Question

I have a header with a particular set of rules that the user must follow. If the user does not want to follow the rules, they must define a preprocessor macro which will disable the checks. If the user is using the library, the user must include the header in every one of their classes that uses any code from the library or explicitly state otherwise by the above mentioned preprocessor (in this case, the header may be included implicitly [using one of the library’s classes] which is fine):

// User using my library, they must include the BaseHeader.h or otherwise use the following define
// #define NOT_USING_BASE_HEADER //(can of course be defined project wide)
#include <BaseHeader.h>
// ... other includes (not necessarily from my library)
class Foo
{
  //...
};

Is there a way to accomplish this? Keep in mind that the base header is also being included

Answer 1

I’m not sure if I understand your question entirely, but do you think this will help?

BaseHeader.h:

#pragma once
#define BASE_HEADER_INCLUDED
...

internal_header.h:

#pragma once

#ifdef NOT_USING_BASE_HEADER
   #ifdef BASE_HEADER_INCLUDED
      #error You can't both define NOT_USING_BASE_HEADER and include the base-header-file.
   #endif
   ... 
#elseif defined BASE_HEADER_INCLUDED
   ...
#else
   #error You have to either define NOT_USING_BASE_HEADER or include the base header file!
#endif

If you want to make it so that it doesn’t take “internal” includes of the baseheader.h, then you have to add/remove a define to make sure that it doesn’t define it:

internal_header.h:

#pragma once

#define SKIP_HEADER_DEFINE
#include 
#undef SKIP_HEADER_DEFINE

And in BaseHeader.h:

#pragma once
#ifndef SKIP_HEADER_DEFINE
   #define BASE_HEADER_INCLUDED
#endif

Cheers,
Simon