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| <td class="back"><a id="top"></a><a href="s9.1.html" rel="prev"><< §9.1 Defining classes with value parameters</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s9.3.html" rel="next">§9.3 Restrictions and limitations >></a></td> |
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| <div class="breadcrumb"><a class="nav" href="s9.html" rel="section">§9 Value Dependent Classes</a></div> |
| <div class="sect depth2" id="s9.2"> |
| <h2 class="sect">§9.2 Using classes with value parameters<a class="img" href="s9.2.html" |
| title="PermaLink to §9.2 Using classes with value parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h2> |
| <div class="syntaxlink"><a href="sA.html#sA.9.2" title="§A.9.2 ActualTypeArgument" |
| class="syntax">→ Syntax §A.9.2</a></div> |
| <p>When using a class which declares one or more value parameters (type anchors) |
| a corresponding <strong>anchor value</strong> has to be provided. |
| |
| </p> |
| <div class="sect depth3" id="s9.2.1"> |
| <h3 class="sect">§9.2.1 Parameter substitution<a class="img" href="s9.2.1.html" |
| title="PermaLink to §9.2.1 Parameter substitution"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §9.2</a></span></h3> |
| <p>Substitution of a type anchor of a class <code>MyClass<YourType p></code> is denoted as |
| <code>MyClass<@v></code>. |
| In this term <code>v</code> must be a value which is conform to the declaration of the value |
| parameter "<code>YourType p</code>", ie., <code>v</code> must have the static type <code>YourType</code>. |
| <br /> |
| The value passed for substituting a type anchor must be a path of variables declared as <code>final</code>. |
| Obviously, only the first element in such a path can be a local variable or a method argument, |
| all other elements have to be fields. The reason for requiring final variables is |
| in type checking as discussed next. |
| |
| </p> |
| <div class="note"> |
| <h5>Note:</h5> |
| Externalized roles as defined in <a href="s1.2.2.b.html" |
| title="§1.2.2.(b) Declaration with anchored type" |
| class="sect">§1.2.2.(b)</a> are |
| a special case of types with a value parameter, where the value |
| is an instance of the enclosing team. |
| |
| </div> |
| <div class="subsect depth4" id="s9.2.1.a"> |
| <h4 class="subsect">(a) <span class="title">Instance constrained type parameters</span><a class="img" href="s9.2.1.a.html" |
| title="PermaLink to (a) Instance constrained type parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>In addition to normal usage, a value parameter can be applied nested to a regular type parameter: |
| |
| </p> |
| <div class="listing plain"><pre><b>class</b> MyClass<YourType aName, DependentParam<<em>@aName></em>> { ...</pre></div> |
| <p> |
| Here the type parameter <code>DependentParam</code> is constrained to be anchored to <code>aName</code>. |
| |
| </p> |
| <p>If a value parameter is used as a constraint for a regular type parameter |
| any substitution for the type parameter must also supply a value matching the value parameter. |
| The class from above could be applied like this: |
| |
| </p> |
| <div class="listing plain"><pre><b>final</b> YourType anchor = <b>new</b> YourType(); |
| MyClass <em><@anchor, YourDependent<<strong>@anchor</strong>>></em></pre></div> |
| <p>Within the declaring element (class or method) |
| applications of the type variable representing the instance constrained type parameter |
| must repeat the anchor verbatim, i.e., no substitutions are performed here. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth3" id="s9.2.2"> |
| <h3 class="sect">§9.2.2 Type conformance<a class="img" href="s9.2.2.html" |
| title="PermaLink to §9.2.2 Type conformance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §9.2</a></span></h3> |
| <p>Two value dependent types (anchored types) are considered conform only if the anchors |
| of both types refer to <i>the same object(s)</i>. The compiler must be able to statically |
| analyze this anchor identity. |
| |
| </p> |
| <div class="subsect depth4" id="s9.2.2.a"> |
| <h4 class="subsect">(a) <span class="title">Substitutions for type anchors</span><a class="img" href="s9.2.2.a.html" |
| title="PermaLink to (a) Substitutions for type anchors"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Only two substitutions are considered for determining anchor identity: |
| |
| </p> |
| <ol> |
| <li>If a method signature uses <code>this</code> as the anchor of any of its types, |
| type checking an application of this method performs the following substitutions: |
| <br /> |
| A simple <code>this</code> expression is substituted by the actual call target |
| of the method application. |
| <br /> |
| A qualified <code>Outer.this</code> expression is substituted by the corresponding |
| enclosing instance of the call target. |
| </li> |
| <li>Assignments from a <code>final</code> identifier to another <code>final</code> |
| identifier are transitively followed, i.e., if <code>t1, t2</code> are final, |
| after an assignment <code>t1=t2</code> the types <code>C<@t1></code> and |
| <code>C<@t2></code> are considered identical. Otherwise <code>C<@t1></code> |
| and <code>C<@t2></code> are incommensurable. |
| <br /> |
| Attaching an actual parameter to a formal parameter in a method call is also considered |
| as an assignment with respect to this rule. |
| </li> |
| </ol> |
| </div> |
| <div class="subsect depth4" id="s9.2.2.b"> |
| <h4 class="subsect">(b) <span class="title">Conformance of raw types</span><a class="img" href="s9.2.2.b.html" |
| title="PermaLink to (b) Conformance of raw types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>After anchors have been proven identical, the raw types are checked for compatibility |
| using the standard Java rules. |
| |
| </p> |
| </div> |
| </div> |
| </div> |
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| <td class="back"><a href="s9.1.html" rel="prev"><< §9.1 Defining classes with value parameters</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s9.3.html" rel="next">§9.3 Restrictions and limitations >></a></td> |
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