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| <div class="section" title="Exception Safety Guarantees"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_optional.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception Safety |
| Guarantees</a> |
| </h2></div></div></div> |
| <p> |
| Because of the current implementation (see <a class="link" href="implementation_notes.html" title="Implementation Notes">Implementation |
| Notes</a>), all of the assignment methods: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" 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></code></li> |
| </ul></div> |
| <p> |
| Can only <span class="emphasis"><em>guarantee</em></span> the <span class="underline">basic |
| exception safety</span>: The lvalue optional is left <span class="underline">uninitialized</span> |
| if an exception is thrown (any previous value is <span class="emphasis"><em>first</em></span> |
| destroyed using <code class="computeroutput"><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>) |
| </p> |
| <p> |
| On the other hand, the <span class="emphasis"><em>uninitializing</em></span> methods: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" 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">detail</span><span class="special">::</span><span class="identifier">none_t</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></code></li> |
| </ul></div> |
| <p> |
| Provide the no-throw guarantee (assuming a no-throw <code class="computeroutput"><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>) |
| </p> |
| <p> |
| However, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> |
| itself doesn't throw any exceptions, the only source for exceptions here are |
| <code class="computeroutput"><span class="identifier">T</span></code>'s constructor, so if you |
| know the exception guarantees for <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span> |
| <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>, you |
| know that <code class="computeroutput"><span class="identifier">optional</span></code>'s assignment |
| and reset has the same guarantees. |
| </p> |
| <pre class="programlisting"><span class="comment">// |
| </span><span class="comment">// Case 1: Exception thrown during assignment. |
| </span><span class="comment">// |
| </span><span class="identifier">T</span> <span class="identifier">v0</span><span class="special">(</span><span class="number">123</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">opt0</span><span class="special">(</span><span class="identifier">v0</span><span class="special">);</span> |
| <span class="keyword">try</span> |
| <span class="special">{</span> |
| <span class="identifier">T</span> <span class="identifier">v1</span><span class="special">(</span><span class="number">456</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">opt1</span><span class="special">(</span><span class="identifier">v1</span><span class="special">);</span> |
| <span class="identifier">opt0</span> <span class="special">=</span> <span class="identifier">opt1</span> <span class="special">;</span> |
| |
| <span class="comment">// If no exception was thrown, assignment succeeded. |
| </span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">*</span><span class="identifier">opt0</span> <span class="special">==</span> <span class="identifier">v1</span> <span class="special">)</span> <span class="special">;</span> |
| <span class="special">}</span> |
| <span class="keyword">catch</span><span class="special">(...)</span> |
| <span class="special">{</span> |
| <span class="comment">// If any exception was thrown, 'opt0' is reset to uninitialized. |
| </span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">!</span><span class="identifier">opt0</span> <span class="special">)</span> <span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="comment">// |
| </span><span class="comment">// Case 2: Exception thrown during reset(v) |
| </span><span class="comment">// |
| </span><span class="identifier">T</span> <span class="identifier">v0</span><span class="special">(</span><span class="number">123</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">opt</span><span class="special">(</span><span class="identifier">v0</span><span class="special">);</span> |
| <span class="keyword">try</span> |
| <span class="special">{</span> |
| <span class="identifier">T</span> <span class="identifier">v1</span><span class="special">(</span><span class="number">456</span><span class="special">);</span> |
| <span class="identifier">opt</span><span class="special">.</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">v1</span> <span class="special">)</span> <span class="special">;</span> |
| |
| <span class="comment">// If no exception was thrown, reset succeeded. |
| </span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v1</span> <span class="special">)</span> <span class="special">;</span> |
| <span class="special">}</span> |
| <span class="keyword">catch</span><span class="special">(...)</span> |
| <span class="special">{</span> |
| <span class="comment">// If any exception was thrown, 'opt' is reset to uninitialized. |
| </span> <span class="identifier">assert</span><span class="special">(</span> <span class="special">!</span><span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <a name="boost_optional.exception_safety_guarantees.swap"></a><h4> |
| <a name="id583497"></a> |
| <a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a> |
| </h4> |
| <p> |
| <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</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">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">)</span></code> has the same exception guarantee as <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code> |
| when both optionals are initialized. If only one of the optionals is initialized, |
| it gives the same <span class="emphasis"><em>basic</em></span> exception guarantee as <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> (since |
| <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></code> doesn't throw). If none of the optionals |
| is initialized, it has no-throw guarantee since it is a no-op. |
| </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<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> |
| </div></td> |
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