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23.2 确立时的typeof函数</div>
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<div class="Element13"><div class="Element12" id="code00927"><pre class="Element12">typeof_function ::= <span style="color: #008000">// 没有包含在附录A中</span>
<strong><span style="color: #FF0000">$typeof(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong>
| <strong><span style="color: #FF0000">$typeof(</span></strong>data_type<strong><span style="color: #FF0000">)</span></strong></pre></div></div>
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<strong>语法 23-1 — typeof函数语法（没有包含在附录A中）</strong></p><p class="Element10">
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<span style="color: #0000FF">$typeof</span>系统函数返回它的自变量的数据类型。<span style="color: #0000FF">$typeof</span>系统函数返回的数据类型可以用来赋值或覆盖一个类型参数，或者在确立过程中与另外一个<span style="color: #0000FF">$typeof</span>系统函数返回的数据类型进行比较。 </p>
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在调用时，如果将表达式作为它的自变量，那么<span style="color: #0000FF">$typeof</span>返回一个代表表达式自决类型结果的类型。表达式的返回类型在确立时确定，但永远不会对其求值。表达式不应包含任何层次化的标识符或对动态对象元素的引用。在所有的关联文中，<span style="color: #0000FF">$typeof</span>与它的自变量一起可以使用在要求确立时常量的任何地方。 </p>
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当使用在比较表达式中的时候，如果操作数是类型等价的（参见<a href="00547.html" target="topic">5.8节</a>中有关类型等价的内容），那么等式(<strong><span style="color: #800080">==</span></strong>)或条件等式(<strong><span style="color: #800080">===</span></strong>)的结果为“真”。 </p>
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例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00928"><pre class="Element12"><span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">12</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> A_bus<span style="color: #800080">,</span> B_bus<span style="color: #800080">;</span>
<span style="color: #0000FF">parameter type</span> bus_t <span style="color: #800080">=</span> <span style="color: #0000FF">$typeof</span><span style="color: #800080">(</span>A_bus<span style="color: #800080">);</span>
<span style="color: #0000FF">generate</span>
<span style="color: #0000FF">case</span> <span style="color: #800080">(</span><span style="color: #0000FF">$typeof</span><span style="color: #800080">(</span>but_t<span style="color: #800080">))</span>
<span style="color: #0000FF">$typeof</span><span style="color: #800080">(</span><span style="color: #0000FF">bit</span><span style="color: #800080">[</span><span style="color: #FF0000">12</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]):</span> addfixed_int <span style="color: #800080">#(</span>bus_t<span style="color: #800080">)</span> <span style="color: #800080">(</span>A_bus<span style="color: #800080">,</span>B_bus<span style="color: #800080">);</span>
<span style="color: #0000FF">$typeof</span><span style="color: #800080">(</span><span style="color: #0000FF">real</span><span style="color: #800080">):</span> add_float <span style="color: #800080">#(</span><span style="color: #0000FF">$typeof</span><span style="color: #800080">(</span>A_bus<span style="color: #800080">))</span> <span style="color: #800080">(</span>A_bus<span style="color: #800080">,</span>B_bus<span style="color: #800080">);</span>
<span style="color: #0000FF">endcase</span>
<span style="color: #0000FF">endgenerate</span></pre></div></div>
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对用户来讲，<span style="color: #0000FF">$typeof</span>返回的真实值是不可见的并且是未定义的。</p></div>
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