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<div class="section">
<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>
      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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&amp;</span> <span class="special">)</span></code>
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&amp;</span>
          <span class="special">)</span></code>
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&amp;</span>
          <span class="special">)</span> </code>
        </li>
<li class="listitem">
          <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
<h4>
<a name="boost_optional.exception_safety_guarantees.h0"></a>
      <span class="phrase"><a name="boost_optional.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>
    </h4>
<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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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 &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<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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