Is #if defined MACRO equivalent to #ifdef MACRO?

Question

I have code that I want to have two modes, debug and verbose. I define them in my header file as,

#define verbose TRUE
#def         


        

Answer 1

No, they are not at all equivalent. An #if MACRO branch is compiled if MACRO evaluates to non-zero. On the other hand, an #ifdef MACRO branch is compiled if MACRO is defined, no matter what it evaluates to. So

#include 

#define VAR_SET_TO_TRUE 1
#define VAR_SET_TO_FALSE 0

int main()
{

#if VAR_SET_TO_TRUE
    printf("#if VAR_SET_TO_TRUE\n");
#endif

#if VAR_SET_TO_FALSE
    printf("#if VAR_SET_TO_FALSE\n");
#endif

#if VAR_UNSET
    printf("#if VAR_UNSET\n");
#endif



#ifdef VAR_SET_TO_TRUE
    printf("#ifdef VAR_SET_TO_TRUE\n");
#endif

#ifdef VAR_SET_TO_FALSE
    printf("#ifdef VAR_SET_TO_FALSE\n");
#endif

#ifdef VAR_UNSET
    printf("#ifdef VAR_UNSET\n");
#endif

}

will output

#if VAR_SET_TO_TRUE
#ifdef VAR_SET_TO_TRUE
#ifdef VAR_SET_TO_FALSE

Note that the line printf("#ifdef VAR_SET_TO_FALSE\n"); is compiled while the line printf("#if VAR_SET_TO_FALSE\n"); is not. The first one is compiled, because VAR_SET_TO_FALSE is defined, even though its value is false.