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| <td class="back"><a id="top"></a><a href="s2.4.html" rel="prev"><< §2.4 Explicit role creation</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s2.6.html" rel="next">§2.6 Explicit base references >></a></td> |
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| <div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a></div> |
| <div class="sect depth2" id="s2.5"> |
| <h2 class="sect">§2.5 Abstract Roles<a class="img" href="s2.5.html" |
| title="PermaLink to §2.5 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="§1.3.1.(e) Dynamic binding of types" |
| class="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) <span class="title">Using abstract classes for creation</span><a class="img" href="s2.5.a.html" |
| title="PermaLink to (a) 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&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. |
| |
| </div> |
| </div> |
| <div class="subsect depth3" id="s2.5.b"> |
| <h4 class="subsect">(b) <span class="title">Relevant roles</span><a class="img" href="s2.5.b.html" title="PermaLink to (b) 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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| <td class="back"><a href="s2.4.html" rel="prev"><< §2.4 Explicit role creation</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s2.6.html" rel="next">§2.6 Explicit base references >></a></td> |
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| <div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a></div> |
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| width="88" /></a><address>© Stephan Herrmann, Christine Hundt, Marco Mosconi</address> |
| OT/J version 1.3 — last modified: 2011-05-12 |
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