#if USING macro | Together C & C++ | Page 1

clear brook Jan 22, 2026, 3:51 PM

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Hello, I'm trying to come up with a way to have an #if USING(SOME_PREPROCESSOR),

SOME_PREPROCESSOR would be either defined as 0, or 1, but the main goal is that I want the statement to be a compile-time error if SOME_PREPROCESSOR isn't defined at all, the #if USING would also need to check the value, which would either be the 0 or 1.

I've tried a few things like the below, but can't seem to get it to work.
Thanks!

#define USING(X) (X && X)

flat krakenBOT Jan 22, 2026, 3:51 PM

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wild rose Jan 22, 2026, 3:52 PM

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defined checks if a preprocesor macro is defined

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there's also ifdef

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So you could do:

#if defined(X)
// what you want to do with it
#else
#error X not defined
#endif

clear brook Jan 22, 2026, 3:54 PM

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That isn't quite what I was looking for, but I probably worded the question wrong. I don't just want to know if the thing exists or not, I want to check the value

wild rose Jan 22, 2026, 3:56 PM

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And error if the macro doesn't exist?

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or just silently fail?

clear brook Jan 22, 2026, 3:56 PM

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If it is not defined, correct

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I thought that

#define USING(X) (X && X) would work, becuse it would just evaluate to #if &&, but I was wrong

wild rose Jan 22, 2026, 3:57 PM

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Oh, do you mean function like macros?

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So if you don't pass X to USING it just emits &&?

clear brook Jan 22, 2026, 3:57 PM

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No, hmmm. Let me get a better example

wild rose Jan 22, 2026, 3:58 PM

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yes please. also, i'm generally against macros, so there's probably a better way within the C++ language itself if you can share the actual problem you're trying to solve

clear brook Jan 22, 2026, 3:58 PM

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The main goal is to just protect against free-standing macros and avoid spelling mistakes.

For example me code defines WITH_EDITOR=0 (or 1), and I want to protect against accidentally doing WITH_EDTIOR or something

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For example, I'm trying to just make a simple helper to protect against these sort of cases. Just making a more strict check.

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The first one should error, as WITH_EDTIOR is not defined

wild rose Jan 22, 2026, 4:00 PM

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I mean, you could alway do:

#if defined(WITH_EDITOR) && WITH_EDITOR == 0
// Do some stuff if the editor is defined to 0
#elif defined(WITH_EDITOR) && WITH_EDITOR == 1
// Do some stuff if editor is defined to 1
#else
// Do some stuff if it's not defined or defined to something else
#endif

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I'd just build out simple macros based on that

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So like:

#define DO_WITH_EDITOR defined(WITH_EDITOR) && WITH_EDITOR == 0

clear brook Jan 22, 2026, 4:03 PM

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I'm fairly certain that macro expansion producing 'defined' has undefined behavior :/

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https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/c5105?view=msvc-170

Compiler Warning C5105

Compiler warning C5105 description and solution.

compact epoch Jan 22, 2026, 4:07 PM

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#if replaces undefined macros with 0, not empty, that's why your X && X doesn't error

clear brook Jan 22, 2026, 4:07 PM

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Yeah :/

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Okay I figured it out.

#define IN_USE ||
#define NOT_IN_USE &&
#define USING(X) (1 X 0)

define your type as either IN_USE or NOT_IN_USE

flat krakenBOT Jan 22, 2026, 4:12 PM

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flat kraken

Thank you and let us know if you have any more questions!

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compact epoch Jan 22, 2026, 4:18 PM

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https://gcc.godbolt.org/z/nxhGbY9cv

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Here's a proper one that handles 0 and 1

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I hate that I somehow ended up learning how to work around legacy MSVC preprocessor bugs 😛

##if USING macro