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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
operators</a>
</h3></div></div></div>
<p>
Type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
<a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>. Two optional
objects containing a value compare in the same as their contained values.
The uninitialized state of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
is treated as a distinct value, equal to itself, and unequal to any value
of type <code class="computeroutput"><span class="identifier">T</span></code>:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o0</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">oN</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="identifier">o1</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="identifier">o0</span><span class="special">);</span>
</pre>
<p>
The converting constructor from <code class="computeroutput"><span class="identifier">T</span></code>
as well as from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> implies the existence and semantics
of the mixed comparison between <code class="computeroutput"><span class="identifier">T</span></code>
and <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> as
well as between <code class="computeroutput"><span class="identifier">none_t</span></code> and
<code class="computeroutput"><span class="identifier">optionl</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>:
</p>
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
</pre>
<p>
This mixed comparison has a practical interpretation, which is occasionally
useful:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">ask_user</span><span class="special">();</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">choice</span> <span class="special">==</span> <span class="number">2</span><span class="special">)</span>
<span class="identifier">start_procedure_2</span><span class="special">();</span>
</pre>
<p>
In the above example, the meaning of the comparison is 'user chose number
2'. If user chose nothing, he didn't choose number 2.
</p>
<p>
In case where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
compared to <code class="computeroutput"><span class="identifier">none</span></code>, it is not
required that <code class="computeroutput"><span class="identifier">T</span></code> be <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
</p>
<p>
In a similar manner, type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. The optional
object containing no value is compared less than any value of <code class="computeroutput"><span class="identifier">T</span></code>. To illustrate this, if the default ordering
of <code class="computeroutput"><span class="identifier">size_t</span></code> is {<code class="computeroutput"><span class="number">0</span></code>, <code class="computeroutput"><span class="number">1</span></code>,
<code class="computeroutput"><span class="number">2</span></code>, ...}, the default ordering
of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span></code>
is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, <code class="computeroutput"><span class="number">0</span></code>,
<code class="computeroutput"><span class="number">1</span></code>, <code class="computeroutput"><span class="number">2</span></code>,
...}. This order does not have a practical interpretation. The goal is to
have any semantically correct default ordering in order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> to
be usable in ordered associative containers (wherever <code class="computeroutput"><span class="identifier">T</span></code>
is usable).
</p>
<p>
Mixed relational operators are the only case where the contained value of
an optional object can be inspected without the usage of value accessing
function (<code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>,
<code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>).
</p>
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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>)
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