Is there a way to prevent a header file without header guards from being included at all at compile time?

Question

I am trying to ban the inclusion of a list of header files from a source file, and thus preventing the dependency on classes defined in those headers.

There are tricks of checking the header guard macros (like the following), but, in my case, the headers to exclude all use #pragma once instead of header guards.

#ifdef A_BANNED_HEADER_FILE_H
static_assert(false);
#endif

Is there a way to enforce 0 inclusion at compile time?

P.S. For the purpose of avoiding dependency to any class, I could define a class of the same qualified name in the source file and let redefinition error scream at accidental inclusion.

Technically header guards is nothing more than a common convention to prevent multiple inclusion into a single translation unit. As such, the compiler and its preprocessor doesn't really deal with it. — Some programmer dude
– Some programmer dude, Commented Jan 7 at 17:25
This might be a job for a script that runs before the compile command. — sweenish
– sweenish, Commented Jan 7 at 17:28
#error seems more appropriate than static_assert(false); BTW — Jarod42
– Jarod42, Commented Jan 7 at 17:28
@PeteBecker, this is an actual situation my team is in. We need to enforce some Clean Code coding guidelines to new code that we add in an existing code base where we cannot change other teams' code. We try to use interfaces instead of directly calling other teams' mockless APIs so we can unit test our core logic. However, the code base's build system cannot prevent undesired header inclusion. We need to find a way to prevent undesired dependency from creeping in. — DXZ
– DXZ, Commented Jan 7 at 23:24

Drew Dormann · Accepted Answer · 2025-01-08 16:02:24Z

4

Preventing a header from being included doesn't necessarily prevent any class from being defined.

But you can explicitly demand that a class is not defined in your source file.

// foo.h must not be included, because class foo must not be defined!
// #include "foo.h"


template<typename T> concept is_defined = requires { sizeof(T); };


static_assert( !is_defined<class foo> );

See it work in Compiler Explorer

Note that you don't want the same concept to evaluate differently in the same program, so it's important to avoid any code that expects a different result for is_defined<class foo>

edited Jan 8 at 16:02

answered Jan 7 at 17:39

Drew Dormann

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5 Comments

DXZ Jan 7 at 17:41

Nice trick using the concept. I assume this will also work for free function dependency.

Jarod42 Jan 8 at 9:02

Don't we have ODR violation if is_defined<foo> is true in one TU and not in another one?

Drew Dormann Jan 8 at 15:19

@Jarod42 this is the closest I can come to an answer. There are rules to determine if a concept is satisfied, and it is IFNDR if this result changes within a program. I take this to mean that there should be no code evaluating is_defined<class foo> to have a different result (making this answer okay). I don't think hypothetical changes to satisfaction would be ill-formed or very many concepts would break.

Jarod42 Jan 8 at 15:31

I meant, if another TU does (and expect) static_assert(is_defined<class foo>);, we are in troubles. Incomplete types are dangerous with concept...

Drew Dormann Jan 8 at 16:03

@Jarod42 we seem to be on the same page then. I'll add a note to this answer.

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