The core idea of a defensive include

We can solve the problem of including a header file twice by defining a unique preprocessor instruction constant that if it is encountered we do not process the body of the header file, i.e.:

#ifndef SOME_ARBITRARY_UNIQUE_CODE
#define SOME_ARBITRARY_UNIQUE_CODE
//.... body of header file goes here.
#endif // end of the block which started at line 2

So how do you make a unique arbitrary code? There is more than one way to solve to solve this problem:

Have convention that maps the file name of the header file to a unique code
Or use something like a Globally Unique IDentifier - GUID.

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The core idea of a defensive include

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