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<td class="back"><a id="top"></a><a href="s2.3.3.html" rel="prev">&lt;&lt;&nbsp;&sect;2.3.3&nbsp;Smart lifting</a></td>
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<div class="breadcrumb"><a class="nav" href="s2.html" rel="section">&sect;2&nbsp;Role Binding</a>&nbsp;&gt;&nbsp;<a class="nav" href="s2.3.html" rel="section">&sect;2.3&nbsp;Lifting</a></div>
<div class="sect depth3" id="s2.3.4">
<h3 class="sect">&sect;2.3.4&nbsp;Binding ambiguities<a class="img" href="s2.3.4.html"
title="PermaLink to &sect;2.3.4&nbsp;Binding ambiguities"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h3>
<p>While all examples so far have only shown 1-to-1 class bindings,
several cases of multiple bindings are allowable. Ambiguities may be
detected at compile time and/or at runtime.
</p>
<div class="subsect depth4" id="s2.3.4.a">
<h4 class="subsect">(a)&nbsp;<span class="title">Potential ambiguity</span><a class="img" href="s2.3.4.a.html"
title="PermaLink to (a)&nbsp;Potential ambiguity"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>A <strong>potential ambiguity</strong> is given,
if two role classes <code>R1</code> and <code>R2</code>
exist such that
</p>
<ul>
<li><code>R1</code> and <code>R2</code> are played by the
same base class <code>B</code>, and
</li>
<li><code>R1</code> and <code>R2</code> have a common
super role <code>R0</code>,
which is also bound to a base class <code>B0</code>, and
</li>
<li>neither role class <code>R1</code> nor
<code>R2</code> is a (indirect) sub-class of the other.
</li>
</ul>
<div class="note">
<h5>Note:</h5>
According to <a href="s2.1.c.html" title="&sect;2.1.(c)&nbsp;Covariant refinement"
class="sect">&sect;2.1.(c)</a>, if <code>B</code> is distinct from <code>B0</code>
it has to be a sub-class of <code>B0</code>.
</div>
<div class="note">
<h5>Effect:</h5>
In this case the compiler issues a warning, stating that the <code>B</code><em> may not be liftable,</em> because both role classes <code>R1</code>
and <code>R2</code> are candidates and there is no reason to prefer one over the other.
<br /><strong>If no potential ambiguity is detected, lifting will always be unambiguous.</strong></div>
<p>In the above situation, trying to lift an instance of type <code>B</code> to the role type
<code>R0</code> is an <strong>illegal lifting request</strong>. If <code>R0</code> is bound
to the same base class <code>B</code> as its sub-roles <code>R1</code> and <code>R2</code> are,
role <code>R0</code> is <strong>unliftable</strong>, meaning that no instance of <code>R0</code>
can ever by obtained by lifting.
</p>
<h5 class="listing">Example code (Potential Ambiguity):</h5>
<div class="listing example frame">
<table class="listing">
<tr class="line odd">
<td class="ln">1</td>
<td><pre><b>team</b> <b>class</b> MyTeam {</pre></td>
</tr>
<tr class="line even">
<td class="ln">2</td>
<td><pre> <b>public</b> <b>class</b> SuperRole <b>playedBy</b> MyBase {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> <b>public</b> <b>class</b> SubRoleA <b>extends</b> SuperRole {...}</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <b>public</b> <b>class</b> SubRoleB <b>extends</b> SuperRole {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre>}</pre></td>
</tr>
</table>
</div>
</div>
<div class="subsect depth4" id="s2.3.4.b">
<h4 class="subsect">(b)&nbsp;<span class="title">Definite ambiguity</span><a class="img" href="s2.3.4.b.html"
title="PermaLink to (b)&nbsp;Definite ambiguity"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>A <strong>definite ambiguity</strong> is given if
</p>
<ul>
<li>the situation of potential ambiguity according to (a)
above is given and
</li>
<li>lifting is requested (either by method binding or explicitly
(<a href="s2.3.2.html" title="&sect;2.3.2&nbsp;Declared lifting" class="sect">&sect;2.3.2</a>)) from the shared base class <code>B</code> to any role
class <code>R0</code> that is a common super role for <code>R1</code> and <code>R2</code>.
</li>
</ul>
<div class="note">
<h5>Effect:</h5>
Definite ambiguity is a compile time error.
</div>
<h5 class="listing">Example code (Definite Ambiguity):</h5>
<div class="listing example frame">
<table class="listing">
<tr class="line odd">
<td class="ln">1</td>
<td><pre><b>team</b> <b>class</b> MyTeam {</pre></td>
</tr>
<tr class="line even">
<td class="ln">2</td>
<td><pre> <b>public</b> <b>class</b> SuperRole <b>playedBy</b> MyBase {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> <b>public</b> <b>class</b> SubRoleA <b>extends</b> SuperRole <b>playedBy</b> SubBase {...}</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <b>public</b> <b>class</b> SubRoleB <b>extends</b> SuperRole <b>playedBy</b> SubBase {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre></pre></td>
</tr>
<tr class="line even">
<td class="ln">6</td>
<td><pre> <b>public</b> <b>void</b> useSuperRole(SubBase <b>as</b> SuperRole r) {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">7</td>
<td><pre>}</pre></td>
</tr>
</table>
</div>
</div>
<div class="subsect depth4" id="s2.3.4.c">
<h4 class="subsect">(c)&nbsp;<span class="title">Actual ambiguity</span><a class="img" href="s2.3.4.c.html"
title="PermaLink to (c)&nbsp;Actual ambiguity"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>At runtime <strong>actual ambiguity</strong> may occur if for the
<em>dynamic type</em> of a base to be lifted the conditions of (b)
above hold accordingly. Actual ambiguity is only possible in cases
reported by the compiler as potential ambiguity.
</p>
<div class="note">
<h5>Effect:</h5>
An actual ambiguity is reported at runtime by throwing a
<code>org.objectteams.LiftingFailedException</code>.
</div>
<h5 class="listing">Example code (Actual Ambiguity):</h5>
<div class="listing example frame">
<table class="listing">
<tr class="line odd">
<td class="ln">1</td>
<td><pre><b>team</b> <b>class</b> MyTeam {</pre></td>
</tr>
<tr class="line even">
<td class="ln">2</td>
<td><pre> <b>public</b> <b>class</b> SuperRole <b>playedBy</b> MyBase {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> <b>public</b> <b>class</b> SubRoleA <b>extends</b> SuperRole <b>playedBy</b> SubBase {...}</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <b>public</b> <b>class</b> SubRoleB <b>extends</b> SuperRole <b>playedBy</b> SubBase {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre> </pre></td>
</tr>
<tr class="line even">
<td class="ln">6</td>
<td><pre> <b>public</b> <b>void</b> useSuperRole(MyBase <b>as</b> SuperRole r) {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">7</td>
<td><pre>}</pre></td>
</tr>
<tr class="line even">
<td class="ln">8</td>
<td><pre><span class="comment">// plus these calls:</span></pre></td>
</tr>
<tr class="line odd">
<td class="ln">9</td>
<td><pre>MyTeam mt = <b>new</b> MyTeam();</pre></td>
</tr>
<tr class="line even">
<td class="ln">10</td>
<td><pre>mt.useSuperRole(<b>new</b> SubBase());</pre></td>
</tr>
</table>
</div>
</div>
<div class="subsect depth4" id="s2.3.4.d">
<h4 class="subsect">(d)&nbsp;<span class="title">Mismatching role</span><a class="img" href="s2.3.4.d.html"
title="PermaLink to (d)&nbsp;Mismatching role"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>In cases of potential ambiguity another runtime error may occur:
a <strong>mismatching role</strong> is encountered when a role is found
in the cache, which is not conform to the required type.
This happens, if the base object has previously been lifted
to a type that is incompatible with the currently requested type.
</p>
<div class="note">
<h5>Effect:</h5>
This is reported by throwing a <code>org.objectteams.WrongRoleException</code>.
</div>
<h5 class="listing">Example code (Mismatching Role):</h5>
<div class="listing example frame">
<table class="listing">
<tr class="line odd">
<td class="ln">1</td>
<td><pre><b>team</b> <b>class</b> MyTeam {</pre></td>
</tr>
<tr class="line even">
<td class="ln">2</td>
<td><pre> <b>public</b> <b>class</b> SuperRole <b>playedBy</b> MyBase {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> <b>public</b> <b>class</b> SubRoleA <b>extends</b> SuperRole {...}</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <b>public</b> <b>class</b> SubRoleB <b>extends</b> SuperRole {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre> </pre></td>
</tr>
<tr class="line even">
<td class="ln">6</td>
<td><pre> <b>public</b> <b>void</b> useRoleA(MyBase <b>as</b> SubRoleA r) {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">7</td>
<td><pre> <b>public</b> <b>void</b> useRoleB(MyBase <b>as</b> SubRoleB r) {...}</pre></td>
</tr>
<tr class="line even">
<td class="ln">8</td>
<td><pre>}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">9</td>
<td><pre><span class="comment">// plus these calls:</span></pre></td>
</tr>
<tr class="line even">
<td class="ln">10</td>
<td><pre>MyTeam mt = <b>new</b> MyTeam();</pre></td>
</tr>
<tr class="line odd">
<td class="ln">11</td>
<td><pre>MyBase b = <b>new</b> MyBase();</pre></td>
</tr>
<tr class="line even">
<td class="ln">12</td>
<td><pre>mt.useRoleA(b); <span class="comment">// creates a SubRoleA for b</span></pre></td>
</tr>
<tr class="line odd">
<td class="ln">13</td>
<td><pre>mt.useRoleB(b); <span class="comment">// finds the SubRoleA which is not compatible</span></pre></td>
</tr>
<tr class="line even">
<td class="ln">14</td>
<td><pre> <span class="comment">// to the expected type SubRoleB.</span></pre></td>
</tr>
</table>
</div>
<p>From the second item of <a href="#s2.3.4.a" title="&sect;2.3.4.(a)&nbsp;Potential ambiguity"
class="sect">&sect;2.3.4.(a)</a> follows, that for binding ambiguities different
role hierarchies are analyzed in isolation.
For this analysis only those role classes are considered that are bound to a
base class (directly using <code>playedBy</code> or by inheriting this relation
from another role class).
I.e., two role classes that have no common bound super role will never cause
any ambiguity.
</p>
</div>
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