define macro expanded to literal # [duplicate]

This question already has answers here: Closed 12 years ago.

Possible Duplicate:

c++ preprocessor macro expansion to another preprocessor directive

My question is very simple: I want to use开发者_开发百科 "#" in the macro expansion, for example, to define a marco 'M(X)':

#define M(X) #ifdef FOO \ 
              X=1 \
             #else \ 
              X=2

I tried to use '\' to escape the '#', but the '\' is interpreted as the newline for macro expansion, and not as the escape character. So how to use '#' in the macro expansion?

Thanks folks!

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Sorry, but you cannot have a macro emit another macro because the pre-processor is single pass. Also from the c99 standard:

(6.10.3.4 paragraph 3):

3 The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, ...

c++ has something similar as well.

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You could do the opposite:

#ifdef FOO
    #define M(X) X=1
#else
    #define M(X) X=2
#endif

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As @Evan points out, macro expansion is done in a single pass, so your example will not work. However, here is an alternative that does what you want, albeit in a few more lines.

#ifdef FOO
#define FOOVAL 1
#else
#define FOOVAL 2
#endif

#define M(X) X=FOOVAL