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11.13 过载成员</div>
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子类对象也是它们的父类对象的有效表示。例如，每一个<i>LinkPacket</i>对象都是一个完全合法的<i>Packet</i>对象。 </p>
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一个<i>LinkedPacket</i>对象的句柄可以赋值到一个<i>Packet</i>变量： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00364"><pre class="Element12">LinkedPacket lp <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span>
Packet p <span style="color: #800080">=</span> lp<span style="color: #800080">;</span></pre></div></div>
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在这种情况下，对<i>p</i>的引用访问<i>Packet</i>类的方法和类属性。因此，例如，如果<i>LinkPacket</i>中的类属性和方法被过载，那么通过<i>p</i>引用这些过载的成员会获得<i>Packet</i>类中的原始成员。从<i>p</i>的角度看，<i>LinkedPacket</i>中的<i>new</i>和所有过载的成员都被隐藏了。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00365"><pre class="Element12"><span style="color: #0000FF">class</span> Packet<span style="color: #800080">;</span>
<span style="color: #0000FF">integer</span> i <span style="color: #800080">=</span> <span style="color: #FF0000">1</span><span style="color: #800080">;</span>
<span style="color: #0000FF">function integer</span> get<span style="color: #800080">();</span>
get <span style="color: #800080">=</span> i<span style="color: #800080">;</span>
<span style="color: #0000FF">endfunction</span>
<span style="color: #0000FF">endclass</span>
<span style="color: #0000FF">class</span> LinkedPacket <span style="color: #0000FF">extends</span> Packet<span style="color: #800080">;</span>
<span style="color: #0000FF">integer</span> i <span style="color: #800080">=</span> <span style="color: #FF0000">2</span><span style="color: #800080">;</span>
<span style="color: #0000FF">function integer</span> get<span style="color: #800080">();</span>
get <span style="color: #800080">=</span> <span style="color: #800080">-</span>i<span style="color: #800080">;</span>
<span style="color: #0000FF">endfunction</span>
<span style="color: #0000FF">endclass</span>
LinkedPacket lp <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span>
Packet p <span style="color: #800080">=</span> lp<span style="color: #800080">;</span>
j <span style="color: #800080">=</span> p<span style="color: #800080">.</span>i<span style="color: #800080">;</span> <span style="color: #008000">// j=1，而不是2</span>
j <span style="color: #800080">=</span> p<span style="color: #800080">.</span>get<span style="color: #800080">();</span> <span style="color: #008000">// j=1，不是-1或–2</span></pre></div></div>
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为了通过一个父类对象（在本例中为<i>p</i>）调用被过载的方法，这个方法需要使用<span style="color: #0000FF">vitual</span>声明（参见<a href="00022.html" target="topic">11.19节</a>）。</p></div>
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