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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>
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<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>
</div>
<div class="section">
<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>
</h2></div></div></div>
<h4>
<a name="string_ref.history.h0"></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>
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Introduced
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<td align="left"><p><small>Last revised: April 07, 2015 at 22:46:52 GMT</small></p></td>
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