What It Means & How to Fix It in UE5 C++

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🧠 Why You’re Seeing This Error

C2027 means you’re trying to use a class or struct that was declared but never defined. The compiler knows the type exists, but it doesn’t know enough about it to access its members or size.

In UE5, this usually happens when:

• You forward-declare a class, but then try to access its members
• You forget to #include the header file that defines the type
• You try to create an instance or dereference a pointer to a forward-declared type

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💥 Example Error Message

error C2027: use of undefined type 'UMyComponent'

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🛠️ Example Code

// MyActor.h

class UMyComponent; // forward declaration

UCLASS()
class AMyActor : public AActor
{
    GENERATED_BODY()

    UMyComponent Component; // ❌ C2027: can't use member of undefined type
};

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✅ How to Fix C2027 in UE5 – Step-by-Step

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✔️ 1. Only Use Forward-Declared Types for Pointers or References

Fix:

UMyComponent* Component; // ✅ OK with forward declaration

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✔️ 2. Include the Full Header When Using the Type Directly

If you need to use a type by value or access its members, include its header.

Fix:

#include "MyComponent.h"
UMyComponent Component; // ✅ Fully defined now

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✔️ 3. Avoid Using Incomplete Types Inside Inline Functions

Inline or header functions that use incomplete types will break unless you include the full definition.

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✅ Summary: How to Fix C2027 in UE5

Cause	Fix Example
Using incomplete type	Include the full header file
Accessing members of forward-declared type	Only allowed with full definition
Forward-declared type used by value	Use pointer/reference or include header