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situations, you will really appreciate the succinctness of <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>.
</p>
<a name="range.reference.adaptors.adaptors_introduction.composition_of_adaptors"></a><h6>
<a name="id3026637"></a>
<a name="id3057363"></a>
<a class="link" href="adaptors_introduction.html#range.reference.adaptors.adaptors_introduction.composition_of_adaptors">Composition
of Adaptors</a>
</h6>
@ -159,7 +159,7 @@
is the design solution to this problem.
</p>
<a name="range.reference.adaptors.adaptors_introduction.range_adaptor_alternative_to_copy_if_algorithm"></a><h6>
<a name="id3026947"></a>
<a name="id3057673"></a>
<a class="link" href="adaptors_introduction.html#range.reference.adaptors.adaptors_introduction.range_adaptor_alternative_to_copy_if_algorithm">Range
Adaptor alternative to copy_if algorithm</a>
</h6>
@ -176,7 +176,7 @@
<p>
</p>
<a name="range.reference.adaptors.adaptors_introduction.range_adaptor_alternative_to_count_if_algorithm"></a><h6>
<a name="id3027116"></a>
<a name="id3057842"></a>
<a class="link" href="adaptors_introduction.html#range.reference.adaptors.adaptors_introduction.range_adaptor_alternative_to_count_if_algorithm">Range
Adaptor alternative to count_if algorithm</a>
</h6>

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<a name="range.reference.adaptors.general_requirements"></a><a class="link" href="general_requirements.html" title="General Requirements"> General
Requirements</a>
</h4></div></div></div>
<p>
In the description of generator expressions, the following notation is
used:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput"><span class="identifier">fwdRng</span></code> is an expression
of a type <code class="computeroutput"><span class="identifier">R</span></code> that models
<code class="computeroutput"><span class="identifier">ForwardRange</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">biRng</span></code> is an expression
of a type <code class="computeroutput"><span class="identifier">R</span></code> that models
<code class="computeroutput"><span class="identifier">BidirectionalRange</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">rndRng</span></code> is an expression
of a type <code class="computeroutput"><span class="identifier">R</span></code> that models
<code class="computeroutput"><span class="identifier">RandomAccessRange</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">pred</span></code> is an expression
of a type that models <code class="computeroutput"><span class="identifier">UnaryPredicate</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">bi_pred</span></code> is an expression
of a type that models <code class="computeroutput"><span class="identifier">BinaryPredicate</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">fun</span></code> is an expression
of a type that models <code class="computeroutput"><span class="identifier">UnaryFunction</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">new_value</span></code> and <code class="computeroutput"><span class="identifier">old_value</span></code>
are objects convertible to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">range_value</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">n</span><span class="special">,</span><span class="identifier">m</span></code> are integer expressions convertible
to <code class="computeroutput"><span class="identifier">range_difference</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
</li>
</ul></div>
<p>
Also note that <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">range_value</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;::</span><span class="identifier">type</span></code> must be implicitly convertible to
the type arguments to <code class="computeroutput"><span class="identifier">pred</span></code>,
<code class="computeroutput"><span class="identifier">bi_pred</span></code> and <code class="computeroutput"><span class="identifier">fun</span></code>.
</p>
<p>
Range Category in the following adaptor descriptions refers to the minimum
range concept required by the range passed to the adaptor. The resultant
range is a model of the same range concept as the input range unless specified
otherwise.
</p>
<p>
Returned Range Category is the concept of the returned range. In some cases
the returned range is of a lesser category than the range passed to the
adaptor. For example, the <code class="computeroutput"><span class="identifier">filtered</span></code>
adaptor returns only a <code class="computeroutput"><span class="identifier">ForwardRange</span></code>
regardless of the input.
</p>
<p>
Furthermore, the following rules apply to any expression of the form
</p>
<pre class="programlisting"><span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">adaptor_generator</span>
</pre>
<p>
</p>
<p>
1. Applying <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>
to a range <code class="computeroutput"><span class="identifier">R</span></code> (always left
argument) and a range adapter <code class="computeroutput"><span class="identifier">RA</span></code>
(always right argument) yields a new range type which may not conform to
the same range concept as <code class="computeroutput"><span class="identifier">R</span></code>.
</p>
<p>
2. The return-type of <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code> is otherwise unspecified.
</p>
<p>
3. <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>
is found by Argument Dependent Lookup (ADL) because a range adaptor is
implemented in namespace <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span></code>.
</p>
<p>
4. <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>
is used to add new behaviour <span class="emphasis"><em><span class="bold"><strong>lazily</strong></span></em></span>
and never modifies its left argument.
</p>
<p>
5. All iterators extracted from the left argument are extracted using qualified
calls to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">()</span></code>
and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">end</span><span class="special">()</span></code>.
</p>
<p>
6. In addition to the <code class="computeroutput"><span class="keyword">throw</span></code>-clauses
below, <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>
may throw exceptions as a result of copying iterators. If such copying
cannot throw an exception, then neither can the whole expression.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2003 -2010 Thorsten Ottosen, Neil Groves<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="titlepage"><div><div><h4 class="title">
<a name="range.reference.adaptors.introduction"></a><a class="link" href="introduction.html" title="Introduction and motivation"> Introduction
and motivation</a>
</h4></div></div></div>
<p>
A <span class="bold"><strong>Range Adaptor</strong></span> is a class that wraps
an existing Range to provide a new Range with different behaviour. Since
the behaviour of Ranges is determined by their associated iterators, a
Range Adaptor simply wraps the underlying iterators with new special iterators.
In this example
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">range</span><span class="special">/</span><span class="identifier">adaptors</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">range</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">vec</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span>
</pre>
<p>
</p>
<p>
the iterators from <code class="computeroutput"><span class="identifier">vec</span></code>
are wrapped <code class="computeroutput"><span class="identifier">reverse_iterator</span></code>s.
The type of the underlying Range Adapter is not documented because you
do not need to know it. All that is relevant is that the expression
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">vec</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span>
</pre>
<p>
</p>
<p>
returns a Range Adaptor where the iterator type is now the iterator type
of the range <code class="computeroutput"><span class="identifier">vec</span></code> wrapped
in <code class="computeroutput"><span class="identifier">reverse_iterator</span></code>. The
expression <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span></code> is called an <span class="bold"><strong>Adaptor
Generator</strong></span>.
</p>
<p>
There are two ways of constructing a range adaptor. The first is by using
<code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>.
This is my preferred technique, however while discussing range adaptors
with others it became clear that some users of the library strongly prefer
a more familiar function syntax, so equivalent functions of the present
tense form have been added as an alternative syntax. The equivalent to
<code class="computeroutput"><span class="identifier">rng</span> <span class="special">|</span>
<span class="identifier">reversed</span></code> is <code class="computeroutput"><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reverse</span><span class="special">(</span><span class="identifier">rng</span><span class="special">)</span></code> for example.
</p>
<p>
Why do I prefer the <code class="computeroutput"><span class="keyword">operator</span><span class="special">|</span></code> syntax? The answer is readability:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reverse</span><span class="special">(</span><span class="identifier">vec</span><span class="special">),</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span>
</pre>
<p>
</p>
<p>
This might not look so bad, but when we apply several adaptors, it becomes
much worse. Just compare
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reverse</span><span class="special">(</span> <span class="identifier">vec</span> <span class="special">)</span> <span class="special">),</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span>
</pre>
<p>
</p>
<p>
to
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">vec</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span>
<span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">uniqued</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">)</span> <span class="special">);</span>
</pre>
<p>
</p>
<p>
Furthermore, some of the adaptor generators take arguments themselves and
these arguments are expressed with function call notation too. In those
situations, you will really appreciate the succinctness of <code class="computeroutput"><span class="keyword">operator</span><span class="special">|()</span></code>.
</p>
<a name="range.reference.adaptors.introduction.composition_of_adaptors"></a><h6>
<a name="id2737533"></a>
<a class="link" href="introduction.html#range.reference.adaptors.introduction.composition_of_adaptors">Composition
of Adaptors</a>
</h6>
<p>
Range Adaptors are a powerful complement to Range algorithms. The reason
is that adaptors are <span class="emphasis"><em><span class="bold"><strong>orthogonal</strong></span></em></span>
to algorithms. For example, consider these Range algorithms:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">out</span> <span class="special">)</span></code></li>
<li class="listitem"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">pred</span> <span class="special">)</span></code></li>
</ul></div>
<p>
What should we do if we only want to copy an element <code class="computeroutput"><span class="identifier">a</span></code>
if it satisfies some predicate, say <code class="computeroutput"><span class="identifier">pred</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>?
And what if we only want to count the elements that satisfy the same predicate?
The naive answer would be to use these algorithms:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy_if</span><span class="special">(</span>
<span class="identifier">rng</span><span class="special">,</span>
<span class="identifier">pred</span><span class="special">,</span>
<span class="identifier">out</span> <span class="special">)</span></code></li>
<li class="listitem"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count_if</span><span class="special">(</span>
<span class="identifier">rng</span><span class="special">,</span>
<span class="identifier">pred</span> <span class="special">)</span></code></li>
</ul></div>
<p>
These algorithms are only defined to maintain a one to one relationship
with the standard library algorithms. This approach of adding algorithm
suffers a combinatorial explosion. Inevitably many algorithms are missing
<code class="computeroutput"><span class="identifier">_if</span></code> variants and there
is redundant development overhead for each new algorithm. The Adaptor Generator
is the design solution to this problem.
</p>
<a name="range.reference.adaptors.introduction.range_adaptor_alternative_to_copy_if_algorithm"></a><h6>
<a name="id2737841"></a>
<a class="link" href="introduction.html#range.reference.adaptors.introduction.range_adaptor_alternative_to_copy_if_algorithm">Range
Adaptor alternative to copy_if algorithm</a>
</h6>
<p>
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy_if</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">pred</span><span class="special">,</span> <span class="identifier">out</span> <span class="special">);</span>
</pre>
<p>
can be expressed as
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">filtered</span><span class="special">(</span><span class="identifier">pred</span><span class="special">),</span> <span class="identifier">out</span> <span class="special">);</span>
</pre>
<p>
</p>
<a name="range.reference.adaptors.introduction.range_adaptor_alternative_to_count_if_algorithm"></a><h6>
<a name="id2738011"></a>
<a class="link" href="introduction.html#range.reference.adaptors.introduction.range_adaptor_alternative_to_count_if_algorithm">Range
Adaptor alternative to count_if algorithm</a>
</h6>
<p>
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count_if</span><span class="special">(</span> <span class="identifier">rng</span><span class="special">,</span> <span class="identifier">pred</span> <span class="special">);</span>
</pre>
<p>
can be expressed as
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">count</span><span class="special">(</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">filtered</span><span class="special">(</span><span class="identifier">pred</span><span class="special">),</span> <span class="identifier">out</span> <span class="special">);</span>
</pre>
<p>
</p>
<p>
What this means is that <span class="emphasis"><em><span class="bold"><strong>no</strong></span></em></span>
algorithm with the <code class="computeroutput"><span class="identifier">_if</span></code>
suffix is needed. Furthermore, it turns out that algorithms with the <code class="computeroutput"><span class="identifier">_copy</span></code> suffix are not needed either. Consider
the somewhat misdesigned <code class="computeroutput"><span class="identifier">replace_copy_if</span><span class="special">()</span></code> which may be used as
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">replace_copy_if</span><span class="special">(</span> <span class="identifier">rng</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">vec</span><span class="special">),</span> <span class="identifier">pred</span> <span class="special">);</span>
</pre>
<p>
</p>
<p>
With adaptors and algorithms we can express this as
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">replaced_if</span><span class="special">(</span><span class="identifier">pred</span><span class="special">,</span> <span class="identifier">new_value</span><span class="special">));</span>
</pre>
<p>
</p>
<p>
The latter code has several benefits:
</p>
<p>
1. it is more <span class="emphasis"><em><span class="bold"><strong>efficient</strong></span></em></span>
because we avoid extra allocations as might happen with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">back_inserter</span></code>
</p>
<p>
2. it is <span class="emphasis"><em><span class="bold"><strong>flexible</strong></span></em></span>
as we can subsequently apply even more adaptors, for example:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span> <span class="identifier">rng</span> <span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">replaced_if</span><span class="special">(</span><span class="identifier">pred</span><span class="special">,</span> <span class="identifier">new_value</span><span class="special">)</span>
<span class="special">|</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">adaptors</span><span class="special">::</span><span class="identifier">reversed</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
3. it is <span class="emphasis"><em><span class="bold"><strong>safer</strong></span></em></span> because
there is no use of an unbounded output iterator.
</p>
<p>
In this manner, the <span class="emphasis"><em><span class="bold"><strong>composition</strong></span></em></span>
of Range Adaptors has the following consequences:
</p>
<p>
1. we no longer need <code class="computeroutput"><span class="identifier">_if</span></code>,
<code class="computeroutput"><span class="identifier">_copy</span></code>, <code class="computeroutput"><span class="identifier">_copy_if</span></code>
and <code class="computeroutput"><span class="identifier">_n</span></code> variants of algorithms.
</p>
<p>
2. we can generate a multitude of new algorithms on the fly, for example,
above we generated <code class="computeroutput"><span class="identifier">reverse_replace_copy_if</span><span class="special">()</span></code>
</p>
<p>
In other words:
</p>
<p>
<span class="bold"><strong>Range Adaptors are to algorithms what algorithms
are to containers</strong></span>
</p>
</div>
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