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11.18 常量类属性</div>
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与任何其它SystemVerilog变量一样，类属性可以通过<span style="color: #0000FF">const</span>声明成只读的。然而，由于类属性是动态的对象，类属性允许两种形式的只读变量：全局常量和实例常量。 </p>
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全局常量类属性是那些在声明中包含了初始值得常量类属性。它们与其它的const变量类似，也就是它们不能在除声明之外的其它地方赋值。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00372"><pre class="Element12"><span style="color: #0000FF">class</span> Jumbo_Packet<span style="color: #800080">;</span>
<span style="color: #0000FF">const int</span> max_size <span style="color: #800080">=</span> <span style="color: #FF0000">9</span> <span style="color: #800080">*</span> <span style="color: #FF0000">1024</span><span style="color: #800080">;</span> <span style="color: #008000">// 全局常量</span>
<span style="color: #0000FF">byte</span> payload<span style="color: #800080">[];</span>
<span style="color: #0000FF">function new</span><span style="color: #800080">(</span><span style="color: #0000FF">int</span> size<span style="color: #800080">);</span>
payload <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><span style="color: #800080">[</span>size <span style="color: #800080">></span> max_size <span style="color: #800080">?</span> max_size <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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实例常量在声明中不包含初始值，仅包含<span style="color: #0000FF">const</span>限定符。这种类型的常量可以在运行时赋一个值，但赋值只能在对应的类构造器中完成一次。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00373"><pre class="Element12"><span style="color: #0000FF">class</span> Big_Packet<span style="color: #800080">;</span>
<span style="color: #0000FF">const int</span> size<span style="color: #800080">;</span> <span style="color: #008000">// 实例常量</span>
<span style="color: #0000FF">byte</span> payload<span style="color: #800080">[];</span>
<span style="color: #0000FF">function new</span><span style="color: #800080">();</span>
size <span style="color: #800080">=</span> <span style="color: #0000FF">$random</span> <span style="color: #800080">%</span> <span style="color: #FF0000">4096</span><span style="color: #800080">;</span> <span style="color: #008000">// 在new中完成一次赋值 -> 正确</span>
payload <span style="color: #800080">=</span> <span style="color: #0000FF">new</span><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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典型情况下，全局常量还被声明成<span style="color: #0000FF">static</span>，因为它们对于类的所有实例都是相同的。然而，一个实例常量不能声明成<span style="color: #0000FF">static</span>，因为它们在构造器不接受所有的赋值。</p></div>
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