What It Means & How to Fix It

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🧠 Why You’re Seeing This Error

C4005 means a macro (usually via #define) is being defined more than once — without using #undef or proper guard logic.

In UE5, this can happen when:

• You define the same macro in multiple files
• You include two headers that both define the same macro
• You redefine an engine/platform macro by accident (like check, TEXT, or PI)

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💥 Example Error Message

error C4005: 'MY_MACRO': macro redefinition

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🛠️ Example Code

#define MY_MACRO 1
#define MY_MACRO 2 // ❌ C4005: Redefinition

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✅ How to Fix C4005 in UE5 – Step-by-Step

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✔️ 1. Use #ifndef or #pragma once in Headers

If you’re defining macros in headers, always guard them.

Fix:

#ifndef MY_MACRO
#define MY_MACRO 1
#endif

Or just ensure your file uses:

#pragma once

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✔️ 2. Use #undef Before Redefining

If you must redefine a macro (rare), explicitly #undef it first.

Fix:

#undef MY_MACRO
#define MY_MACRO 2

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✔️ 3. Avoid Redefining Engine Macros

UE5 has a lot of built-in macros like check, PI, TEXT, etc. Redefining them will cause chaos.

Never do this:

#define PI 3.14 // ❌ UE5 already defines PI

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✅ Summary: How to Fix C4005 in UE5

Cause	Fix Example
Defining the same macro twice	Use #ifndef, #pragma once, or #undef
Including headers with clashing macros	Remove duplicate includes or guard properly
Redefining engine macros	Avoid overriding UE macros like PI, TEXT, etc.