#if vs #ifndef vs #ifdef

Question

My problem is first of all, understanding #ifndef and #ifdef. I also want to understand the difference between #if, #ifndef , and #ifdef. I understand that #if is basically an if statement. For example:

#include<iostream>
#define LINUX_GRAPHICS 011x101

int main(){
 long Compare = LINUX_GRAPHICS;
 #if Compare == LINUX_GRAPHICS
   std::cout << "True" << std::endl;
 #endif
}

But the others, although I read about them I can't comprehend. They also seem like very similar terms, but I doubt they work similarly. Help would be greatly appreciated.

Answer 1

#if doesn't have any notion about Compare or the value it contains, so it probably doesn't do what you intend.

Remember the preprocessor does plain text replacement.

The statement will expand as seen from #if as

#if Compare == 011x101

and being expanded as

#if 0 == 011x101

which certainly won't yield true at the preprocessing stage.

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The #ifdef and #ifndef directives check if a preprocessor symbol was #define'd at all, either using that (<--) preprocessor directive, or your compilers preprocessor option (most commonly -D<preprocessor-symbol>).
These don't care if the preprocessor symbol carries an empty value or something. A simple

#define MY_CONDITION

or

-DMY_CONDITION

is enough to satisfy

#ifdef MY_CONDITION

to expand the text coming afterwards (or hide it with #ifndef).

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The Compare declaration isn't a preprocessor symbol and can't be used reasonably with #ifdef or #ifndef either.