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<title>Exception Safety Guarantees</title>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception
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Safety Guarantees</a>
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</h3></div></div></div>
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<p>
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This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
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destructor does not throw exceptions. If it does, the behaviour of many operations
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on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
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undefined.
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</p>
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<p>
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The following mutating operations never throw exceptions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span>
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<span class="special">)</span> <span class="keyword">noexcept</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
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</li>
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</ul></div>
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<p>
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In addition, the following constructors and the destructor never throw exceptions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
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<span class="keyword">noexcept</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span>
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<span class="identifier">none_t</span> <span class="special">)</span>
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<span class="keyword">noexcept</span></code>
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</li>
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</ul></div>
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<p>
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Regarding the following assignment functions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
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<span class="keyword">const</span><span class="special">&</span>
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<span class="special">)</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
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<span class="keyword">const</span><span class="special">&</span>
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<span class="special">)</span> </code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
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</li>
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</ul></div>
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<p>
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They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
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constructors or assignments (depending on whether the optional object is
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initialized or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s
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constructor and the assignment provide strong exception safety guarantee,
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
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assignment also provides strong exception safety guarantee; otherwise we
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only get the basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s constructor and the assignment never
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throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
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assignment also never throws.
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</p>
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<p>
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Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or
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assignment throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
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do not throw anything else on its own. A throw during assignment never changes
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the initialization state of any optional object involved:
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</p>
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<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
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<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
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<span class="keyword">try</span>
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<span class="special">{</span>
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<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
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<span class="special">}</span>
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<span class="keyword">catch</span><span class="special">(...)</span>
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<span class="special">{</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
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<span class="special">}</span>
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</pre>
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<p>
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This also applies to move assignments/constructors. However, move operations
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are made no-throw more often.
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</p>
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<p>
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Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
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basic exception safety guarantee. If it throws, the optional object becomes
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uninitialized regardless of its initial state, and its previous contained
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value (if any) is destroyed. It doesn't call any assignment or move/copy
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constructor on <code class="computeroutput"><span class="identifier">T</span></code>.
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</p>
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<h5>
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<a name="boost_optional.tutorial.exception_safety_guarantees.h0"></a>
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<span class="phrase"><a name="boost_optional.tutorial.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.tutorial.exception_safety_guarantees.swap">Swap</a>
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</h5>
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<p>
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Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
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customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
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<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor
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(depending on the initialization state of the optional objects). Thus, if
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both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
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and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
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on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
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throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
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<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor
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offer strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code>
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on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
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offers a strong guarantee.
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</p>
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<p>
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In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional
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is customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
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move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
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default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
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(This is more useful on older compilers that do not support move semantics,
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when one wants to acheive stronger exception safety guarantees.) In this
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case the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
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are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
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<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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