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7.15 标签联合体表达式与成员访问</div>
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<div class="Element13"><div class="Element12" id="code00235"><pre class="Element12">expression ::= <span style="color: #008000">// 引用自附录A.8.3</span>
...
| tagged_union_expression
tagged_union_expression ::=
<strong><span style="color: #FF0000">tagged</span></strong> member_identifier [expression]</pre></div></div>
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<strong>语法 7-2 — 标签联合体语法（摘录自附录A）</strong></p><p class="Element10">
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一个标签联合体表达式（压缩的或非压缩的）可以使用关键字<span style="color: #0000FF">tagged</span>紧跟着一个标签联合体标识符，然后再跟着一个代表对应成员值的表达式来表示。对于<span style="color: #0000FF">void</span>类型的成员，成员值表达式被忽略。 </p>
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例子： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00236"><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 vi1<span style="color: #800080">,</span> vi2<span style="color: #800080">;</span>
vi1 <span style="color: #800080">=</span> <span style="color: #0000FF">tagged</span> Valid <span style="color: #800080">(</span><span style="color: #FF0000">23</span><span style="color: #800080">+</span><span style="color: #FF0000">34</span><span style="color: #800080">);</span> <span style="color: #008000">// 产生Valid int</span>
vi2 <span style="color: #800080">=</span> <span style="color: #0000FF">tagged</span> Invalid<span style="color: #800080">;</span> <span style="color: #008000">// 产生一个Invalid值</span></pre></div></div>
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在下面的标签联合体表达式中，花括号内的表达式是结构体表达式（参见<a href="00560.html" target="topic">7.14 结构体表达式</a> ）。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00237"><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 i1<span style="color: #800080">,</span> i2<span style="color: #800080">;</span>
<span style="color: #008000">// 产生具有3个寄存器域的Add指令</span>
i1 <span style="color: #800080">= (</span>e
<span style="color: #800080">?</span> <span style="color: #0000FF">tagged</span> Add <span style="color: #800080">{</span>e1<span style="color: #800080">,</span> <span style="color: #FF0000">4</span><span style="color: #800080">,</span> ed<span style="color: #800080">};</span> <span style="color: #008000">// 通过位置设置结构体成员</span>
<span style="color: #800080">?</span> <span style="color: #0000FF">tagged</span> Add <span style="color: #800080">{</span>reg2<span style="color: #800080">:</span>e2<span style="color: #800080">,</span> regd<span style="color: #800080">:</span><span style="color: #FF0000">3</span><span style="color: #800080">,</span> reg1<span style="color: #800080">:</span><span style="color: #FF0000">19</span><span style="color: #800080">});</span> <span style="color: #008000">// 通过名字（顺序无关的）</span>
<span style="color: #008000">// 产生一个具有“无条件”子操作码的Jump指令</span>
i1 <span style="color: #800080">=</span> <span style="color: #0000FF">tagged</span> Jmp <span style="color: #800080">(</span><span style="color: #0000FF">tagged</span> JmpU <span style="color: #FF0000">239</span><span style="color: #800080">);</span>
<span style="color: #008000">// 产生一个具有“有条件”子操作码的Jump指令</span>
i2 <span style="color: #800080">=</span> <span style="color: #0000FF">tagged</span> Jmp <span style="color: #800080">(</span><span style="color: #0000FF">tagged</span> JmpC <span style="color: #800080">{</span><span style="color: #FF0000">2</span><span style="color: #800080">,</span> <span style="color: #FF0000">83</span><span style="color: #800080">});</span> <span style="color: #008000">// 通过位置设置内层结构体</span>
i2 <span style="color: #800080">=</span> <span style="color: #0000FF">tagged</span> Jmp <span style="color: #800080">(</span><span style="color: #0000FF">tagged</span> JmpC <span style="color: #800080">{</span>cc<span style="color: #800080">:</span><span style="color: #FF0000">2</span><span style="color: #800080">,</span> addr<span style="color: #800080">:</span><span style="color: #FF0000">83</span><span style="color: #800080">});</span> <span style="color: #008000">// 通过名字设置内层结构体</span></pre></div></div>
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标签联合体表达式的类型必须能够从它的上下文中获知（例如，使用在为类型已知变量赋值的语句的右侧、具有一个强制类型转换、或者是使用在一个已知类型的另外一个表达式中）。表达式计算成该类型的标签联合体的值。标签联合体表达式可以执行完全静态的类型检查：<span style="color: #0000FF">tagged</span>关键字之后的成员名字必须具有表达式的类型，并且成员表达式必须具有对应的成员类型。 </p>
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一个未初始化的标签联合体类型的变量应该是未定义的（包括标签位）。标签联合体变量可以使用标签联合体表达式初始化，只要成员值的表达式具有合法的成员类型。 </p>
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标签联合体的成员可以使用通常的点号来读取或赋值。这样的访问需要完全的类型检查，也就是说，读取或赋值的值必须与当前的标签一致。通常，这要求运行时的检查。试图读取或赋值一个类型与标签不一致的值会导致一个运行时错误。 </p>
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只要指令变量<i>instr</i>当前具有<i>Add</i>标签，那么下面的例子就是合法的： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00238"><pre class="Element12">x <span style="color: #800080">=</span> i1<span style="color: #800080">.</span>Add<span style="color: #800080">.</span>reg1<span style="color: #800080">;</span>
i1<span style="color: #800080">.</span>Add <span style="color: #800080">= {</span><span style="color: #FF0000">19</span><span style="color: #800080">,</span> <span style="color: #FF0000">4</span><span style="color: #800080">,</span> <span style="color: #FF0000">3</span><span style="color: #800080">};</span>
i1<span style="color: #800080">.</span>Add<span style="color: #800080">.</span>reg2 <span style="color: #800080">=</span> <span style="color: #FF0000">4</span><span style="color: #800080">;</span></pre></div></div>
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