plugins/org.eclipse.objectteams.otdt.ui.help/guide/otjld/xdef/s5.4.2.html - objectteams/org.eclipse.objectteams - Gitiles

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                <td class="back"><a id="top"></a><a href="s5.4.1.html" rel="prev">&lt;&lt;&nbsp;&sect;5.4.1&nbsp;Regular guards</a></td>
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          <div class="breadcrumb"><a class="nav" href="s5.html" rel="section">&sect;5&nbsp;Team Activation</a>&nbsp;&gt;&nbsp;<a class="nav" href="s5.4.html" rel="section">&sect;5.4&nbsp;Guard predicates</a></div>
          <div class="sect depth3" id="s5.4.2">
             <h3 class="sect">&sect;5.4.2&nbsp;Base guards<a class="img" href="s5.4.2.html"
                   title="PermaLink to &sect;5.4.2&nbsp;Base guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                        alt="" /></a></h3>
             <p>The intention behind base guards is to prevent lifting of a callin-target
                		if a guard evaluates to <code>false</code> and thus refuses to invoke the
                		callin bound role method. Using base guards it is easier to prevent any
                		side-effects caused by a callin binding, because lifting could cause side-effects
                		at two levels:

             </p>
             <ul>
                <li>Creating a role on-demand already is a side-effect (observable e.g.
                   			by the reflective function <code><a href="s6.1.html" title="&sect;6.1&nbsp;Reflection" class="sect">hasRole (&sect;6.1)</a></code>)

                </li>
                <li>Role creation triggers execution of a role constructor
                   			(see <a href="s2.3.1.c.html"
                      title="&sect;2.3.1.(c)&nbsp;Custom lifting constructor"
                      class="sect">custom lifting constructor (&sect;2.3.1.(c))</a>)
                   			which could produce arbitrary side-effects.

                </li>
             </ul>
             <p>Both kinds of side-effects can be avoided using a base guard which prevents
                		unnecessary lifting.

             </p>
             <p>Any guard (5.4.1 (b)-(e)) can be turned into a base guard by adding
                		the modifier <code>base</code> as in:

             </p>
             <div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole <b>playedBy</b> MyBase
   <em><b>base</b> when</em> (base.value &gt; MyTeam.this.threshold)
 {
     <i><b>class</b> body declarations</i>
 }</pre></div>
             <p>However, different scoping rules apply for the identifiers
                		that can be used in a base guard:

             </p>
             <div class="subsect depth4" id="s5.4.2.a">
                <h4 class="subsect">(a)&nbsp;<span class="title">Base object reference</span><a class="img" href="s5.4.2.a.html"
                      title="PermaLink to (a)&nbsp;Base object reference"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>In all base guard predicates the special identifier <code>base</code>
                   			can be used to denote the base object that is about to be lifted.

                </p>
             </div>
             <div class="subsect depth4" id="s5.4.2.b">
                <h4 class="subsect">(b)&nbsp;<span class="title">Method binding guards</span><a class="img" href="s5.4.2.b.html"
                      title="PermaLink to (b)&nbsp;Method binding guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>A base method binding guard may access parameters as passed to the
                   			base method. Parameter mappings are not considered.<br />
                   Additionally, for <code>after</code> callin bindings, the identifier <code>result</code>
                   may be used to refer to the result of the base method (if any).

                </p>
                <div class="note">
                   <h5>Note:</h5>
                   <p>In order to achieve the same effect of accessing
                      	the base method's result, a regular binding guard (not a base guard)
                      	must use a suitable parameter mapping (see <a href="s4.4.c.html"
                         title="&sect;4.4.(c)&nbsp;Mapping the result of a base method"
                         class="sect">&sect;4.4.(c)</a>).

                   </p>
                </div>
             </div>
             <div class="subsect depth4" id="s5.4.2.c">
                <h4 class="subsect">(c)&nbsp;<span class="title">Method guards</span><a class="img" href="s5.4.2.c.html" title="PermaLink to (c)&nbsp;Method guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>In contrast to regular method guards, a <em>base</em> guard attached to a role method
                   			cannot access any method parameters. See the next item (d) for values that are actually in scope.

                </p>
             </div>
             <div class="subsect depth4" id="s5.4.2.d">
                <h4 class="subsect">(d)&nbsp;<span class="title">Role level guards</span><a class="img" href="s5.4.2.d.html"
                      title="PermaLink to (d)&nbsp;Role level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>Role level base guards may use these values:

                </p>
                <ul>
                   <li>The base instance using the special identifier <code>base</code>.
                   </li>
                   <li>The team instance using a qualified this references (<code>MyTeam.this</code>).
                   </li>
                </ul>
             </div>
             <div class="subsect depth4" id="s5.4.2.e">
                <h4 class="subsect">(e)&nbsp;<span class="title">Team level guards</span><a class="img" href="s5.4.2.e.html"
                      title="PermaLink to (e)&nbsp;Team level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>Team level base guards have the same scope as role level base guards (d).
                   			However, the type of the role instance is not known here, i.e., here <code>base</code>
                   			has the static type <code>java.lang.Object</code>.

                </p>
             </div>
             <div class="subsect depth4" id="s5.4.2.f">
                <h4 class="subsect">(f)&nbsp;<span class="title">Unbound roles</span><a class="img" href="s5.4.2.f.html" title="PermaLink to (f)&nbsp;Unbound roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
                           alt="" /></a></h4>
                <p>In contrast to regular guards, base guards cannot be attached to
                   	unbound role classes nor to their methods.<br />
                   	Only team level base guards are independent of role binding.

                </p>
             </div><img src="../images/guards.png" alt="Overview: Guard predicates" /></div>
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          			OT/J version 1.3 &mdash; last modified: 2010-05-18
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	<!DOCTYPE html
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	<tr>
	<td class="back"><a id="top"></a><a href="s5.4.1.html" rel="prev"><< §5.4.1 Regular guards</a></td>
	<td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td>
	<td class="next"><a href="s5.4.3.html" rel="next">§5.4.3 Multiple guards >></a></td>
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	<div class="breadcrumb"><a class="nav" href="s5.html" rel="section">§5 Team Activation</a> > <a class="nav" href="s5.4.html" rel="section">§5.4 Guard predicates</a></div>
	<div class="sect depth3" id="s5.4.2">
	<h3 class="sect">§5.4.2 Base guards<a class="img" href="s5.4.2.html"
	title="PermaLink to §5.4.2 Base guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h3>
	<p>The intention behind base guards is to prevent lifting of a callin-target
	if a guard evaluates to <code>false</code> and thus refuses to invoke the
	callin bound role method. Using base guards it is easier to prevent any
	side-effects caused by a callin binding, because lifting could cause side-effects
	at two levels:

	</p>
	<ul>
	<li>Creating a role on-demand already is a side-effect (observable e.g.
	by the reflective function <code><a href="s6.1.html" title="§6.1 Reflection" class="sect">hasRole (§6.1)</a></code>)

	</li>
	<li>Role creation triggers execution of a role constructor
	(see <a href="s2.3.1.c.html"
	title="§2.3.1.(c) Custom lifting constructor"
	class="sect">custom lifting constructor (§2.3.1.(c))</a>)
	which could produce arbitrary side-effects.

	</li>
	</ul>
	<p>Both kinds of side-effects can be avoided using a base guard which prevents
	unnecessary lifting.

	</p>
	<p>Any guard (5.4.1 (b)-(e)) can be turned into a base guard by adding
	the modifier <code>base</code> as in:

	</p>
	<div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole <b>playedBy</b> MyBase
	<em><b>base</b> when</em> (base.value > MyTeam.this.threshold)
	{
	<i><b>class</b> body declarations</i>
	}</pre></div>
	<p>However, different scoping rules apply for the identifiers
	that can be used in a base guard:

	</p>
	<div class="subsect depth4" id="s5.4.2.a">
	<h4 class="subsect">(a) <span class="title">Base object reference</span><a class="img" href="s5.4.2.a.html"
	title="PermaLink to (a) Base object reference"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In all base guard predicates the special identifier <code>base</code>
	can be used to denote the base object that is about to be lifted.

	</p>
	</div>
	<div class="subsect depth4" id="s5.4.2.b">
	<h4 class="subsect">(b) <span class="title">Method binding guards</span><a class="img" href="s5.4.2.b.html"
	title="PermaLink to (b) Method binding guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>A base method binding guard may access parameters as passed to the
	base method. Parameter mappings are not considered.<br />
	Additionally, for <code>after</code> callin bindings, the identifier <code>result</code>
	may be used to refer to the result of the base method (if any).

	</p>
	<div class="note">
	<h5>Note:</h5>
	<p>In order to achieve the same effect of accessing
	the base method's result, a regular binding guard (not a base guard)
	must use a suitable parameter mapping (see <a href="s4.4.c.html"
	title="§4.4.(c) Mapping the result of a base method"
	class="sect">§4.4.(c)</a>).

	</p>
	</div>
	</div>
	<div class="subsect depth4" id="s5.4.2.c">
	<h4 class="subsect">(c) <span class="title">Method guards</span><a class="img" href="s5.4.2.c.html" title="PermaLink to (c) Method guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In contrast to regular method guards, a <em>base</em> guard attached to a role method
	cannot access any method parameters. See the next item (d) for values that are actually in scope.

	</p>
	</div>
	<div class="subsect depth4" id="s5.4.2.d">
	<h4 class="subsect">(d) <span class="title">Role level guards</span><a class="img" href="s5.4.2.d.html"
	title="PermaLink to (d) Role level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Role level base guards may use these values:

	</p>
	<ul>
	<li>The base instance using the special identifier <code>base</code>.
	</li>
	<li>The team instance using a qualified this references (<code>MyTeam.this</code>).
	</li>
	</ul>
	</div>
	<div class="subsect depth4" id="s5.4.2.e">
	<h4 class="subsect">(e) <span class="title">Team level guards</span><a class="img" href="s5.4.2.e.html"
	title="PermaLink to (e) Team level guards"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>Team level base guards have the same scope as role level base guards (d).
	However, the type of the role instance is not known here, i.e., here <code>base</code>
	has the static type <code>java.lang.Object</code>.

	</p>
	</div>
	<div class="subsect depth4" id="s5.4.2.f">
	<h4 class="subsect">(f) <span class="title">Unbound roles</span><a class="img" href="s5.4.2.f.html" title="PermaLink to (f) Unbound roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></h4>
	<p>In contrast to regular guards, base guards cannot be attached to
	unbound role classes nor to their methods.<br />
	Only team level base guards are independent of role binding.

	</p>
	</div><img src="../images/guards.png" alt="Overview: Guard predicates" /></div>
	<table class="nav">
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	<td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td>
	<td class="next"><a href="s5.4.3.html" rel="next">§5.4.3 Multiple guards >></a></td>
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	<div class="breadcrumb"><a class="nav" href="s5.html" rel="section">§5 Team Activation</a> > <a class="nav" href="s5.4.html" rel="section">§5.4 Guard predicates</a></div>
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