Is there a case where including the same header twice is actually helpful?

Question

Creating header guards for my h/hpp files has always been standard practice for me, but I wonder, why is it even possible to include de same file twice? Is there a case where you actually need unprotected headers?

Answer 1

"Parameterized" header files can be used to simulate C++-ish-style templates in C. In such cases the header file will depend on a number of macros ("template parameters"). It will generate different code depending on the actual "value" of these macros.

So, the typical usage of such header would look as a sequence of "template parameter" macro definitions followed by the #include directive, followed by another sequence of "template parameter" macro definitions followed by the same #include, and so on.

https://stackoverflow.com/a/7186396/187690

When using this technique you will see header files without any include guards or header files with include guards that cover only a portion of the file.

Answer 2

Stuff like Boost.PP does lots of tricks by including headers multiple times. It essentially allows for primitive forms of loops.

Also, X-Macros are designed to be included multiple times.

Answer 3

In C:

#undef NDEBUG
#include <assert.h>

...code using active asserts...

#define NDEBUG
#include <assert.h>

...code using disabled asserts...

Rinse and repeat. The analogue in C++ uses the header <cassert> instead.

So, sometimes there are reasons to include a header twice. Not often, but there are reasons to do so.

Answer 4

Cases like these are rare, and when they do exist, a re-design is better suited anyway. One I can think of is headers that amass declarations:

//functions.h
virtual void foo();
virtual void goo();

//classes.h
class A : public Base
{
    #include "functions.h"
};

class B : public Base
{
    #include "functions.h"
};

This wouldn't work if functions.h had include guards, but then again, this is pretty awkward code...