude smaller than n) the current implementation
169            provides acceptable efficiency.
170            Even the C++ standard defines the required complexity for search algorithm as O(n * m).
171            It is possible that a future version of library will also contain algorithms with linear
172            complexity as an option
173        </p>
174</div>
175<div class="section" lang="en">
176<div class="titlepage"><div><div><h3 class="title">
177<a name="string_algo.replace"></a>Replace Algorithms</h3></div></div></div>
178<p>
179            The implementation of replace algorithms follows the layered structure of the library. The
180            lower layer implements generic substitution of a range in the input sequence.
181            This layer takes a <a href="concept.html#string_algo.finder_concept" title="Finder Concept">Finder</a> object and a
182            <a href="concept.html#string_algo.formatter_concept" title="Formatter concept">Formatter</a> object as an input. These two
183            functors define what to replace and what to replace it with. The upper layer functions
184            are just wrapping calls to the lower layer. Finders are shared with the find and split facility.
185        </p>
186<p>
187            As usual, the implementation of the lower layer is designed to work with a generic sequence while
188            taking advantage of specific features if possible
189            (by using <a href="design.html#string_algo.sequence_traits" title="Sequence Traits">Sequence traits</a>)
190        </p>
191</div>
192<div class="section" lang="en">
193<div class="titlepage"><div><div><h3 class="title">
194<a name="string_algo.split"></a>Find Iterators &amp; Split Algorithms</h3></div></div></div>
195<p>
196            Find iterators are a logical extension of the <a href="design.html#string_algo.find" title="Find Algorithms">find facility</a>.
197            Instead of searching for one match, the whole input can be iteratively searched for multiple matches.
198            The result of the search is then used to partition the input. It depends on the algorithms which parts
199            are returned as the result. They can be the matching parts (<code class="computeroutput"><a href="../boost/algorithm/find_iterator.html" title="Class template find_iterator">find_iterator</a></code>) of the parts in
200            between (<code class="computeroutput"><a href="../boost/algorithm/split_iterator.html" title="Class template split_iterator">split_iterator</a></code>).
201        </p>
202<p>
203            In addition the split algorithms like <code class="computeroutput"><a href="../boost/algorithm/find_all.html" title="Function template find_all">find_all()</a></code> and <code class="computeroutput"><a href="../id691162-bb.html" title="Function template split">split()</a></code>
204            can simplify the common operations. They use a find iterator to search the whole input and copy the
205            matches they found into the supplied container.
206        </p>
207</div>
208<div class="section" lang="en">
209<div class="titlepage"><div><div><h3 class="title">
210<a name="string_algo.exception"></a>Exception Safety</h3></div></div></div>
211<p>
212            The library requires that all operations on types used as template
213            or function arguments provide the <span class="emphasis"><em>basic exception-safety guarantee</em></span>.
214            In turn, all functions and algorithms in this library, except where stated
215            otherwise, will provide the <span class="emphasis"><em>basic exception-safety guarantee</em></span>.
216            In other words:
217            The library maintains its invariants and does not leak resources in
218            the face of exceptions.  Some library operations give stronger
219            guarantees, which are documented on an individual basis.
220        </p>
221<p>
222            Some functions can provide the <span class="emphasis"><em>strong exception-safety guarantee</em></span>.
223            That means that following statements are true:   
224            </p>
225<div class="itemizedlist"><ul type="disc">
226<li>
227                    If an exception is thrown, there are no effects other than those
228                    of the function
229                </li>
230<li>
231                    If an exception is thrown other than by the function, there are no effects
232                </li>
233</ul></div>
234<p>
235            This guarantee can be provided under the condition that the operations
236            on the types used for arguments for these functions either
237            provide the strong exception guarantee or do not alter the global state .
238         </p>
239<p>
240            In the reference, under the term <span class="emphasis"><em>strong exception-safety guarantee</em></span>, we mean the
241            guarantee as defined above.           
242        </p>
243<p>
244            For more information about the exception safety topics, follow this
245            <a href="../../../more/generic_exception_safety.html" target="_top">link</a>
246        </p>
247</div>
248</div>
249<table width="100%"><tr>
250<td align="left"><small><p>Last revised: August 16, 2006 at 07:10:48 GMT</p></small></td>
251<td align="right"><small>Copyright © 2002-2004 Pavol Droba</small></td>
252</tr></table>
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