ceph/src/boost/libs/optional/doc/html/boost_optional/tutorial/optional_references/rebinding_semantics_for_assignment_of_optional_references.html

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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
  28         semantics for assignment of optional references</a>
  29 </h4></div></div></div>
  30 <p>
  31           If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code> the effect is to bind (for the
  32           first time) to the object. Clearly, there is no other choice.
  33         </p>
  34 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
  35 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
  36 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">ora</span> <span class="special">;</span>
  37 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
  38 <span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
  39 <span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
  40 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
  41 </pre>
  42 <p>
  43           If you assign to a bare C++ reference, the assignment is forwarded to the
  44           referenced object; its value changes but the reference is never rebound.
  45         </p>
  46 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
  47 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
  48 <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
  49 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
  50 <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
  51 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
  52 <span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
  53 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
  54 </pre>
  55 <p>
  56           Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code>,
  57           the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
  58           instead of assigning the referee. This is unlike bare C++ references.
  59         </p>
  60 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
  61 <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
  62 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
  63 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
  64 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
  65 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
  66 <span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
  67 <span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
  68 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
  69 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
  70 </pre>
  71 <h6>
  72 <a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
  73           <span class="phrase"><a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
  74         </h6>
  75 <p>
  76           Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
  77           <code class="computeroutput"><span class="identifier">optional</span></code> references has
  78           been chosen to provide <span class="bold"><strong>consistency among initialization
  79           states</strong></span> even at the expense of lack of consistency with the semantics
  80           of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span></code> strives to behave as much as possible
  81           as <code class="computeroutput"><span class="identifier">U</span></code> does whenever it is
  82           initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
  83           is <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span></code>,
  84           doing so would result in inconsistent behavior w.r.t to the lvalue initialization
  85           state.
  86         </p>
  87 <p>
  88           Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code>
  89           forwarding assignment to the referenced object (thus changing the referenced
  90           object value but not rebinding), and consider the following code:
  91         </p>
  92 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
  93 <span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
  94 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
  95 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
  96 <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
  97 </pre>
  98 <p>
  99           What does the assignment do?
 100         </p>
 101 <p>
 102           If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
 103           the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
 104           (we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
 105           But what if <code class="computeroutput"><span class="identifier">a</span></code> is already
 106           <span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced
 107           object (whatever that is); which is inconsistent with the other possible
 108           case.
 109         </p>
 110 <p>
 111           If <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code>
 112           would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span></code> does, you would never be able to use
 113           Optional's assignment without explicitly handling the previous initialization
 114           state unless your code is capable of functioning whether after the assignment,
 115           <code class="computeroutput"><span class="identifier">a</span></code> aliases the same object
 116           as <code class="computeroutput"><span class="identifier">b</span></code> or not.
 117         </p>
 118 <p>
 119           That is, you would have to discriminate in order to be consistent.
 120         </p>
 121 <p>
 122           If in your code rebinding to another object is not an option, then it is
 123           very likely that binding for the first time isn't either. In such case,
 124           assignment to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code> shall be prohibited. It is quite
 125           possible that in such a scenario it is a precondition that the lvalue must
 126           be already initialized. If it isn't, then binding for the first time is
 127           OK while rebinding is not which is IMO very unlikely. In such a scenario,
 128           you can assign the value itself directly, as in:
 129         </p>
 130 <pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
 131 <span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
 132 </pre>
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 136 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<p>
 137         Distributed under the Boost Software License, Version 1.0. (See accompanying
 138         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>)
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