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| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.tutorial.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> |
| </h3></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> |
| <h5> |
| <a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.h0"></a> |
| <span class="phrase"><a name="boost_optional.tutorial.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.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a> |
| </h5> |
| <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> |
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| 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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