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<td class="back"><a id="top"></a><a href="s2.4.html" rel="prev">&lt;&lt;&nbsp;&sect;2.4&nbsp;Explicit role creation</a></td>
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<div class="breadcrumb"><a class="nav" href="s2.html" rel="section">&sect;2&nbsp;Role Binding</a></div>
<div class="sect depth2" id="s2.5">
<h2 class="sect">&sect;2.5&nbsp;Abstract Roles<a class="img" href="s2.5.html"
title="PermaLink to &sect;2.5&nbsp;Abstract Roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
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<p>Overriding of role classes and dynamic binding of role types (<a href="s1.3.1.e.html" title="&sect;1.3.1.(e)&nbsp;Dynamic binding of types"
class="sect">&sect;1.3.1.(e)</a>)
adds new cases to <strong>creation</strong> with respect to abstract classes.
</p>
<div class="subsect depth3" id="s2.5.a">
<h4 class="subsect">(a)&nbsp;<span class="title">Using abstract classes for creation</span><a class="img" href="s2.5.a.html"
title="PermaLink to (a)&nbsp;Using abstract classes for creation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>Abstract role classes can indeed be used for object creation.
The effect of such a statement is that the team must be
marked <code>abstract</code>. Only those sub-teams are concrete
that provide concrete versions for all role classes used in
creation expressions.<br />
This includes the case, where a
super-team has a concrete role class and creates
instances of this role class and only the sub-team changes
the status of this role class to abstract. Also here
the sub-team must be marked abstract, because it contains
an abstract role class that is used in creation expressions.
</p>
<div class="note">
<h5>Interpretation:</h5>
Since the type in a role creation expression is late-bound relative to the enclosing team instance, abstract role classes
can be seen
as the hook in a <strong>template&amp;hook pattern</strong> that is raised from the method level to the class level:
A super-team may already refer to the constructor of an abstract role class,
only the sub-team will provide the concrete role class to fill the hook with the necessary implementation.
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<div class="subsect depth3" id="s2.5.b">
<h4 class="subsect">(b)&nbsp;<span class="title">Relevant roles</span><a class="img" href="s2.5.b.html" title="PermaLink to (b)&nbsp;Relevant roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>A team must be marked <code>abstract</code> if one of its <strong>relevant roles</strong> is abstract.
<br />
A role is relevant in this sense if
</p>
<ul>
<li>the role class is public <em>or if</em></li>
<li>an explicit <code>new</code> expression
would require to create instances of the role class, <em>or if</em></li>
<li>any of the lifting methods of the enclosing team
would require to create instances of the role class.<br />
A role is irrelevant with respect to lifting
if either of the following holds:
<ul>
<li>It is not bound to a base class, neither directly nor
by an inherited <code>playedBy</code> clause.
</li>
<li>It has a sub-role without a <code>playedBy</code> clause.
</li>
<li>It is bound to an abstract base class, and for all concrete
sub-classes of the base class, a binding to a more specific role class exists.
</li>
</ul>
</li>
</ul>
<p>If neither property, relevance nor irrelevance, can be shown for an abstract role,
a warning is given in case the enclosing team is not abstract.
</p>
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