What It Means & How to Fix It

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🧠 Why You’re Seeing This Error

C4668 means you’re using an #if or #elif with a macro (e.g., #if WITH_EDITOR) — but the macro was never defined, so the compiler assumes it evaluates to 0.

In UE5, this often happens when:

• You use engine macros (WITH_EDITOR, WITH_EDITORONLY_DATA, PLATFORM_WINDOWS, etc.) outside their intended scope
• You write your own macros but forget to define them in your .Build.cs or via #define
• You use #if instead of #ifdef or #if defined(...)

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💥 Example Error Message

warning C4668: 'WITH_EDITOR' is not defined as a preprocessor macro, replacing with '0' for '#if/#elif'

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🛠️ Example Code

#if WITH_EDITOR
// Editor-only logic here
#endif

But in this case, WITH_EDITOR wasn’t defined.

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✅ How to Fix C4668 in UE5 – Step-by-Step

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✔️ 1. Use #if defined(MACRO) or #ifdef

Fix:

#if defined(WITH_EDITOR) && WITH_EDITOR
// ✅ Now you're safe
#endif

Or more simply:

#ifdef WITH_EDITOR
// ✅ Works only if macro is defined
#endif

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✔️ 2. Define Custom Macros Manually If Needed

For your own macros, make sure they’re #defined before usage:

#define MY_FEATURE_ENABLED 1

#if MY_FEATURE_ENABLED
// Do stuff
#endif

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✔️ 3. Don’t Panic – This Is Just a Warning

Unless you’re treating warnings as errors (/WX), C4668 won’t stop your build — but you should still clean it up.

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✅ Summary: How to Fix C4668 in UE5

Cause	Fix Example
Using undefined macro in #if	Use #ifdef or #if defined(...)
Custom macro not declared	Add #define for your macro
Relying on platform/engine macro	Wrap with safe checks (#if defined(...) && ...)