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<div><h2 class="title">
<a name="stl_interfaces"></a>Chapter 39. Boost.STLInterfaces</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Zach</span> <span class="surname">Laine</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2019 T. Zachary Laine</p></div>
<div><div class="legalnotice">
<a name="stl_interfaces.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="stl_interfaces.html#boost_stlinterfaces.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/this_library_s_relationship_to_boost_iterator.html">This
Library's Relationship to Boost.Iterator</a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/tutorial___iterator_interface_.html">Tutorial:
<code class="computeroutput"><span class="identifier">iterator_interface</span></code></a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/tutorial___view_interface_.html">Tutorial:
<code class="computeroutput"><span class="identifier">view_interface</span></code></a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/tutorial___sequence_container_interface_.html">Tutorial:
<code class="computeroutput"><span class="identifier">sequence_container_interface</span></code></a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/tutorial___reverse_iterator_.html">Tutorial:
<code class="computeroutput"><span class="identifier">reverse_iterator</span></code></a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/examples.html">Examples</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_stlinterfaces/examples.html#boost_stlinterfaces.examples.random_access_iterator">Random
Access Iterator</a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/examples.html#boost_stlinterfaces.examples.mutable_and_constant_iterator_interoperability">Mutable
and Constant Iterator Interoperability</a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/examples.html#boost_stlinterfaces.examples.zip_iterator___proxy_iterator">Zip
Iterator / Proxy Iterator</a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/examples.html#boost_stlinterfaces.examples.reimplementing__back_insert_iterator_">Reimplementing
<code class="computeroutput"><span class="identifier">back_insert_iterator</span></code></a></span></dt>
<dt><span class="section"><a href="boost_stlinterfaces/examples.html#boost_stlinterfaces.examples.reimplementing__reverse_iterator_">Reimplementing
<code class="computeroutput"><span class="identifier">reverse_iterator</span></code></a></span></dt>
</dl></dd>
<dt><span class="section"><a href="stl_interfaces/compiler_support.html">Compiler Support</a></span></dt>
<dt><span class="section"><a href="stl_interfaces/reference.html">Reference</a></span></dt>
<dd><dl><dt><span class="section"><a href="stl_interfaces/reference.html#headers">Headers</a></span></dt></dl></dd>
<dt><span class="section"><a href="boost_stlinterfaces/rationale.html">Rationale</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_stlinterfaces.introduction"></a><a class="link" href="stl_interfaces.html#boost_stlinterfaces.introduction" title="Introduction">Introduction</a>
</h2></div></div></div>
<p>
Writing STL iterators, views, and containers is surprisingly hard. There are
a lot of things that can subtly go wrong. It is also very tedious, which of
course makes it error-prone.
</p>
<p>
Iterators have numerous typedefs and operations, even though all the operations
of a given iterator can be implemented in terms of at most four operations
(and usually only three). Writing all the other operations yields very similar-looking
code that is hard to review, and all but requires that you write full-coverage
tests for each iterator.
</p>
<p>
Writing view types like those found in <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ranges</span></code>
is also laborious, considering that most of each view type's API can be derived
from <code class="computeroutput"><span class="identifier">begin</span><span class="special">()</span></code>
and <code class="computeroutput"><span class="identifier">end</span><span class="special">()</span></code>.
C++20 has a template that does exactly this, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ranges</span><span class="special">::</span><span class="identifier">view_interface</span></code>;
Boost.STLInterfaces provides a pre-C++20-friendly implementation.
</p>
<p>
Most daunting of all is the task of writing a type or template that meets the
container requirements in the standard. Boost.STLInterfaces provides another
template called <code class="computeroutput"><a class="link" href="boost/stl_interfaces/v1/sequ_1_3_40_11_2_5_1_1_1_1.html" title="Struct template sequence_container_interface">sequence_container_interface</a></code>
that reduces the implementation and testing burden dramatically.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
C++20 versions of <code class="computeroutput"><a class="link" href="boost/stl_interfaces/v1/iterator_interface.html" title="Struct template iterator_interface">iterator_interface</a></code> and <code class="computeroutput"><a class="link" href="boost/stl_interfaces/v1/sequ_1_3_40_11_2_5_1_1_1_1.html" title="Struct template sequence_container_interface">sequence_container_interface</a></code>
are provided (C++20 provides <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">view_interface</span></code>).
These are constrained templates using C++20 concepts. These are in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">stl_interfaces</span><span class="special">::</span><span class="identifier">v2</span></code> namespace, and are considered experimental,
because at the time of this writing, no C++20-conforming compiler exists.
</p></td></tr>
</table></div>
<h4>
<a name="boost_stlinterfaces.introduction.h0"></a>
<span class="phrase"><a name="boost_stlinterfaces.introduction.a_quick_example"></a></span><a class="link" href="stl_interfaces.html#boost_stlinterfaces.introduction.a_quick_example">A
Quick Example</a>
</h4>
<p>
Here is an example of the iterator portion of the library. Let's say that we
wanted to make a random access iterator that represents a string of arbitrary
length constructed by repeating a shorter string. Let's call this iterator
<code class="computeroutput"><span class="identifier">repeated_chars_iterator</span></code>. Here
it is in action:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">repeated_chars_iterator</span> <span class="identifier">first</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">0</span><span class="special">);</span> <span class="comment">// 3 is the length of "foo", 0 is this iterator's position.</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">last</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">7</span><span class="special">);</span> <span class="comment">// Same as above, but now the iterator's position is 7.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">result</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">back_inserter</span><span class="special">(</span><span class="identifier">result</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">result</span> <span class="special">==</span> <span class="string">"foofoof"</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
There's nothing in the standard library that gets us that kind of behavior,
so we have to write it. This library seeks to turn what we write from this:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">repeated_chars_iterator</span>
<span class="special">{</span>
<span class="keyword">using</span> <span class="identifier">value_type</span> <span class="special">=</span> <span class="keyword">char</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">difference_type</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">pointer</span> <span class="special">=</span> <span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*;</span>
<span class="keyword">using</span> <span class="identifier">reference</span> <span class="special">=</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">iterator_category</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">random_access_iterator_tag</span><span class="special">;</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">:</span>
<span class="identifier">first_</span><span class="special">(</span><span class="keyword">nullptr</span><span class="special">),</span>
<span class="identifier">size_</span><span class="special">(</span><span class="number">0</span><span class="special">),</span>
<span class="identifier">n_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
<span class="special">{}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span><span class="special">(</span>
<span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">first</span><span class="special">,</span>
<span class="identifier">difference_type</span> <span class="identifier">size</span><span class="special">,</span>
<span class="identifier">difference_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">:</span>
<span class="identifier">first_</span><span class="special">(</span><span class="identifier">first</span><span class="special">),</span>
<span class="identifier">size_</span><span class="special">(</span><span class="identifier">size</span><span class="special">),</span>
<span class="identifier">n_</span><span class="special">(</span><span class="identifier">n</span><span class="special">)</span>
<span class="special">{}</span>
<span class="keyword">constexpr</span> <span class="identifier">reference</span> <span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">first_</span><span class="special">[</span><span class="identifier">n_</span> <span class="special">%</span> <span class="identifier">size_</span><span class="special">];</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">value_type</span> <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">difference_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">first_</span><span class="special">[(</span><span class="identifier">n_</span> <span class="special">+</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">%</span> <span class="identifier">size_</span><span class="special">];</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">++()</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="special">++</span><span class="identifier">n_</span><span class="special">;</span>
<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="keyword">operator</span><span class="special">++(</span><span class="keyword">int</span><span class="special">)</span><span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">retval</span> <span class="special">=</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">++*</span><span class="keyword">this</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">retval</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">+=(</span><span class="identifier">difference_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="identifier">n_</span> <span class="special">+=</span> <span class="identifier">n</span><span class="special">;</span>
<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">--()</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="special">--</span><span class="identifier">n_</span><span class="special">;</span>
<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="keyword">operator</span><span class="special">--(</span><span class="keyword">int</span><span class="special">)</span><span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">retval</span> <span class="special">=</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">--*</span><span class="keyword">this</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">retval</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">-=(</span><span class="identifier">difference_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="identifier">n_</span> <span class="special">-=</span> <span class="identifier">n</span><span class="special">;</span>
<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">lhs</span><span class="special">.</span><span class="identifier">first_</span> <span class="special">==</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">first_</span> <span class="special">&amp;&amp;</span> <span class="identifier">lhs</span><span class="special">.</span><span class="identifier">n_</span> <span class="special">==</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">n_</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!=(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="special">!(</span><span class="identifier">lhs</span> <span class="special">==</span> <span class="identifier">rhs</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&lt;(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">lhs</span><span class="special">.</span><span class="identifier">first_</span> <span class="special">==</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">first_</span> <span class="special">&amp;&amp;</span> <span class="identifier">lhs</span><span class="special">.</span><span class="identifier">n_</span> <span class="special">&lt;</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">n_</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&lt;=(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">lhs</span> <span class="special">==</span> <span class="identifier">rhs</span> <span class="special">||</span> <span class="identifier">lhs</span> <span class="special">&lt;</span> <span class="identifier">rhs</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&gt;(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">rhs</span> <span class="special">&lt;</span> <span class="identifier">lhs</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&gt;=(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">rhs</span> <span class="special">&lt;=</span> <span class="identifier">lhs</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span>
<span class="keyword">operator</span><span class="special">+(</span><span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">difference_type</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">lhs</span> <span class="special">+=</span> <span class="identifier">rhs</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span>
<span class="keyword">operator</span><span class="special">+(</span><span class="identifier">difference_type</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">rhs</span> <span class="special">+=</span> <span class="identifier">lhs</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span>
<span class="keyword">operator</span><span class="special">-(</span><span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">difference_type</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">lhs</span> <span class="special">-=</span> <span class="identifier">rhs</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">friend</span> <span class="keyword">constexpr</span> <span class="identifier">difference_type</span> <span class="keyword">operator</span><span class="special">-(</span>
<span class="identifier">repeated_chars_iterator</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">repeated_chars_iterator</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">lhs</span><span class="special">.</span><span class="identifier">n_</span> <span class="special">-</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">n_</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">first_</span><span class="special">;</span>
<span class="identifier">difference_type</span> <span class="identifier">size_</span><span class="special">;</span>
<span class="identifier">difference_type</span> <span class="identifier">n_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
(that's a lot of code!) into this:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">repeated_chars_iterator</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">stl_interfaces</span><span class="special">::</span><span class="identifier">iterator_interface</span><span class="special">&lt;</span>
<span class="identifier">repeated_chars_iterator</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">random_access_iterator_tag</span><span class="special">,</span>
<span class="keyword">char</span><span class="special">,</span>
<span class="keyword">char</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">:</span>
<span class="identifier">first_</span><span class="special">(</span><span class="keyword">nullptr</span><span class="special">),</span>
<span class="identifier">size_</span><span class="special">(</span><span class="number">0</span><span class="special">),</span>
<span class="identifier">n_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
<span class="special">{}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span><span class="special">(</span>
<span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">difference_type</span> <span class="identifier">size</span><span class="special">,</span> <span class="identifier">difference_type</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">:</span>
<span class="identifier">first_</span><span class="special">(</span><span class="identifier">first</span><span class="special">),</span>
<span class="identifier">size_</span><span class="special">(</span><span class="identifier">size</span><span class="special">),</span>
<span class="identifier">n_</span><span class="special">(</span><span class="identifier">n</span><span class="special">)</span>
<span class="special">{}</span>
<span class="keyword">constexpr</span> <span class="keyword">char</span> <span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">first_</span><span class="special">[</span><span class="identifier">n_</span> <span class="special">%</span> <span class="identifier">size_</span><span class="special">];</span> <span class="special">}</span>
<span class="keyword">constexpr</span> <span class="identifier">repeated_chars_iterator</span> <span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">+=(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ptrdiff_t</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="identifier">n_</span> <span class="special">+=</span> <span class="identifier">i</span><span class="special">;</span>
<span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">constexpr</span> <span class="keyword">auto</span> <span class="keyword">operator</span><span class="special">-(</span><span class="identifier">repeated_chars_iterator</span> <span class="identifier">other</span><span class="special">)</span> <span class="keyword">const</span> <span class="keyword">noexcept</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">n_</span> <span class="special">-</span> <span class="identifier">other</span><span class="special">.</span><span class="identifier">n_</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="keyword">char</span> <span class="keyword">const</span> <span class="special">*</span> <span class="identifier">first_</span><span class="special">;</span>
<span class="identifier">difference_type</span> <span class="identifier">size_</span><span class="special">;</span>
<span class="identifier">difference_type</span> <span class="identifier">n_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
Ah, that's better. Both of these definitions for <code class="computeroutput"><span class="identifier">repeated_chars_iterator</span></code>
have the same semantics and performance profile. It's just a lot less code
to write the second one, and writing the second one is more novice-friendly.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Boost.STLInterfaces's <code class="computeroutput"><span class="identifier">iterator_interface</span></code>
implements iterators that model the C++20 iterator concepts.
</p></td></tr>
</table></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: April 13, 2021 at 16:32:31 GMT</small></p></td>
<td align="right"><div class="copyright-footer"></div></td>
</tr></table>
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