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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Exception Safety
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Guarantees</title>
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<div class="section" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.exception_safety_guarantees"></a><a href="exception_safety_guarantees.html" title="Exception Safety
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Guarantees">Exception Safety
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Guarantees</a>
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</h2></div></div></div>
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<p>
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Because of the current implementation (see <a href="../index.html#optional_implementation_notes">Implementation
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Notes</a>), all of the assignment methods:
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</p>
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<div class="itemizedlist"><ul type="disc">
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<li><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>
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<span class="keyword">const</span><span class="special">&</span>
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<span class="special">)</span></code></li>
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<li><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>
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<li><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>
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<span class="keyword">const</span><span class="special">&</span>
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<span class="special">)</span></code></li>
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<li><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></li>
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<li><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></li>
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<li><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>
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</ul></div>
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<p>
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Can only <span class="emphasis"><em>guarantee</em></span> the <span class="underline">basic
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exception safety</span>: The lvalue optional is left <span class="underline">uninitialized</span>
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if an exception is thrown (any previous value is <span class="emphasis"><em>first</em></span>
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destroyed using <code class="computeroutput"><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>)
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</p>
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<p>
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On the other hand, the <span class="emphasis"><em>uninitializing</em></span> methods:
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</p>
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<div class="itemizedlist"><ul type="disc">
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<li><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>
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<li><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>
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</ul></div>
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<p>
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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>)
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</p>
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<p>
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However, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
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itself doesn't throw any exceptions, the only source for exceptions here are
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<code class="computeroutput"><span class="identifier">T</span></code>'s constructor, so if you
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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>
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<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>, you
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know that <code class="computeroutput"><span class="identifier">optional</span></code>'s assignment
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and reset has the same guarantees.
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</p>
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<pre class="programlisting">
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<span class="comment">//
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</span><span class="comment">// Case 1: Exception thrown during assignment.
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</span><span class="comment">//
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</span><span class="identifier">T</span> <span class="identifier">v0</span><span class="special">(</span><span class="number">123</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">opt0</span><span class="special">(</span><span class="identifier">v0</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">T</span> <span class="identifier">v1</span><span class="special">(</span><span class="number">456</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">opt1</span><span class="special">(</span><span class="identifier">v1</span><span class="special">);</span>
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<span class="identifier">opt0</span> <span class="special">=</span> <span class="identifier">opt1</span> <span class="special">;</span>
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<span class="comment">// If no exception was thrown, assignment succeeded.
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</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>
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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="comment">// If any exception was thrown, 'opt0' is reset to uninitialized.
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</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>
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<span class="special">}</span>
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<span class="comment">//
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</span><span class="comment">// Case 2: Exception thrown during reset(v)
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</span><span class="comment">//
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</span><span class="identifier">T</span> <span class="identifier">v0</span><span class="special">(</span><span class="number">123</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">opt</span><span class="special">(</span><span class="identifier">v0</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">T</span> <span class="identifier">v1</span><span class="special">(</span><span class="number">456</span><span class="special">);</span>
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<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>
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<span class="comment">// If no exception was thrown, reset succeeded.
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</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>
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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="comment">// If any exception was thrown, 'opt' is reset to uninitialized.
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</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>
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<span class="special">}</span>
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</pre>
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<a name="boost_optional.exception_safety_guarantees.swap"></a><h4>
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<a name="id2644335"></a>
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<a href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>
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</h4>
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<p>
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<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>
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<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>
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when both optionals are initialized. If only one of the optionals is initialized,
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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
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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></code> doesn't throw). If none of the optionals
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is initialized, it has no-throw guarantee since it is a no-op.
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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"><small>Copyright <20> 2003 -2007 Fernando Luis Cacciola Carballal</small></td>
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</tr></table>
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