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<a href="00863.html" target="topic">第三章 数据类型</a></div>
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3.9 用户定义的类型</div>
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<div class="Element13"><div class="Element12" id="code00043"><pre class="Element12">type_declaration ::= <span style="color: #008000">// 引用自附录A.2.1.3</span>
<strong><span style="color: #FF0000">typedef</span></strong> data_type type_identifier variable_dimension<strong><span style="color: #FF0000">;</span></strong>
| <strong><span style="color: #FF0000">typedef</span></strong> interface_instance_identifier<strong><span style="color: #FF0000">.</span></strong>type_identifier type_identifer<strong><span style="color: #FF0000">;</span></strong>
| <span style="color: #FF0000">typedef</span> [<strong><span style="color: #FF0000">enum</span></strong>|<strong><span style="color: #FF0000">struct</span></strong>|<strong><span style="color: #FF0000">union</span></strong>|<strong><span style="color: #FF0000">class</span></strong>] type_identifier<strong><span style="color: #FF0000">;</span></strong></pre></div></div>
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<strong>语法3-2 — 用户定义的类型（摘录自附录A）</strong></p><p class="Element10">
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象C语言一样，用户可以使用<span style="color: #0000FF">typedef</span>定义一个新的数据类型。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00044"><pre class="Element12"><span style="color: #0000FF">tyepdef</span> <span style="color: #0000FF">int</span> intP<span style="color: #800080">;</span></pre></div></div>
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这个新类型可以进行如下的实例化操作： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00045"><pre class="Element12">intP a<span style="color: #800080">,</span> b<span style="color: #800080">;</span></pre></div></div>
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如果一个类型首先使用空的<span style="color: #0000FF">typedef</span>标识为类型，那么它就可以在其定义之前使用。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00046"><pre class="Element12"><span style="color: #0000FF">typedef</span> foo<span style="color: #800080">;</span>
foo f <span style="color: #800080">=</span> <span style="color: #FF0000">1</span><span style="color: #800080">;</span>
<span style="color: #0000FF">typedef</span> <span style="color: #0000FF">int</span> foo<span style="color: #800080">;</span></pre></div></div>
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注意：这种方法不适合于枚举值，枚举类型必须在使用之前定义。 </p>
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除了不允许层次化地引用用户定义的类型标识符之外，用户定义的类型标识符与数据标识符具有相同的作用范围规则。SystemVerilog允许通过端口引用在接口内定义的类型标识符，只要它们在使用之前在本地重新定义。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00047"><pre class="Element12"><span style="color: #0000FF">interface</span> intf_i<span style="color: #800080">;</span>
<span style="color: #0000FF">typedef</span> <span style="color: #0000FF">int</span> data_t<span style="color: #800080">;</span>
<span style="color: #0000FF">endinterface</span>
<span style="color: #0000FF">module</span> sub <span style="color: #800080">(</span>intf_i p<span style="color: #800080">)</span>
<span style="color: #0000FF">typedef</span> p<span style="color: #800080">.</span>data_t mydata_t<span style="color: #800080">;</span>
my_data_t data<span style="color: #800080">;</span>
<span style="color: #008000">// 当'data'连接到上述接口的时候，它的类型是int</span>
<span style="color: #0000FF">endmodule</span></pre></div></div>
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因为强制类型转换（参见<a href="00385.html" target="topic">3.14节</a>）中仅允许使用简单类型名，因此对强制类型转换中的复合数据类型必须使用用户定义的类型名。另外，当使用非压缩数组的时候，用户定义的数据类型必须作为类型参数值使用。 </p>
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有时一个用户定义的类型需要在类型的内容被定义之前声明。这对于由<span style="color: #0000FF">enum</span>、<span style="color: #0000FF">struct</span>、<span style="color: #0000FF">union</span>和<span style="color: #0000FF">class</span>派生出的用户定义类型很有用处。对于这种应用的例子参看<a href="00028.html" target="topic">11.24节</a>。这种方式通过下列的<span style="color: #0000FF">typedef</span>格式来提供支持。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00048"><pre class="Element12"><span style="color: #0000FF">tyepdef</span> <span style="color: #0000FF">enum</span> type_declaration_identifier<span style="color: #800080">;</span>
<span style="color: #0000FF">tyepdef</span> <span style="color: #0000FF">struct</span> type_declaration_identifier<span style="color: #800080">;</span>
<span style="color: #0000FF">tyepdef</span> <span style="color: #0000FF">union</span> type_declaration_identifier<span style="color: #800080">;</span>
<span style="color: #0000FF">tyepdef</span> <span style="color: #0000FF">class</span> type_declaration_identifier<span style="color: #800080">;</span>
<span style="color: #0000FF">tyepdef</span> type_declaration_identifier<span style="color: #800080">;</span></pre></div></div>
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注意：这种方式在<a href="00028.html" target="topic">11.24节</a>中展示的类耦合定义中很有用，但它不能使用在结构体的耦合定义中，因为结构体是静态声明的，并且不支持结构体的指针。 </p>
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最后一个形式表明，用户定义类型中的类型不必在提前声明中定义。 </p>
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在<span style="color: #0000FF">generate</span>内部的<span style="color: #0000FF">typedef</span>不应该定义一个在提前定义范围外的提前定义的真实类型。</p></div>
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