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