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8.4.1.1 case语句中的模式匹配</div>
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在一个模式匹配<span style="color: #0000FF">case</span>语句中，圆括号内的表达式紧跟着关键字<span style="color: #0000FF">matches</span>，并且语句包含了一系列“<i>case_pattern_items</i>”。在一个<span style="color: #0000FF">case</span>条目的左侧，“<span style="color: #800080">:</span>”之前的部分，包含了一个模式以及一个可选的<span style="color: #800080">&amp;&amp;</span>操作符，在<span style="color: #800080">&amp;&amp;</span>操作符之后紧跟一个“<i>filter</i>”布尔表达式。<span style="color: #0000FF">case</span>条目的右侧是一条语句。每一个模式都引入了一个新的作用范围，在这个范围中会隐式地声明它的模式标识符；这个作用范围扩展到可选的过滤器表达式以及同一个case条目右侧的语句。因此，不同的case条目可以复用模式标识符。&nbsp;</p>
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&nbsp;</p>
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所有的模式均执行完全静态的类型检查。在模式匹配<span style="color: #0000FF">case</span>语句中进行测试的表达式必须具有已知的类型，它与每个<span style="color: #0000FF">case</span>条目中模式的类型相同。&nbsp;</p>
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&nbsp;</p>
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模式匹配<span style="color: #0000FF">case</span>语句中圆括号内的表达式应该仅计算一次。它的值<i>V</i>应该以指定的顺序一次一个条目地与case条目的左侧进行匹配，如果有<span style="color: #0000FF">default case</span>条目的话，会将其忽略。在这个线性搜索过程中，如果一个<span style="color: #0000FF">case</span>条目被选择，那么它对应的语句会被执行，然后中止这个线性搜索。如果没有<span style="color: #0000FF">case</span>条目被选择，那么会执行<span style="color: #0000FF">default case</span>条目中的语句。如果既没有<span style="color: #0000FF">case</span>条目被选择也没有指定<span style="color: #0000FF">default case</span>条目的话，那么不执行任何语句。&nbsp;</p>
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对于一个被选择的<span style="color: #0000FF">case</span>条目，值<i>V</i>必须与模式匹配（模式标识符被赋值为<i>V</i>中对应的成员），并且，布尔过滤器表达式的计算结果必须为“真”（非零的确定值）。&nbsp;</p>
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&nbsp;</p>
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例子： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00268"><pre class="Element12"><span style="color: #0000FF">typedef union tagged</span> <span style="color: #800080">{</span>
    <span style="color: #0000FF">void</span> Invalid<span style="color: #800080">;</span>
    <span style="color: #0000FF">int</span> Valid<span style="color: #800080">;</span>
<span style="color: #800080">}</span> VInt<span style="color: #800080">;</span>

...
VInt v<span style="color: #800080">;</span>
...

<span style="color: #0000FF">case</span> <span style="color: #800080">(</span>v<span style="color: #800080">)</span> <span style="color: #0000FF">matches</span>
    <span style="color: #0000FF">tagged</span> Invalid <span style="color: #800080">:</span> <span style="color: #0000FF">$display</span> <span style="color: #800080">(</span><span style="color: #575D94">&quot;v is Invalid&quot;</span><span style="color: #800080">);</span>
    <span style="color: #0000FF">tagged</span> Valid n <span style="color: #800080">:</span> <span style="color: #0000FF">$display</span> <span style="color: #800080">(</span><span style="color: #575D94">&quot;v is Valid with value %d&quot;</span><span style="color: #800080">,</span> n<span style="color: #800080">);</span>
<span style="color: #0000FF">endcase</span></pre></div></div>
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在上述的case语句中，如果<i>v</i>当前具有<i>Invalid</i>标签，那么第一个模式形成匹配。否则，它必须具有<i>Valid</i>标签，并且第二个模式形成匹配。标识符<i>n</i>被绑定到<i>Valid</i>成员的值，并且这个值被显示。当标签为<i>Invalid</i>的时候，我们不可能访问整数成员值<i>n</i>。&nbsp;</p>
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&nbsp;</p>
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例子： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00269');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00269"><pre class="Element12"><span style="color: #0000FF">typedef union tagged</span> <span style="color: #800080">{</span>
    <span style="color: #0000FF">struct</span> <span style="color: #800080">{</span>
        <span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">4</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> reg1<span style="color: #800080">,</span> reg2<span style="color: #800080">,</span> regd<span style="color: #800080">;</span>
    <span style="color: #800080">}</span> Add<span style="color: #800080">;</span>
    <span style="color: #0000FF">union tagged</span> <span style="color: #800080">{</span>
        <span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">9</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> JmpU<span style="color: #800080">;</span>
        <span style="color: #0000FF">struct</span> <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">0</span><span style="color: #800080">]</span> cc<span style="color: #800080">;</span>
            <span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">9</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">]</span> addr<span style="color: #800080">;</span>
        <span style="color: #800080">}</span> JmpC<span style="color: #800080">;</span>
    <span style="color: #800080">}</span> Jmp<span style="color: #800080">;</span>
<span style="color: #800080">}</span> Instr<span style="color: #800080">;</span>
...
Instr instr;
...
<span style="color: #0000FF">case</span> <span style="color: #800080">(</span>instr<span style="color: #800080">)</span> <span style="color: #0000FF">matches</span>
    <span style="color: #0000FF">tagged</span> Add <span style="color: #800080">{</span>r1<span style="color: #800080">,</span>r2<span style="color: #800080">,</span>rd<span style="color: #800080">}</span> <span style="color: #800080">&amp;&amp; (</span>rd <span style="color: #800080">!=</span> <span style="color: #FF0000">0</span><span style="color: #800080">):</span> rf<span style="color: #800080">[</span>rd<span style="color: #800080">] =</span> rf<span style="color: #800080">[</span>r1<span style="color: #800080">] + </span>rf<span style="color: #800080">[</span>r2<span style="color: #800080">];</span>
    <span style="color: #0000FF">tagged</span> Jmp j <span style="color: #800080">:</span> <span style="color: #0000FF">case</span> <span style="color: #800080">(</span>j<span style="color: #800080">)</span> <span style="color: #0000FF">matches</span>
                       <span style="color: #0000FF">tagged</span> JmpU a <span style="color: #800080">:</span> pc <span style="color: #800080">=</span> pc <span style="color: #800080">+</span> a<span style="color: #800080">;</span>
                       <span style="color: #0000FF">tagged</span> JmpC <span style="color: #800080">{</span>c<span style="color: #800080">,</span>a<span style="color: #800080">}:</span> <span style="color: #0000FF">if</span> <span style="color: #800080">(</span>rf<span style="color: #800080">[</span>c<span style="color: #800080">])</span> pc <span style="color: #800080">=</span> a<span style="color: #800080">;</span>
<span style="color: #0000FF">                   endcase</span>
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