c宏 # ##

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1. #include <stdio.h>
   
2. 
   
3. 
   
4. struct test {
   
5. 
   
6.     int  n0;
   
7.     int  n1;
   
8.     char ch;
   
9. };
   
10. 
    
11. #define add_test_n(num) do { t.n##num ++; } while (0)
    
12. #define add_test(obj) do { t.obj ++; } while (0)         /* 非注释 */
    
13. //#define add_test(obj) do { t.##obj ++; } while (0)  /* 注释 */
    
14. 
    
15. int main()
    
16. {
    
17.     struct test t = { 100, 100, 'a' };
    
18. 
    
19.     add_test_n(1);  // 在t.n后面连1 ->  t.n1
    
20.     add_test(ch);    // 按非注释的，直接将obj替换成ch当然没问题；按注释的add_test()，是在t.后面连ch，也是t.ch，为什么报错？
    
21.     printf("%d, %d, %c\n", t.n0, t.n1, t.ch);
    
22. 
    
23.     return 0;
    
24. }

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// 按非注释的，直接将obj替换成ch当然没问题；按注释的add_test()，是在t.后面连ch，也是t.ch，为什么报错？

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http://stackoverflow.com/questio ... preprocessing-token

nswcfd

因为t.ch不是个合法的identifier。

nswcfd

It is often useful to merge two tokens into one while expanding macros.
This is called "token pasting" or "token concatenation".  The `##'
preprocessing operator performs token pasting.  When a macro is
expanded, the two tokens on either side of each `##' operator are
combined into a single token, which then replaces the `##' and the two
original tokens in the macro expansion.  Usually both will be
identifiers, or one will be an identifier and the other a preprocessing
number.  When pasted, they make a longer identifier.  This isn't the
only valid case.  It is also possible to concatenate two numbers (or a
number and a name, such as `1.5' and `e3') into a number.  Also,
multi-character operators such as `+=' can be formed by token pasting.

   However, two tokens that don't together form a valid token cannot be
pasted together.  For example, you cannot concatenate `x' with `+' in
either order.  If you try, the preprocessor issues a warning and emits
the two tokens.  Whether it puts white space between the tokens is
undefined.  It is common to find unnecessary uses of `##' in complex
macros.  If you get this warning, it is likely that you can simply
remove the `##'.

----------------------------------------------

Preprocessing tokens fall into five broad classes: identifiers,
preprocessing numbers, string literals, punctuators, and other.  An
"identifier" is the same as an identifier in C: any sequence of
letters, digits, or underscores, which begins with a letter or
underscore.  Keywords of C have no significance to the preprocessor;
they are ordinary identifiers.  You can define a macro whose name is a
keyword, for instance.  The only identifier which can be considered a
preprocessing keyword is `defined'.  *Note Defined::.

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回复 3# nswcfd

终于又出现了