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📁 这是一本关于verilog编程语言的教程,对学习verilog语言有帮助
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<a href="#" onmousedown="showPopup(this, 'popup00199');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
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11.11 赋值、重命名与拷贝</div>
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声明一个类变量仅仅产生一个名字，这个名字表示了一个对象。因此： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00355"><pre class="Element12">Packet p1<span style="color: #800080">;</span></pre></div></div>
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上面的例子产生了一个变量，<i>p1</i>，它保存了一个类型为<i>Packet</i>的类对象的句柄，但<i>p1</i>的初始值为<span style="color: #FF0000">null</span>。在<i>Packet</i>类型的一个实例被产生之前，对象并不存在，并且<i>p1</i>没有包含一个真实的句柄： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00356"><pre class="Element12">p1 <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span></pre></div></div>
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因此，如果声明了另外一个变量，并且将先前的句柄赋值为新的句柄，就像下面的例子所示： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00357"><pre class="Element12">Packet p2<span style="color: #800080">;</span>
p2 <span style="color: #800080">= </span>p1<span style="color: #800080">;</span></pre></div></div>
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那么，仍然只有一个对象，它可以使用<i>p1</i>或<i>p2</i>引用。注意：<span style="color: #0000FF">new</span>仅仅执行一次，所以只产生了一个对象。&nbsp;</p>
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然而，如果上面的例子按如下的方式重写，那么会产生一个<i>p1</i>的拷贝： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00358"><pre class="Element12">Packet p1<span style="color: #800080">;</span>
Packet p2<span style="color: #800080">;</span>
p1 <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span>
p2 <span style="color: #800080">=</span> <span style="color: #0000FF">new</span> p1<span style="color: #800080">;</span></pre></div></div>
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最后一条语句使得<span style="color: #0000FF">new</span>第二次执行，因此产生了一个新的对象<i>p2</i>，它的类属性拷贝自<i>p1</i>。这种方式的拷贝被称作浅拷贝（<strong><i>shallow copy</i></strong>）。所有的变量都被拷贝：整数、字符串、实例句柄等。然而其中包含的对象没有被拷贝，拷贝的是它的句柄；与前面的例子一样，产生了相同对象的两个名字。即使类声明中包含了操作符<span style="color: #0000FF">new</span>的实例化的时候也是如此： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00359"><pre class="Element12"><span style="color: #0000FF">class</span> A<span style="color: #800080">;</span>
    <span style="color: #0000FF">integer</span> j <span style="color: #800080">=</span> <span style="color: #FF0000">5</span><span style="color: #800080">;</span>
<span style="color: #0000FF">endclass</span>

<span style="color: #0000FF">class</span> B<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>
    A a <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span>
<span style="color: #0000FF">endclass</span>

<span style="color: #0000FF">function integer</span> test<span style="color: #800080">;</span>
    B b1 <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">;</span>    <span style="color: #008000">// 产生一个B类的对象</span>
    B b2 <span style="color: #800080">=</span> <span style="color: #0000FF">new</span> b1<span style="color: #800080">;</span> <span style="color: #008000">// 产生一个拷贝自b1的对象</span>
    b2<span style="color: #800080">.</span>i <span style="color: #800080">=</span> <span style="color: #FF0000">10</span><span style="color: #800080">;</span>     <span style="color: #008000">// i在b2中改变，然而在b1中没有改变</span>
    b2<span style="color: #800080">.</span>a<span style="color: #800080">.</span>j <span style="color: #800080">=</span> <span style="color: #FF0000">50</span><span style="color: #800080">;</span>   <span style="color: #008000">// 改变a.j，为b1和b2共享</span>
    test <span style="color: #800080">=</span> b1<span style="color: #800080">.</span>i<span style="color: #800080">;</span>   <span style="color: #008000">// test被设置成1（b1.i没有改变）</span>
    test <span style="color: #800080">=</span> b1<span style="color: #800080">.</span>a<span style="color: #800080">.</span>j<span style="color: #800080">;</span> <span style="color: #008000">// test被设置成50（a.j已经改变了）</span>
<span style="color: #0000FF">endfunction</span></pre></div></div>
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有几件事需要注意。第一，类属性和实例对象可以直接在类声明中初始化。第二，shallow copy不会拷贝对象。第三，在需要的时候实例限定可以被串接起来以便到达对象或通过对象到达： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00360"><pre class="Element12">b1<span style="color: #800080">.</span>a<span style="color: #800080">.</span>j               <span style="color: #008000">// reaches into a, which is a property of b1</span>
p<span style="color: #800080">.</span>next<span style="color: #800080">.</span>next<span style="color: #800080">.</span>next<span style="color: #800080">.</span>val <span style="color: #008000">// 通过一系列句柄的串接来获得val</span></pre></div></div>
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为了实现完整（深度）拷贝，其中每一个东西（包括嵌套的对象）都被拷贝，典型情况下需要定制代码。例如： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00361');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00361"><pre class="Element12">Packet p1 <span style="color: #800080">= </span><span style="color: #0000FF">new</span><span style="color: #800080">;</span>
Packet p2 <span style="color: #800080">= </span><span style="color: #0000FF">new</span><span style="color: #800080">;</span>
p2<span style="color: #800080">.</span>copy<span style="color: #800080">(</span>p1<span style="color: #800080">);</span></pre></div></div>
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其中<i>copy<span style="color: #800080">(</span>Packet p<span style="color: #800080">)</span></i>是一个定制的方法，它用来拷贝作为参数传递给它的实例的对象。</p></div>
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