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| <td class="back"><a id="top"></a><a href="s2.6.html" rel="prev"><< §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="s2.7"> |
| <h2 class="sect">§2.7 Advanced structures<a class="img" href="s2.7.html" |
| title="PermaLink to §2.7 Advanced structures"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
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| <p>This section discusses how role containment and the playedBy relationship can be combined. |
| It does not define new rules, but illustrates rules defined above. The central idea is that any class |
| can have more than one of the three flavors <em>team, role, </em>and<em> base</em>. |
| |
| </p> |
| <div class="subsect depth3" id="s2.7.a"> |
| <h4 class="subsect">(a) <span class="title">Nesting</span><a class="img" href="s2.7.a.html" title="PermaLink to (a) Nesting"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a role (contained in a team) is also a team (marked with the <code>team</code> modifier) |
| it is a <strong>nested team</strong>. The depth of nesting is not restricted. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s2.7.b"> |
| <h4 class="subsect">(b) <span class="title">Stacking</span><a class="img" href="s2.7.b.html" title="PermaLink to (b) Stacking"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If the base class to which a role is bound using <code>playedBy</code> is a team, |
| the role is said to be <strong>stacked</strong> on the base team. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s2.7.c"> |
| <h4 class="subsect">(c) <span class="title">Layering</span><a class="img" href="s2.7.c.html" title="PermaLink to (c) Layering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If roles of a team <code>Secondary</code> are played by roles of another team <code>Primary</code> |
| (i.e., base classes are roles), the team <code>Secondary</code> defines a <strong>layer</strong> over the team <code>Primary</code>. |
| Such layering requires a final reference <code>anchor</code> from <code>Secondary</code> to an instance of <code>Primary</code>. |
| All playedBy declarations within <code>Secondary</code> specify their base classes anchored to that final link <code>anchor</code>. |
| |
| </p><img src="../images/Layering.png" alt="Team layering example" /><p>Due to the anchored base types, layered teams implicitly support the following guarantee: |
| all base objects of roles of <code>Secondary</code> are contained within the team instance specified by the link <code>anchor</code>. |
| If roles of <code>Secondary</code> contain any callin bindings to non-static base methods, these will be triggered only |
| when a base method is invoked on a base instance contained in the team specified by <code>anchor</code>. |
| <br /> |
| In accordance with <a href="s2.6.a.html" |
| title="§2.6.(a) Externalized roles of a base team" |
| class="sect">§2.6.(a)</a> the anchor in such anchored playedBy declarations |
| could also be the pseudo identifier <code>base</code>, provided that <code>Secondary</code> is a nested team, |
| which has a playedBy binding to <code>Primary</code> as its base class. |
| This situation is part of the second example <a href="#s2.7.d" title="§2.7.(d) Implicit playedBy specialization" |
| class="sect">below (§2.7.(d))</a> (see <code>T1 playedBy TB1</code>). |
| |
| </p> |
| </div> |
| <div class="newpage"></div> |
| <div class="subsect depth3" id="s2.7.d"> |
| <h4 class="subsect">(d) <span class="title">Implicit playedBy specialization</span><a class="img" href="s2.7.d.html" |
| title="PermaLink to (d) Implicit playedBy specialization"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>According to <a href="s2.1.d.html" title="§2.1.(d) No-variance" class="sect">§2.1.(d)</a> an implicit sub-role may <em>implicitly</em> specialize an existing <code>playedBy</code> relation. |
| This requires the base class to be specified relative to some implicit (<code>OuterTeam.this</code>) or explicit (<code>OuterTeam.base</code>) team anchor. |
| Specializing that team anchor automatically specializes the playedBy declaration, too. |
| This rule never requires any action from a programmer but only explains the interpretation of a playedBy declaration in |
| complex situations. |
| |
| </p> |
| <h5>Two advanced examples demonstrating the above are:</h5> |
| <table border="0"> |
| <colgroup span="1"> |
| <col align="left" span="1" /> |
| <col align="left" span="1" /> |
| </colgroup> |
| <tr> |
| <td rowspan="1" colspan="1"> |
| <ul> |
| <li>If a role <code>TOuter1.T.R</code> of a <strong>nested team </strong><code>TOuter1.T</code> is played by |
| another role of the outer enclosing team <code>TOuter1.B</code>, subclassing the outer team <code>TOuter1</code> to <code>TOuter2</code> |
| will produce a new role <code>TOuter2.T.R</code> which is automatically played by <code>TOuter2.B</code>, |
| an implicit sub class of the original base class <code>TOuter1.B</code>. |
| </li> |
| </ul> |
| </td> |
| <td rowspan="1" colspan="1"><img src="../images/implicitly_overriding_playedby.png" |
| alt="Implicitly overriding playedBy" /></td> |
| </tr> |
| <tr> |
| <td rowspan="1" colspan="1"> |
| <ul> |
| <li>Consider the case where a <strong>nested </strong><code>T1</code> as a role of <code>TOuter</code> is <strong>stacked</strong> |
| on a base team <code>TB1</code>. Also, <code>T1</code> is a <strong>layered team</strong> over <code>TB1</code> |
| because its role <code>R</code> adapts role <code>TB1.B</code>. |
| <br /> |
| In this situation the playedBy relation of role <code>TOuter.T1.R</code> is given by a base-anchored type <code>B<@T1.base></code>. |
| If furthermore <code>TOuter.T1</code> is subclassed to <code>TOuter.T2</code> which covariantly refines the inherited |
| playedBy declaration to <code>TB2</code>, then <code>TOuter.T2.R</code> will automatically refine the inherited playedBy relation |
| to <code>TB2.B</code> to follow the new interpretation of the <code>base</code> anchor. |
| </li> |
| </ul> |
| </td> |
| <td rowspan="1" colspan="1"><img src="../images/implicitly_overriding_playedby_base.png" |
| alt="Implicitly overriding playedBy base" /></td> |
| </tr> |
| </table> |
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| <td class="back"><a href="s2.6.html" rel="prev"><< §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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