How to undefine a define at commandline using gcc

Question

How do I at compile time undefine a compiler macro using gcc. I tried some compile args to gcc like -D but I can't get to see the "not defined" message.

Thanks

#include <iostream>  #define MYDEF   int main(){ #ifdef MYDEF   std::cout<<"defined\n"; #else   std::cout<<"not defined\n"; #endif  } 

Answer 1

You can use the -U option with gcc, but it won't undefine a macro defined in your source code. As far as I know, there's no way to do that.

Answer 2

You should wrap the MYDEF definition in a preprocessor macro, the presence of which (defined on the command line) would then prevent MYDEF from being defined. A bit convoluted to be sure but you can then control the build in the way you want from the command line (or Makefile). Example:

#ifndef DONT_DEFINE_MYDEF #define MYDEF #endif 

Then from the command line when you don't want MYDEF:

gcc -DDONT_DEFINE_MYDEF ...