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| <div class="section" title="Rebinding semantics for assignment of optional references"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_optional.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 |
| semantics for assignment of optional references</a> |
| </h2></div></div></div> |
| <p> |
| If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> |
| the effect is to bind (for the first time) to the object. Clearly, there is |
| no other choice. |
| </p> |
| <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> |
| <span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span> |
| <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span> |
| <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span> |
| <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><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><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> |
| </pre> |
| <p> |
| If you assign to a bare C++ reference, the assignment is forwarded to the referenced |
| object; it's value changes but the reference is never rebound. |
| </p> |
| <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> |
| <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span> |
| <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> |
| <span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> |
| <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><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> |
| <span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> |
| <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></pre> |
| <p> |
| Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>, |
| the effect is to <span class="bold"><strong>rebind</strong></span> to the new object |
| instead of assigning the referee. This is unlike bare C++ references. |
| </p> |
| <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> |
| <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> |
| <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span> |
| <span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> |
| <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span> |
| <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span> |
| <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><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><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> |
| <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> |
| </pre> |
| <a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a><h4> |
| <a name="id579234"></a> |
| <a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a> |
| </h4> |
| <p> |
| Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span> |
| <code class="computeroutput"><span class="identifier">optional</span></code> references has been |
| chosen to provide <span class="bold"><strong>consistency among initialization states</strong></span> |
| even at the expense of lack of consistency with the semantics of bare C++ references. |
| It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> strives |
| to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code> |
| does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code> |
| is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, |
| doing so would result in inconsistent behavior w.r.t to the lvalue initialization |
| state. |
| </p> |
| <p> |
| Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> |
| forwarding assignment to the referenced object (thus changing the referenced |
| object value but not rebinding), and consider the following code: |
| </p> |
| <pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span> |
| <span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span> |
| <span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span> |
| <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span> |
| <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> |
| </pre> |
| <p> |
| What does the assignment do? |
| </p> |
| <p> |
| If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>, |
| the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code> |
| (we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>). |
| But what if <code class="computeroutput"><span class="identifier">a</span></code> is already <span class="emphasis"><em>initialized</em></span>? |
| it would change the value of the referenced object (whatever that is); which |
| is inconsistent with the other possible case. |
| </p> |
| <p> |
| If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> |
| would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> |
| does, you would never be able to use Optional's assignment without explicitly |
| handling the previous initialization state unless your code is capable of functioning |
| whether after the assignment, <code class="computeroutput"><span class="identifier">a</span></code> |
| aliases the same object as <code class="computeroutput"><span class="identifier">b</span></code> |
| or not. |
| </p> |
| <p> |
| That is, you would have to discriminate in order to be consistency. |
| </p> |
| <p> |
| If in your code rebinding to another object is not an option, then is very |
| likely that binding for the fist time isn't either. In such case, assignment |
| to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> |
| shall be prohibited. It is quite possible that in such scenario the precondition |
| that the lvalue must be already initialized exist. If it doesn't, then binding |
| for the first time is OK while rebinding is not which is IMO very unlikely. |
| In such scenario, you can assign the value itself directly, as in: |
| </p> |
| <pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span> |
| <span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span> |
| </pre> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003 -2007 Fernando Luis Cacciola Carballal<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>) |
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