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<div><h2 class="title">
<a name="string_ref"></a>String_Ref</h2></div>
<div><div class="authorgroup"><div class="author"><h3 class="author">
<span class="firstname">Marshall</span> <span class="surname">Clow</span>
</h3></div></div></div>
<div><p class="copyright">Copyright &#169; 2012 Marshall Clow</p></div>
<div><div class="legalnotice">
<a name="string_ref.legal"></a><p>
        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" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></div>
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</div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="string_ref.html#string_ref.overview">Overview</a></span></dt>
<dt><span class="section"><a href="string_ref.html#string_ref.examples">Examples</a></span></dt>
<dt><span class="section"><a href="string_ref.html#string_ref.reference">Reference </a></span></dt>
<dt><span class="section"><a href="string_ref.html#string_ref.history">History</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="string_ref.overview"></a><a class="link" href="string_ref.html#string_ref.overview" title="Overview">Overview</a>
</h2></div></div></div>
<p>
      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:
      string_ref: a non-owning reference to a string</a>.
    </p>
<p>
      When you are parsing/processing strings from some external source, frequently
      you want to pass a piece of text to a procedure for specialized processing.
      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>,
      but that has certain drawbacks:
    </p>
<p>
      1) If you are processing a buffer of text (say a HTTP response or the contents
      of a file), then you have to create the string from the text you want to pass,
      which involves memory allocation and copying of data.
    </p>
<p>
      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>
      and wants to pass a portion of that string to another routine, then it must
      create a new string of that substring.
    </p>
<p>
      3) A routine receives a constant <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
      and wants to return a portion of the string, then it must create a new string
      to return.
    </p>
<p>
      <code class="computeroutput"><span class="identifier">string_ref</span></code> is designed to solve
      these efficiency problems. A <code class="computeroutput"><span class="identifier">string_ref</span></code>
      is a read-only reference to a contiguous sequence of characters, and provides
      much of the functionality of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
      A <code class="computeroutput"><span class="identifier">string_ref</span></code> is cheap to create,
      copy and pass by value, because it does not actually own the storage that it
      points to.
    </p>
<p>
      A <code class="computeroutput"><span class="identifier">string_ref</span></code> is implemented
      as a small struct that contains a pointer to the start of the character data
      and a count. A <code class="computeroutput"><span class="identifier">string_ref</span></code> is
      cheap to create and cheap to copy.
    </p>
<p>
      <code class="computeroutput"><span class="identifier">string_ref</span></code> acts as a container;
      it includes all the methods that you would expect in a container, including
      iteration support, <code class="computeroutput"><span class="keyword">operator</span> <span class="special">[]</span></code>,
      <code class="computeroutput"><span class="identifier">at</span></code> and <code class="computeroutput"><span class="identifier">size</span></code>.
      It can be used with any of the iterator-based algorithms in the STL - as long
      as you don't need to change the underlying data (<code class="computeroutput"><span class="identifier">sort</span></code>
      and <code class="computeroutput"><span class="identifier">remove</span></code>, for example, will
      not work)
    </p>
<p>
      Besides generic container functionality, <code class="computeroutput"><span class="identifier">string_ref</span></code>
      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>.
      This makes it easy to replace parameters of type <code class="computeroutput"><span class="keyword">const</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&amp;</span></code>
      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>,
      <code class="computeroutput"><span class="identifier">string_ref</span></code> has a static member
      variable named <code class="computeroutput"><span class="identifier">npos</span></code> to denote
      the result of failed searches, and to mean "the end".
    </p>
<p>
      Because a <code class="computeroutput"><span class="identifier">string_ref</span></code> does not
      own the data that it "points to", it introduces lifetime issues into
      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
      <code class="computeroutput"><span class="identifier">string_ref</span></code> does.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="string_ref.examples"></a><a class="link" href="string_ref.html#string_ref.examples" title="Examples">Examples</a>
</h2></div></div></div>
<p>
      Integrating <code class="computeroutput"><span class="identifier">string_ref</span></code> into
      your code is fairly simple. Wherever you pass a <code class="computeroutput"><span class="keyword">const</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&amp;</span></code>
      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
      for passing a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">string_ref</span></code>.
    </p>
<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>
    <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>
    <span class="special">}</span>

<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>
</pre>
<p>
      Let's figure out what happens in this (contrived) example.
    </p>
<p>
      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)
      to the routine <code class="computeroutput"><span class="identifier">extract_part</span></code>.
      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>
      and returned to <code class="computeroutput"><span class="identifier">extract_part</span></code>
      (this copy may be elided by RVO). Then <code class="computeroutput"><span class="identifier">extract_part</span></code>
      returns that string back to the caller (again this copy may be elided). The
      first temporary string is deallocated, and <code class="computeroutput"><span class="identifier">front</span></code>
      is called on the second string, and then it is deallocated as well.
    </p>
<p>
      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.
      That's (potentially) four memory allocations and deallocations, and the associated
      copying of data.
    </p>
<p>
      Now let's look at the same code with <code class="computeroutput"><span class="identifier">string_ref</span></code>:
    </p>
<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>
    <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>
    <span class="special">}</span>

<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>
</pre>
<p>
      No memory allocations. No copying of character data. No changes to the code
      other than the types. There are two <code class="computeroutput"><span class="identifier">string_ref</span></code>s
      created, and two <code class="computeroutput"><span class="identifier">string_ref</span></code>s
      copied, but those are cheap operations.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="string_ref.reference"></a><a class="link" href="string_ref.html#string_ref.reference" title="Reference">Reference </a>
</h2></div></div></div>
<p>
      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>,
      and four specializations - for <code class="computeroutput"><span class="keyword">char</span></code>
      / <code class="computeroutput"><span class="keyword">wchar_t</span></code> / <code class="computeroutput"><span class="keyword">char16_t</span></code>
      / <code class="computeroutput"><span class="keyword">char32_t</span></code> .
    </p>
<p>
      <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>
    </p>
<p>
      Construction and copying:
    </p>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
</pre>
<p>
      <code class="computeroutput"><span class="identifier">string_ref</span></code> does not define
      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.
    </p>
<p>
      Basic container-like functions:
    </p>
<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>
<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>
<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>
<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>

<span class="comment">// All iterators are const_iterators</span>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
<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>
</pre>
<p>
      Access to the individual elements (all of which are const):
    </p>
<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>
<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>
<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>
<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>
<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>
</pre>
<p>
      Modifying the <code class="computeroutput"><span class="identifier">string_ref</span></code> (but
      not the underlying data):
    </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">clear</span><span class="special">();</span>
<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>
<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>
</pre>
<p>
      Searching:
    </p>
<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>
<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>
<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>
<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>
<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>
<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>

<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>
<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>
<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>
<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>
<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>
<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>
</pre>
<p>
      String-like operations:
    </p>
<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>
<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>
<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>
<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>
<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>
</pre>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="string_ref.history"></a><a class="link" href="string_ref.html#string_ref.history" title="History">History</a>
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<h4>
<a name="string_ref.history.h0"></a>
      <span class="phrase"><a name="string_ref.history.boost_1_71"></a></span><a class="link" href="string_ref.html#string_ref.history.boost_1_71">boost
      1.71</a>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          Glen Fernandes updated the implementation of the stream insertion operator
          to write directly to the <code class="computeroutput"><span class="identifier">basic_streambuf</span></code>
          and refactored that functionality into a common utility.
        </li></ul></div>
<h4>
<a name="string_ref.history.h1"></a>
      <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
      1.53</a>
    </h4>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          Introduced
        </li></ul></div>
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