#define中的#、## && #@
前些一段时间在看WinCE的Code时发现在宏定义中有用到##,如下所示
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在#define中,标准只定义了#和##两种操作。#用来把参数转换成字符串,##则用来连接两个前后两个参数,把它们变成一个字符串。
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#define ToString(a) #a
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ToString( A b Cd ); //A b Cd
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ToString( A\n b Cd ); //A
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// b Cd
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ToString( A\ n b Cd ); //A n b Cd
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#define ConCat(x, y) x ## y
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ConCat( "ABC", "DEF" );
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ConCat( 123, 4);
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ConCat( 123.0, 5);
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//=================================
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// Cat( 123.0, 5.5);
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// Cat( 'a', 'b' );
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// Cat( "ABC", 'd' );
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// Cat( 'a', 1234 );
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// Cat( "ABC", 1234 );
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//===== above can't compile =======
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另外,在网上搜到还有一种用法:#@,把参数转换成字符
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#define ToChar(a) #@a
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ToChar(a); // a
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ToChar(ab); // b
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ToChar(abc); // c
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ToChar(abcd); // d
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//ToChar(abcde); // too many characters in constant
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ToChar(1.); // .
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这个没在标准上看到。上述测试是在VS studio环境下做的。不知道GCC中是否有同样的用法。