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- <div class="section">
- <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; its 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>
- <h4>
- <a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
- <span class="phrase"><a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><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 consistent.
- </p>
- <p>
- If in your code rebinding to another object is not an option, then it is very
- likely that binding for the first 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 a scenario it is a precondition
- that the lvalue must be already initialized. If it isn't, then binding for
- the first time is OK while rebinding is not which is IMO very unlikely. In
- such a 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>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<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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