Purpose of #ifndef FILENAME....#endif in header file

Question

I know it to prevent multiple inclusion of header file. But suppose I ensure that I will include this file in only one .cpp file only once. Are there still scenarios in which I would require this safe-guard?

Answer 1

No, that's the only purpose of the include guards, but using them should be a no-brainer: doing it requires little time and potentially saves a lot.

Answer 2

You can guarantee that your code only includes it once, but can you guarantee that anyone's code will include it once?

Furthermore, imagine this:

// a.h
typedef struct { int x; int y; } type1;

// b.h
#include "a.h"
typedef struct { type1 old; int z; } type2;

// main.c
#include "a.h"
#include "b.h"

Oh, no! Our main.c only included each once, but b.h includes a.h, so we got a.h twice, despite our best efforts.

Now imagine this hidden behind three or more layers of #includes and it's a minor internal-use-only header that gets included twice and it's a problem because one of the headers #undefed a macro that it defined but the second header #defined it again and broke some code and it takes a couple hours to figure out why there are conflicting definitions of things.