Why You’re Seeing This Error

LNK2005 means the linker has found multiple definitions of the same symbol — often a function or variable — and doesn’t know which one to use.

In UE5, this often happens when:

• You define a variable or function in a header file without making it inline or guarding it
• You accidentally include the same symbol multiple times
• You define a static or const variable globally without extern
• You don’t use #pragma once or include guards in your headers
• You define a USTRUCT() or UENUM() in multiple translation units

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💥 Example Error Message

LNK2005: "int32 MyGlobalValue" already defined in MyActor.cpp.obj

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🛠️ Example Code

// MyGlobals.h
int32 MyGlobalValue = 100; // ❌ This creates a definition in every .cpp that includes this header

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✅ How to Fix LNK2005 in UE5 – Step-by-Step

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✔️ 1. Declare Global Variables with extern in Headers, Define in .cpp

Header (MyGlobals.h):

extern int32 MyGlobalValue; // ✅ Declaration only

Source (MyGlobals.cpp):

int32 MyGlobalValue = 100; // ✅ Single definition

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✔️ 2. Use #pragma once or Include Guards in Headers

Every header file should start with:

#pragma once

Or, the old-school way:

#ifndef MY_HEADER_H
#define MY_HEADER_H
// ...
#endif

This prevents duplicate includes that cause linker conflicts.

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✔️ 3. Mark Functions inline If Defined in Headers

If you must define a function in a header, mark it inline to avoid multiple definitions.

Fix:

inline int32 GetDefaultValue() { return 42; }

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✔️ 4. Avoid Defining USTRUCTs or UENUMs in Multiple Files

UE5 doesn’t allow splitting the same USTRUCT() across multiple files. Keep your types in one place.

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✔️ 5. Don’t Define Non-Static Member Variables in Header Files

If you’re putting logic into header files, be careful not to create global duplicate symbols.

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✅ Summary: How to Fix LNK2005 in UE5

Cause	Fix Example
Global variable defined in header	Use extern in header, define in one .cpp
Header included in multiple .cpp files	Add #pragma once to all headers
Non-inline function defined in header	Mark it inline, or move definition to .cpp
Multiple USTRUCT()/UENUM() declarations	Keep them in a single header/source file
Static const improperly scoped	Define static const values inside .cpp if reused