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Comment by jmatt | Hacker News Reader

continued "fun" (read dangerous) redefines:

  #define true false
  #define false true

  #define true 0
  #define false !true

  #define if while
  #define continue break

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  #define true false
  #define false true

Did you know that if you do both of those, they cancel each other out? True story.

My head hurts. Explanation, please?

Dunno about explanation, but experiment shows it to work at least with two preprocessors - Microsoft's and GCC:

First make a file a.c that has in it:

#define true false

#define false true

int A = true;

int B = false;

then from the command line, do (with the Microsoft compiler): D:\>cl -E a.c

int A = true;

int B = false;

Now with gcc (I'm using cygwin's gcc): D:\>sh -c "gcc -E a.c"

int A = true;

int B = false;

I've stripped the other stuff.

There's a rule in the preprocessor to prevent infinite recursion: only replace each symbol once. The first time it encounters 'true', it's replaced with 'false'. The first time it encounters 'false, it's replace with 'true'. It's already seen 'true', so it throws up its hands.

The consequence of this rule is that cycles of #defines don't do anything. Edit: Well, that's only true if there aren't other symbols introduced during expansion. Here's a counterexample that prints 3:

   #include <stdio.h>

   int x = 1;

   #define x 1 + y
   #define y 1 + x

   int main(int argc, char *argv[])
   {
     printf("%d\n",x);
     return 0;
   }

Ah .. Makes sense. Thanks.

  #define TRUE random()%2

You are truly devious

#define struct union

That is by far the evilest of the bunch. }:-D

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