Question

Can we write a portable include guard that doesn’t use the preprocessor in C++? If so how could that be done?

Answer 1

No.

1. You cannot use #include without the preprocessor.
2. Without preprocessor directives, including the same file twice will always result in the same sequence of tokens.

There are a couple non-portable ways to do this (both use the preprocessor), such as:

#pragma once

and

#import "file.h"

But header guards work everywhere, and your compiler is probably optimized to check for header guards so it won’t even bother processing a duplicate #include directive.