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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
| 27 | <a name="boost_optional.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception Safety | |
| 28 | Guarantees</a> | |
| 29 | </h2></div></div></div> | |
| 30 | <p> | |
| 31 | This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 32 | destructor does not throw exceptions. If it does, the behaviour of many operations | |
| 33 | on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is | |
| 34 | undefined. | |
| 35 | </p> | |
| 36 | <p> | |
| 37 | The following mutating operations never throw exceptions: | |
| 38 | </p> | |
| 39 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 40 | <li class="listitem"> | |
| 41 | <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> | |
| 42 | </li> | |
| 43 | <li class="listitem"> | |
| 44 | <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> | |
| 45 | </li> | |
| 46 | </ul></div> | |
| 47 | <p> | |
| 48 | In addition, the following constructors and the destructor never throw exceptions: | |
| 49 | </p> | |
| 50 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 51 | <li class="listitem"> | |
| 52 | <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> | |
| 53 | <span class="keyword">noexcept</span></code> | |
| 54 | </li> | |
| 55 | <li class="listitem"> | |
| 56 | <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> | |
| 57 | </li> | |
| 58 | </ul></div> | |
| 59 | <p> | |
| 60 | Regarding the following assignment functions: | |
| 61 | </p> | |
| 62 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 63 | <li class="listitem"> | |
| 64 | <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> | |
| 65 | </li> | |
| 66 | <li class="listitem"> | |
| 67 | <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> | |
| 68 | </li> | |
| 69 | <li class="listitem"> | |
| 70 | <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> | |
| 71 | </li> | |
| 72 | <li class="listitem"> | |
| 73 | <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> | |
| 74 | <span class="keyword">const</span><span class="special">&</span> | |
| 75 | <span class="special">)</span></code> | |
| 76 | </li> | |
| 77 | <li class="listitem"> | |
| 78 | <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> | |
| 79 | <span class="keyword">const</span><span class="special">&</span> | |
| 80 | <span class="special">)</span> </code> | |
| 81 | </li> | |
| 82 | <li class="listitem"> | |
| 83 | <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> | |
| 84 | </li> | |
| 85 | </ul></div> | |
| 86 | <p> | |
| 87 | They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 88 | constructors or assignments (depending on whether the optional object is initialized | |
| 89 | or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s constructor | |
| 90 | 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 | |
| 91 | also provides strong exception safety guarantee; otherwise we only get the | |
| 92 | basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 93 | 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 | |
| 94 | assignment also never throws. | |
| 95 | </p> | |
| 96 | <p> | |
| 97 | Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or assignment | |
| 98 | 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> | |
| 99 | do not throw anything else on its own. A throw during assignment never changes | |
| 100 | the initialization state of any optional object involved: | |
| 101 | </p> | |
| 102 | <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> | |
| 103 | <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> | |
| 104 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span> | |
| 105 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span> | |
| 106 | ||
| 107 | <span class="keyword">try</span> | |
| 108 | <span class="special">{</span> | |
| 109 | <span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span> | |
| 110 | <span class="special">}</span> | |
| 111 | <span class="keyword">catch</span><span class="special">(...)</span> | |
| 112 | <span class="special">{</span> | |
| 113 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span> | |
| 114 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span> | |
| 115 | <span class="special">}</span> | |
| 116 | </pre> | |
| 117 | <p> | |
| 118 | This also applies to move assignments/constructors. However, move operations | |
| 119 | are made no-throw more often. | |
| 120 | </p> | |
| 121 | <p> | |
| 122 | Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides | |
| 123 | basic exception safety guarantee. If it throws, the optional object becomes | |
| 124 | uninitialized regardless of its initial state, and its previous contained value | |
| 125 | (if any) is destroyed. It doesn't call any assignment or move/copy constructor | |
| 126 | on <code class="computeroutput"><span class="identifier">T</span></code>. | |
| 127 | </p> | |
| 128 | <h4> | |
| 129 | <a name="boost_optional.exception_safety_guarantees.h0"></a> | |
| 130 | <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> | |
| 131 | </h4> | |
| 132 | <p> | |
| 133 | Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is | |
| 134 | customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 135 | <code class="computeroutput"><span class="identifier">swap</span></code> or move constructor (depending | |
| 136 | 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> | |
| 137 | and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code> | |
| 138 | on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never | |
| 139 | throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 140 | <code class="computeroutput"><span class="identifier">swap</span></code> and move constructor offer | |
| 141 | strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code> on | |
| 142 | <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also | |
| 143 | offers a strong guarantee. | |
| 144 | </p> | |
| 145 | <p> | |
| 146 | In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional is | |
| 147 | customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 148 | move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 149 | default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>. | |
| 150 | (This is more useful on older compilers that do not support move semantics, | |
| 151 | when one wants to acheive stronger exception safety guarantees.) In this case | |
| 152 | the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code> | |
| 153 | are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 154 | <code class="computeroutput"><span class="identifier">swap</span></code> and default constructor | |
| 155 | </p> | |
| 156 | </div> | |
| 157 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 158 | <td align="left"></td> | |
| 159 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p> | |
| 160 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 161 | 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>) | |
| 162 | </p> | |
| 163 | </div></td> | |
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