gistfile1.cpp

#ifdef DEBUG
#define debug printf("Debug: ");printf
#else
#define debug //
#endif

#include <cstdio>

int main()
{
  debug("Hello World!\n");
  return 0;
}

//Use debug function like printf
//To show debugging messages, compile with -DDEBUG flag, eg, g++ main.cpp -o main.out -O2 -DDEBUG
//To hide debugging messages, compile without -DDEBUG flag, eg, g++ main.cpp -o main.out -O2