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  24 <a name="string_ref"></a>String_Ref</h2></div>
  25 <div><div class="authorgroup"><div class="author"><h3 class="author">
  26 <span class="firstname">Marshall</span> <span class="surname">Clow</span>
  27 </h3></div></div></div>
  28 <div><p class="copyright">Copyright &#169; 2012 Marshall Clow</p></div>
  29 <div><div class="legalnotice">
  30 <a name="string_ref.legal"></a><p>
  31         Distributed under the Boost Software License, Version 1.0. (See accompanying
  32         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
  33       </p>
  34 </div></div>
  35 </div>
  36 <hr>
  37 </div>
  38 <div class="toc">
  39 <p><b>Table of Contents</b></p>
  40 <dl class="toc">
  41 <dt><span class="section"><a href="string_ref.html#string_ref.overview">Overview</a></span></dt>
  42 <dt><span class="section"><a href="string_ref.html#string_ref.examples">Examples</a></span></dt>
  43 <dt><span class="section"><a href="string_ref.html#string_ref.reference">Reference </a></span></dt>
  44 <dt><span class="section"><a href="string_ref.html#string_ref.history">History</a></span></dt>
  45 </dl>
  46 </div>
  47 <div class="section">
  48 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  49 <a name="string_ref.overview"></a><a class="link" href="string_ref.html#string_ref.overview" title="Overview">Overview</a>
  50 </h2></div></div></div>
  51 <p>
  52       Boost.StringRef is an implementation of Jeffrey Yaskin's <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3442.html" target="_top">N3442:
  53       string_ref: a non-owning reference to a string</a>.
  54     </p>
  55 <p>
  56       When you are parsing/processing strings from some external source, frequently
  57       you want to pass a piece of text to a procedure for specialized processing.
  58       The canonical way to do this is as a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>,
  59       but that has certain drawbacks:
  60     </p>
  61 <p>
  62       1) If you are processing a buffer of text (say a HTTP response or the contents
  63       of a file), then you have to create the string from the text you want to pass,
  64       which involves memory allocation and copying of data.
  65     </p>
  66 <p>
  67       2) if a routine receives a constant <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
  68       and wants to pass a portion of that string to another routine, then it must
  69       create a new string of that substring.
  70     </p>
  71 <p>
  72       3) A routine receives a constant <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
  73       and wants to return a portion of the string, then it must create a new string
  74       to return.
  75     </p>
  76 <p>
  77       <code class="computeroutput"><span class="identifier">string_ref</span></code> is designed to solve
  78       these efficiency problems. A <code class="computeroutput"><span class="identifier">string_ref</span></code>
  79       is a read-only reference to a contiguous sequence of characters, and provides
  80       much of the functionality of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
  81       A <code class="computeroutput"><span class="identifier">string_ref</span></code> is cheap to create,
  82       copy and pass by value, because it does not actually own the storage that it
  83       points to.
  84     </p>
  85 <p>
  86       A <code class="computeroutput"><span class="identifier">string_ref</span></code> is implemented
  87       as a small struct that contains a pointer to the start of the character data
  88       and a count. A <code class="computeroutput"><span class="identifier">string_ref</span></code> is
  89       cheap to create and cheap to copy.
  90     </p>
  91 <p>
  92       <code class="computeroutput"><span class="identifier">string_ref</span></code> acts as a container;
  93       it includes all the methods that you would expect in a container, including
  94       iteration support, <code class="computeroutput"><span class="keyword">operator</span> <span class="special">[]</span></code>,
  95       <code class="computeroutput"><span class="identifier">at</span></code> and <code class="computeroutput"><span class="identifier">size</span></code>.
  96       It can be used with any of the iterator-based algorithms in the STL - as long
  97       as you don't need to change the underlying data (<code class="computeroutput"><span class="identifier">sort</span></code>
  98       and <code class="computeroutput"><span class="identifier">remove</span></code>, for example, will
  99       not work)
 100     </p>
 101 <p>
 102       Besides generic container functionality, <code class="computeroutput"><span class="identifier">string_ref</span></code>
 103       provides a subset of the interface of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
 104       This makes it easy to replace parameters of type <code class="computeroutput"><span class="keyword">const</span>
 105       <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&amp;</span></code>
 106       with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">string_ref</span></code>. Like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>,
 107       <code class="computeroutput"><span class="identifier">string_ref</span></code> has a static member
 108       variable named <code class="computeroutput"><span class="identifier">npos</span></code> to denote
 109       the result of failed searches, and to mean "the end".
 110     </p>
 111 <p>
 112       Because a <code class="computeroutput"><span class="identifier">string_ref</span></code> does not
 113       own the data that it "points to", it introduces lifetime issues into
 114       code that uses it. The programmer must ensure that the data that a <code class="computeroutput"><span class="identifier">string_ref</span></code> refers to exists as long as the
 115       <code class="computeroutput"><span class="identifier">string_ref</span></code> does.
 116     </p>
 117 </div>
 118 <div class="section">
 119 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 120 <a name="string_ref.examples"></a><a class="link" href="string_ref.html#string_ref.examples" title="Examples">Examples</a>
 121 </h2></div></div></div>
 122 <p>
 123       Integrating <code class="computeroutput"><span class="identifier">string_ref</span></code> into
 124       your code is fairly simple. Wherever you pass a <code class="computeroutput"><span class="keyword">const</span>
 125       <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&amp;</span></code>
 126       or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> as a parameter, that's a candidate
 127       for passing a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">string_ref</span></code>.
 128     </p>
 129 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">extract_part</span> <span class="special">(</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&amp;</span><span class="identifier">bar</span> <span class="special">)</span> <span class="special">{</span>
 130     <span class="keyword">return</span> <span class="identifier">bar</span><span class="special">.</span><span class="identifier">substr</span> <span class="special">(</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span> <span class="special">);</span>
 131     <span class="special">}</span>
 132
 133 <span class="keyword">if</span> <span class="special">(</span> <span class="identifier">extract_part</span> <span class="special">(</span> <span class="string">"ABCDEFG"</span> <span class="special">).</span><span class="identifier">front</span><span class="special">()</span> <span class="special">==</span> <span class="char">'C'</span> <span class="special">)</span> <span class="special">{</span> <span class="comment">/* do something */</span> <span class="special">}</span>
 134 </pre>
 135 <p>
 136       Let's figure out what happens in this (contrived) example.
 137     </p>
 138 <p>
 139       First, a temporary string is created from the string literal <code class="computeroutput"><span class="string">"ABCDEFG"</span></code>, and it is passed (by reference)
 140       to the routine <code class="computeroutput"><span class="identifier">extract_part</span></code>.
 141       Then a second string is created in the call <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">substr</span></code>
 142       and returned to <code class="computeroutput"><span class="identifier">extract_part</span></code>
 143       (this copy may be elided by RVO). Then <code class="computeroutput"><span class="identifier">extract_part</span></code>
 144       returns that string back to the caller (again this copy may be elided). The
 145       first temporary string is deallocated, and <code class="computeroutput"><span class="identifier">front</span></code>
 146       is called on the second string, and then it is deallocated as well.
 147     </p>
 148 <p>
 149       Two <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>s are created, and two copy operations.
 150       That's (potentially) four memory allocations and deallocations, and the associated
 151       copying of data.
 152     </p>
 153 <p>
 154       Now let's look at the same code with <code class="computeroutput"><span class="identifier">string_ref</span></code>:
 155     </p>
 156 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">string_ref</span> <span class="identifier">extract_part</span> <span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">string_ref</span> <span class="identifier">bar</span> <span class="special">)</span> <span class="special">{</span>
 157     <span class="keyword">return</span> <span class="identifier">bar</span><span class="special">.</span><span class="identifier">substr</span> <span class="special">(</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span> <span class="special">);</span>
 158     <span class="special">}</span>
 159
 160 <span class="keyword">if</span> <span class="special">(</span> <span class="identifier">extract_part</span> <span class="special">(</span> <span class="string">"ABCDEFG"</span> <span class="special">).</span><span class="identifier">front</span><span class="special">()</span> <span class="special">==</span> <span class="string">"C"</span> <span class="special">)</span> <span class="special">{</span> <span class="comment">/* do something */</span> <span class="special">}</span>
 161 </pre>
 162 <p>
 163       No memory allocations. No copying of character data. No changes to the code
 164       other than the types. There are two <code class="computeroutput"><span class="identifier">string_ref</span></code>s
 165       created, and two <code class="computeroutput"><span class="identifier">string_ref</span></code>s
 166       copied, but those are cheap operations.
 167     </p>
 168 </div>
 169 <div class="section">
 170 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 171 <a name="string_ref.reference"></a><a class="link" href="string_ref.html#string_ref.reference" title="Reference">Reference </a>
 172 </h2></div></div></div>
 173 <p>
 174       The header file "string_ref.hpp" defines a template <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">basic_string_ref</span></code>,
 175       and four specializations - for <code class="computeroutput"><span class="keyword">char</span></code>
 176       / <code class="computeroutput"><span class="keyword">wchar_t</span></code> / <code class="computeroutput"><span class="keyword">char16_t</span></code>
 177       / <code class="computeroutput"><span class="keyword">char32_t</span></code> .
 178     </p>
 179 <p>
 180       <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">string_ref</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
 181     </p>
 182 <p>
 183       Construction and copying:
 184     </p>
 185 <pre class="programlisting"><span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">basic_string_ref</span> <span class="special">();</span>    <span class="comment">// Constructs an empty string_ref</span>
 186 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">basic_string_ref</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">charT</span><span class="special">*</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// Constructs from a NULL-terminated string</span>
 187 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">basic_string_ref</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">charT</span><span class="special">*</span> <span class="identifier">str</span><span class="special">,</span> <span class="identifier">size_type</span> <span class="identifier">len</span><span class="special">);</span> <span class="comment">// Constructs from a pointer, length pair</span>
 188 <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Allocator</span><span class="special">&gt;</span>
 189 <span class="identifier">basic_string_ref</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_string</span><span class="special">&lt;</span><span class="identifier">charT</span><span class="special">,</span> <span class="identifier">traits</span><span class="special">,</span> <span class="identifier">Allocator</span><span class="special">&gt;&amp;</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// Constructs from a std::string</span>
 190 <span class="identifier">basic_string_ref</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">basic_string_ref</span> <span class="special">&amp;</span><span class="identifier">rhs</span><span class="special">);</span>
 191 <span class="identifier">basic_string_ref</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">basic_string_ref</span> <span class="special">&amp;</span><span class="identifier">rhs</span><span class="special">);</span>
 192 </pre>
 193 <p>
 194       <code class="computeroutput"><span class="identifier">string_ref</span></code> does not define
 195       a move constructor nor a move-assignment operator because copying a <code class="computeroutput"><span class="identifier">string_ref</span></code> is just a cheap as moving one.
 196     </p>
 197 <p>
 198       Basic container-like functions:
 199     </p>
 200 <pre class="programlisting"><span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">size_type</span> <span class="identifier">size</span><span class="special">()</span>     <span class="keyword">const</span> <span class="special">;</span>
 201 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">size_type</span> <span class="identifier">length</span><span class="special">()</span>   <span class="keyword">const</span> <span class="special">;</span>
 202 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">size_type</span> <span class="identifier">max_size</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 203 <span class="identifier">BOOST_CONSTEXPR</span> <span class="keyword">bool</span> <span class="identifier">empty</span><span class="special">()</span>         <span class="keyword">const</span> <span class="special">;</span>
 204
 205 <span class="comment">// All iterators are const_iterators</span>
 206 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">const_iterator</span>  <span class="identifier">begin</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 207 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">const_iterator</span> <span class="identifier">cbegin</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 208 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">const_iterator</span>    <span class="identifier">end</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 209 <span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">const_iterator</span>   <span class="identifier">cend</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 210 <span class="identifier">const_reverse_iterator</span>         <span class="identifier">rbegin</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 211 <span class="identifier">const_reverse_iterator</span>        <span class="identifier">crbegin</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 212 <span class="identifier">const_reverse_iterator</span>           <span class="identifier">rend</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 213 <span class="identifier">const_reverse_iterator</span>          <span class="identifier">crend</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 214 </pre>
 215 <p>
 216       Access to the individual elements (all of which are const):
 217     </p>
 218 <pre class="programlisting"><span class="identifier">BOOST_CONSTEXPR</span> <span class="keyword">const</span> <span class="identifier">charT</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">size_type</span> <span class="identifier">pos</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 219 <span class="keyword">const</span> <span class="identifier">charT</span><span class="special">&amp;</span> <span class="identifier">at</span><span class="special">(</span><span class="identifier">size_t</span> <span class="identifier">pos</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 220 <span class="identifier">BOOST_CONSTEXPR</span> <span class="keyword">const</span> <span class="identifier">charT</span><span class="special">&amp;</span> <span class="identifier">front</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span>
 221 <span class="identifier">BOOST_CONSTEXPR</span> <span class="keyword">const</span> <span class="identifier">charT</span><span class="special">&amp;</span> <span class="identifier">back</span><span class="special">()</span>  <span class="keyword">const</span> <span class="special">;</span>
 222 <span class="identifier">BOOST_CONSTEXPR</span> <span class="keyword">const</span> <span class="identifier">charT</span><span class="special">*</span> <span class="identifier">data</span><span class="special">()</span>  <span class="keyword">const</span> <span class="special">;</span>
 223 </pre>
 224 <p>
 225       Modifying the <code class="computeroutput"><span class="identifier">string_ref</span></code> (but
 226       not the underlying data):
 227     </p>
 228 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">clear</span><span class="special">();</span>
 229 <span class="keyword">void</span> <span class="identifier">remove_prefix</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">);</span>
 230 <span class="keyword">void</span> <span class="identifier">remove_suffix</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">);</span>
 231 </pre>
 232 <p>
 233       Searching:
 234     </p>
 235 <pre class="programlisting"><span class="identifier">size_type</span> <span class="identifier">find</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">s</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 236 <span class="identifier">size_type</span> <span class="identifier">find</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 237 <span class="identifier">size_type</span> <span class="identifier">rfind</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">s</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 238 <span class="identifier">size_type</span> <span class="identifier">rfind</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 239 <span class="identifier">size_type</span> <span class="identifier">find_first_of</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 240 <span class="identifier">size_type</span> <span class="identifier">find_last_of</span> <span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 241
 242 <span class="identifier">size_type</span> <span class="identifier">find_first_of</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">s</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 243 <span class="identifier">size_type</span> <span class="identifier">find_last_of</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">s</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 244 <span class="identifier">size_type</span> <span class="identifier">find_first_not_of</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">s</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 245 <span class="identifier">size_type</span> <span class="identifier">find_first_not_of</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 246 <span class="identifier">size_type</span> <span class="identifier">find_last_not_of</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">s</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 247 <span class="identifier">size_type</span> <span class="identifier">find_last_not_of</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 248 </pre>
 249 <p>
 250       String-like operations:
 251     </p>
 252 <pre class="programlisting"><span class="identifier">BOOST_CONSTEXPR</span> <span class="identifier">basic_string_ref</span> <span class="identifier">substr</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">pos</span><span class="special">,</span> <span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">=</span><span class="identifier">npos</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <span class="comment">// Creates a new string_ref</span>
 253 <span class="keyword">bool</span> <span class="identifier">starts_with</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 254 <span class="keyword">bool</span> <span class="identifier">starts_with</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 255 <span class="keyword">bool</span> <span class="identifier">ends_with</span><span class="special">(</span><span class="identifier">charT</span> <span class="identifier">c</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 256 <span class="keyword">bool</span> <span class="identifier">ends_with</span><span class="special">(</span><span class="identifier">basic_string_ref</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span>
 257 </pre>
 258 </div>
 259 <div class="section">
 260 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 261 <a name="string_ref.history"></a><a class="link" href="string_ref.html#string_ref.history" title="History">History</a>
 262 </h2></div></div></div>
 263 <h4>
 264 <a name="string_ref.history.h0"></a>
 265       <span class="phrase"><a name="string_ref.history.boost_1_53"></a></span><a class="link" href="string_ref.html#string_ref.history.boost_1_53">boost
 266       1.53</a>
 267     </h4>
 268 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 269           Introduced
 270         </li></ul></div>
 271 </div>
 272 </div>
 273 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 274 <td align="left"><p><small>Last revised: October 30, 2014 at 10:17:16 GMT</small></p></td>
 275 <td align="right"><div class="copyright-footer"></div></td>
 276 </tr></table>
 277 <hr>
 278 <div class="spirit-nav"></div>
 279 </body>
 280 </html>