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| 2 | <head> | |
| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>Exception Safety Guarantees</title> | |
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| 23 | <a accesskey="p" href="gotchas/false_positive_with__wmaybe_uninitialized.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="type_requirements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> | |
| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_optional.tutorial.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception | |
| 28 | Safety Guarantees</a> | |
| 29 | </h3></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> | |
| 42 | <span class="special">)</span> <span class="keyword">noexcept</span></code> | |
| 43 | </li> | |
| 44 | <li class="listitem"> | |
| 45 | <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> | |
| 46 | </li> | |
| 47 | </ul></div> | |
| 48 | <p> | |
| 49 | In addition, the following constructors and the destructor never throw exceptions: | |
| 50 | </p> | |
| 51 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 52 | <li class="listitem"> | |
| 53 | <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> | |
| 54 | <span class="keyword">noexcept</span></code> | |
| 55 | </li> | |
| 56 | <li class="listitem"> | |
| 57 | <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> | |
| 58 | <span class="identifier">none_t</span> <span class="special">)</span> | |
| 59 | <span class="keyword">noexcept</span></code> | |
| 60 | </li> | |
| 61 | </ul></div> | |
| 62 | <p> | |
| 63 | Regarding the following assignment functions: | |
| 64 | </p> | |
| 65 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 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">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> | |
| 68 | </li> | |
| 69 | <li class="listitem"> | |
| 70 | <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> | |
| 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">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> | |
| 74 | </li> | |
| 75 | <li class="listitem"> | |
| 76 | <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> | |
| 77 | <span class="keyword">const</span><span class="special">&</span> | |
| 78 | <span class="special">)</span></code> | |
| 79 | </li> | |
| 80 | <li class="listitem"> | |
| 81 | <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> | |
| 82 | <span class="keyword">const</span><span class="special">&</span> | |
| 83 | <span class="special">)</span> </code> | |
| 84 | </li> | |
| 85 | <li class="listitem"> | |
| 86 | <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> | |
| 87 | </li> | |
| 88 | </ul></div> | |
| 89 | <p> | |
| 90 | They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 91 | constructors or assignments (depending on whether the optional object is | |
| 92 | initialized or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 93 | constructor and the assignment provide strong exception safety guarantee, | |
| 94 | <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s | |
| 95 | assignment also provides strong exception safety guarantee; otherwise we | |
| 96 | only get the basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s constructor and the assignment never | |
| 97 | throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s | |
| 98 | assignment also never throws. | |
| 99 | </p> | |
| 100 | <p> | |
| 101 | Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or | |
| 102 | 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> | |
| 103 | do not throw anything else on its own. A throw during assignment never changes | |
| 104 | the initialization state of any optional object involved: | |
| 105 | </p> | |
| 106 | <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> | |
| 107 | <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> | |
| 108 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span> | |
| 109 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span> | |
| 110 | ||
| 111 | <span class="keyword">try</span> | |
| 112 | <span class="special">{</span> | |
| 113 | <span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span> | |
| 114 | <span class="special">}</span> | |
| 115 | <span class="keyword">catch</span><span class="special">(...)</span> | |
| 116 | <span class="special">{</span> | |
| 117 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span> | |
| 118 | <span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span> | |
| 119 | <span class="special">}</span> | |
| 120 | </pre> | |
| 121 | <p> | |
| 122 | This also applies to move assignments/constructors. However, move operations | |
| 123 | are made no-throw more often. | |
| 124 | </p> | |
| 125 | <p> | |
| 126 | Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides | |
| 127 | basic exception safety guarantee. If it throws, the optional object becomes | |
| 128 | uninitialized regardless of its initial state, and its previous contained | |
| 129 | value (if any) is destroyed. It doesn't call any assignment or move/copy | |
| 130 | constructor on <code class="computeroutput"><span class="identifier">T</span></code>. | |
| 131 | </p> | |
| 132 | <h5> | |
| 133 | <a name="boost_optional.tutorial.exception_safety_guarantees.h0"></a> | |
| 134 | <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> | |
| 135 | </h5> | |
| 136 | <p> | |
| 137 | Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is | |
| 138 | customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 139 | <code class="computeroutput"><span class="identifier">swap</span></code> or move constructor | |
| 140 | (depending on the initialization state of the optional objects). Thus, if | |
| 141 | both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code> | |
| 142 | and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code> | |
| 143 | on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never | |
| 144 | throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 145 | <code class="computeroutput"><span class="identifier">swap</span></code> and move constructor | |
| 146 | offer strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code> | |
| 147 | on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also | |
| 148 | offers a strong guarantee. | |
| 149 | </p> | |
| 150 | <p> | |
| 151 | In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional | |
| 152 | is customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 153 | move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 154 | default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>. | |
| 155 | (This is more useful on older compilers that do not support move semantics, | |
| 156 | when one wants to acheive stronger exception safety guarantees.) In this | |
| 157 | case the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code> | |
| 158 | are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s | |
| 159 | <code class="computeroutput"><span class="identifier">swap</span></code> and default constructor | |
| 160 | </p> | |
| 161 | </div> | |
| 162 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 163 | <td align="left"></td> | |
| 164 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
| 165 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 166 | 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>) | |
| 167 | </p> | |
| 168 | </div></td> | |
| 169 | </tr></table> | |
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