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Improved documentation. Added some noexcept.
This commit is contained in:
Andrzej Krzemienski
@ -28,8 +28,36 @@
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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 class="link" href="implementation_notes.html" title="Implementation Notes">Implementation
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Notes</a>), all of the assignment methods:
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This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
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destructor does not throw exceptions. If it does, the behaviour of many operations
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on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
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undefined.
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</p>
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<p>
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The following mutating operations never throw exceptions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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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="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>
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</li>
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<li class="listitem">
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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> <span class="keyword">noexcept</span></code>
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</li>
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</ul></div>
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<p>
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In addition, the following constructors and the destructor never throw exceptions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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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">optional</span><span class="special">()</span>
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<span class="keyword">noexcept</span></code>
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</li>
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<li class="listitem">
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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">optional</span><span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
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</li>
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</ul></div>
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<p>
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Regarding the following assignment functions:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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@ -52,88 +80,71 @@
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<span class="special">)</span> </code>
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</li>
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<li class="listitem">
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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> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&)</span></code>
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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> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
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</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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They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
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constructors or assignments (depending on whether the optional object is initialized
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or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s constructor
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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
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also provides strong exception safety guarantee; otherwise we only get the
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basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s
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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
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assignment also never throws.
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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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Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or assignment
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throws, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> does
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not throw anything else on its own. A throw during assignment never changes
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the initialization state of any optional object involved:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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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="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>
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</li>
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<li class="listitem">
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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>
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</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"><span class="comment">//</span>
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<span class="comment">// Case 1: Exception thrown during assignment.</span>
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<span class="comment">//</span>
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<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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<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>
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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">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</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.</span>
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<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="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</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.</span>
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<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">//</span>
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<span class="comment">// Case 2: Exception thrown during reset(v)</span>
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<span class="comment">//</span>
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<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.</span>
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<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.</span>
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<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="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
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<span class="special">}</span>
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</pre>
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<p>
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This also applies to move assignments/constructors. However, move operations
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are made no-throw more often.
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</p>
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<h4>
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<a name="boost_optional.exception_safety_guarantees.h0"></a>
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<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>
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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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Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
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customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
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<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor (depending
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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>
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and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
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on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
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throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
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<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor offer
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strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code> on
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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></code> also
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offers a strong guarantee.
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</p>
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<p>
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In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
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customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
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move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
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default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
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(This is more useful on older compilers that do not support move semantics,
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when one wants to acheive stronger exception safety guarantees.) In this case
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the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
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are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
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<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
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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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