Passing the value of a variable to macro in C

Question

I\'m trying to pass the value of a variable to a macro in C, but I don\'t know if this is possible. Example:

#include 

#define CONCVAR(_n) x          


        

Answer 1

No, it won't work. The C/C++ pre-processor is just that, a "pre-compile" time text processor. As such, it operates on the text found as-is in your source code.

That's why, it takes the literal text "i", pass it into your macro, expanding that into the literal text "xi" in your source code. Then that gets passed into the compiler. The compiler then starts parsing the post-processed text, finding the literal token "xi" as an undeclared variable, going belly up in the process.

You can take your sample source code and pass it to a gcc compiler (I used gcc under cygwin for example, pasting your code into a file I named pimp.c for lack of a better name). Then you'll get the following:

$ gcc pimp.c
pimp.c: In function `main':
pimp.c:9: error: `xi' undeclared (first use in this function)
pimp.c:9: error: (Each undeclared identifier is reported only once
pimp.c:9: error: for each function it appears in.)

In short, no, you cannot do that. To be able to do just that, then the pre-processor would have to act as an interpreter. C and C++ are (generally) not interpreted languages, and the pre-processor is not an interpreter. My suggestion would be to get very clear on the differences between compilers and interpreters (and between compiled and interpreted languages.)

Regards.