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11.23 参数化的类</div>
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如果定义一个基本类，它的对象在实例化的时候可以具有不同的数组尺寸或数据类型，那么这种功能通常很有用。我们可以不必为每一个尺寸或类型编写相同的代码，并且可以为本质上不同并且不可互换的对象使用同一个说明（这与C++的类模板类似）。 </p>
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普通的Verilog参数机制可以用来参数化一个类： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00383');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00383"><pre class="Element12"><span style="color: #0000FF">class</span> vector <span style="color: #800080">#(</span><span style="color: #0000FF">int</span> size <span style="color: #800080">=</span> <span style="color: #FF0000">1</span><span style="color: #800080">);</span>
<span style="color: #0000FF">bit</span> <span style="color: #800080">[</span>size<span style="color: #800080">-</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> a<span style="color: #800080">;</span>
<span style="color: #0000FF">endclass</span></pre></div></div>
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接下来，这个类的实例就可以像模块或接口一样实例化： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00384"><pre class="Element12">vector <span style="color: #800080">#(</span><span style="color: #FF0000">10</span><span style="color: #800080">)</span> vten<span style="color: #800080">;</span> <span style="color: #008000">// 尺寸为10的vector对象</span>
vector <span style="color: #800080">#(.</span>size<span style="color: #800080">(</span><span style="color: #FF0000">2</span><span style="color: #800080">))</span> vtwo<span style="color: #800080">;</span> <span style="color: #008000">// 尺寸为2的vector对象</span>
<span style="color: #0000FF">typedef</span> vector<span style="color: #800080">#(</span><span style="color: #FF0000">4</span><span style="color: #800080">)</span> Vfour<span style="color: #800080">;</span> <span style="color: #008000">// vector尺寸为4的类</span></pre></div></div>
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当将类型作为参数的时候，这个功能特别有用： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00385"><pre class="Element12"><span style="color: #0000FF">class</span> stack <span style="color: #800080">#(</span><span style="color: #0000FF">type</span> T <span style="color: #800080">=</span> <span style="color: #0000FF">int</span><span style="color: #800080">);</span>
<span style="color: #0000FF">local</span> T items<span style="color: #800080">[];</span>
<span style="color: #0000FF">task</span> push<span style="color: #800080">(</span>T a<span style="color: #800080">);</span>
...
<span style="color: #0000FF">endtask</span>
<span style="color: #0000FF"> task</span> pop<span style="color: #800080">(</span><span style="color: #0000FF">ref</span> T a<span style="color: #800080">);</span>
...
<span style="color: #0000FF">endtask</span>
<span style="color: #0000FF">endclass</span></pre></div></div>
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上面的类定义了一个基本的<i>stack</i>类，它可以使用任意类型实例化： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00386"><pre class="Element12">stack is<span style="color: #800080">;</span> <span style="color: #008000">// 缺省：int类型的堆栈</span>
stack<span style="color: #800080">#(</span><span style="color: #0000FF">bit</span><span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">10</span><span style="color: #800080">])</span> bs<span style="color: #800080">;</span> <span style="color: #008000">// 10位向量的堆栈</span>
stack<span style="color: #800080">#(</span><span style="color: #0000FF">real</span><span style="color: #800080">)</span> rs<span style="color: #800080">;</span> <span style="color: #008000">// 实数的堆栈</span></pre></div></div>
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任何类型都可以用作参数，包括用户定义的类型，例如类或结构体。 </p>
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基本类和真实参数值的组合被称作一个特例（或变种）。类的每一个特例又具有一组单独的静态成员变量（这与C++模板类一致）。为了在多个类特例间共享静态成员变量，这些变量必须放置在一个非参数化的基类中。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00387"><pre class="Element12"><span style="color: #0000FF">class</span> vector <span style="color: #800080">#(</span><span style="color: #0000FF">int</span> size <span style="color: #800080">=</span> <span style="color: #FF0000">1</span><span style="color: #800080">);</span>
<span style="color: #0000FF">bit</span> <span style="color: #800080">[</span>size<span style="color: #800080">-</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> a<span style="color: #800080">;</span>
<span style="color: #0000FF">static int</span> count <span style="color: #800080">=</span> <span style="color: #FF0000">0</span><span style="color: #800080">;</span>
<span style="color: #0000FF">function void</span> disp_count<span style="color: #800080">();</span>
<span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">"count: %d of size %d"</span><span style="color: #800080">,</span> count<span style="color: #800080">,</span> size<span style="color: #800080">);</span>
<span style="color: #0000FF">endfunction</span>
<span style="color: #0000FF">endclass</span></pre></div></div>
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上面例子中的变量<i>count</i>只能通过对应的<i>disp_count</i>方法访问。对于<i>vector</i>类的每一个特例，<i>count</i>都具有它自己的唯一的拷贝。 </p>
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为了避免必须在声明中或产生该类的参数中重复特例，我们应该使用<span style="color: #0000FF">typedef</span>： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00388"><pre class="Element12"><span style="color: #0000FF">typedef</span> vector<span style="color: #800080">#(</span><span style="color: #FF0000">4</span><span style="color: #800080">)</span> Vfour<span style="color: #800080">;</span>
<span style="color: #0000FF">typedef</span> stack<span style="color: #800080">#(</span>Vfour<span style="color: #800080">)</span> Stack4<span style="color: #800080">;</span>
Stack4 s1<span style="color: #800080">,</span> s2<span style="color: #800080">;</span> <span style="color: #008000">// 声明Stack4类型的对象</span></pre></div></div>
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一个参数化的类可以扩展另外一个参数化的类，例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00389"><pre class="Element12"><span style="color: #0000FF">class</span> C <span style="color: #800080">#(</span><span style="color: #0000FF">type</span> T <span style="color: #800080">=</span> <span style="color: #0000FF">bit</span><span style="color: #800080">);</span> ... <span style="color: #0000FF">endclass</span> <span style="color: #008000">// 基类</span>
<span style="color: #0000FF">class</span> D1 <span style="color: #800080">#(</span><span style="color: #0000FF">type</span> P <span style="color: #800080">=</span> <span style="color: #0000FF">real</span><span style="color: #800080">)</span> <span style="color: #0000FF">extends</span> C<span style="color: #800080">;</span> <span style="color: #008000">// T的类型为bit（缺省）</span>
<span style="color: #0000FF">class</span> D2 <span style="color: #800080">#(</span><span style="color: #0000FF">type</span> P <span style="color: #800080">=</span> <span style="color: #0000FF">real</span><span style="color: #800080">)</span> <span style="color: #0000FF">extends</span> C <span style="color: #800080">#(</span><span style="color: #0000FF">integer</span><span style="color: #800080">);</span> <span style="color: #008000">// T的类型为integer</span>
<span style="color: #0000FF">class</span> D3 <span style="color: #800080">#(</span><span style="color: #0000FF">type</span> P <span style="color: #800080">=</span> <span style="color: #0000FF">real</span><span style="color: #800080">)</span> <span style="color: #0000FF">extends</span> C <span style="color: #800080">#(</span>P<span style="color: #800080">);</span> <span style="color: #008000">// T的类型为P</span></pre></div></div>
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<i>D1</i>类使用基类的缺省类型（<span style="color: #0000FF">bit</span>）参数扩展了基类<i>C</i>。 <i>D2</i>类使用一个<span style="color: #0000FF">integer</span>参数扩展了基类。<i>D3</i>类使用参数化类型（<i>P</i>，它使得扩展类被参数化）扩展了基类<i>C</i>。</p></div>
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