可变参数宏__VA_ARGS__

「爱情、让人受尽委屈。」 2022-07-21 01:44 165阅读 0赞

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                  在 GNU C 中，宏可以接受可变数目的参数，就象函数一样，例如： 
                 <br style=""> #define pr_debug(fmt,arg...) \ 
                 <br style=""> printk(KERN_DEBUG fmt,##arg) 
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                 <br style=""> 
                 <br style=""> 
                 <br style=""> 
                 <span style="font-family:arial,helvetica; font-size:24px; word-wrap:normal; word-break:normal; line-height:36px"><strong>用可变参数宏(variadic macros)传递可变参数表</strong></span> 
                 <br style=""> 你可能很熟悉在函数中使用可变参数表，如： 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> <span style="color:#0066FF; word-wrap:normal; word-break:normal; line-height:18px">void printf(const char* format, …);</span></p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> 直到最近，可变参数表还是只能应用在真正的函数中，不能使用在宏中。</p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> C99编译器标准终于改变了这种局面，它允许你可以定义可变参数宏(variadic macros)，这样你就可以使用拥有可以变化的参数表的宏。可变参数宏就像下面这个样子：</p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> <span style="color:#0066FF; word-wrap:normal; word-break:normal; line-height:18px">#define debug(…) printf(__VA_ARGS__)</span></p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> 缺省号代表一个可以变化的参数表。使用保留名 __VA_ARGS__ 把参数传递给宏。当宏的调用展开时，实际的参数就传递给&nbsp;<em>printf()</em>了。例如：</p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> <span style="color:#0066FF; word-wrap:normal; word-break:normal; line-height:18px">Debug(“Y = %d\n”, y);</span></p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> 而处理器会把宏的调用替换成：</p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> <span style="color:#0066FF; word-wrap:normal; word-break:normal; line-height:18px">printf(“Y = %d\n”, y);</span></p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> 因为<em>debug()</em>是一个可变参数宏，你能在每一次调用中传递不同数目的参数：</p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> <span style="color:#0066FF; word-wrap:normal; word-break:normal; line-height:18px">debug(“test”);&nbsp;<span style="color:#006600; word-wrap:normal; word-break:normal">//一个参数</span></span></p> 
                 <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> 可变参数宏不被ANSI/ISO C++ 所正式支持。因此，你应当检查你的编译器，看它是否支持这项技术。</p> 
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