What It Means & How to Fix It

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🧠 Why You’re Seeing This Error

The LNK2019 linker error means the compiler found a function or variable declaration, but when it tried to link everything together, it couldn’t find the actual definition — in other words, the function is declared but never implemented (or not linked properly).

In UE5, this happens when:

• You declare a function in your .h file but never define it in your .cpp
• You forgot to include or link against a needed module
• You misspelled a function or class name
• You forgot to mark a method with UFUNCTION() but called it in Blueprint
• You improperly used inline, template, or static variables

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💥 Example Error Message

LNK2019: unresolved external symbol "public: void AMyActor::DoSomething(void)" referenced in function "public: void AMyGameMode::BeginPlay(void)"

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🛠️ Example Code

// MyActor.h
UCLASS()
class AMyActor : public AActor
{
    GENERATED_BODY()
public:
    void DoSomething(); // ✅ Declared
};

// MyGameMode.cpp
void AMyGameMode::BeginPlay()
{
    AMyActor* Actor = GetWorld()->SpawnActor();
    Actor->DoSomething(); // ❌ LNK2019: linker can't find DoSomething()
}

But… you forgot to actually define DoSomething() in the .cpp file.

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✅ How to Fix LNK2019 in UE5 – Step-by-Step

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✔️ 1. Make Sure You Defined the Function

Fix:

// MyActor.cpp
void AMyActor::DoSomething()
{
    // Logic here
}

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✔️ 2. Check for Typos Between .h and .cpp

If the signature differs even slightly, UE5 will treat it like an entirely different function.

Wrong:

// Header
void DoSomething(int32 Value);

// Source
void AMyActor::DoSomething(float Value) // ❌ LNK2019

Fix:

void AMyActor::DoSomething(int32 Value); // ✅ Matches declaration

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✔️ 3. Check for Missing Module Dependencies

If you’re referencing code from another module, like OnlineSubsystem or AIModule, and didn’t list it in your .Build.cs, linking will fail.

Fix:

PublicDependencyModuleNames.AddRange(new string[] { "Core", "Engine", "AIModule" });

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✔️ 4. Don’t Declare Functions Without Defining Them (Even if Unused)

Even if you never call the function, if it’s declared but not defined, the linker can still throw this error when something indirectly references it.

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✔️ 5. Blueprint Callable but Not Implemented

If you mark a function UFUNCTION(BlueprintCallable) and call it in Blueprint, but forgot to define it in C++, the linker will explode.

Fix: Make sure any UFUNCTION() you expose to Blueprint is implemented in C++.

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✅ Summary: How to Fix LNK2019 in UE5

Cause	Fix Example
Function declared but not defined	Define it in the matching .cpp file
Mismatched signature	Match parameters and return type exactly
Missing module or lib dependency	Add missing module to .Build.cs
Blueprint-exposed function missing	Implement all BlueprintCallable methods
Typo in method or class name	Double-check spelling and class scope