gistfile1.cpp

// .h file
#pragma once

#include "Ship_Component_Interface.generated.h"

/**
 * 
 */
UINTERFACE(MinimalAPI)
class UShip_Component_Interface : public UInterface
{
	GENERATED_UINTERFACE_BODY()
};

class IShip_Component_Interface
{
	GENERATED_IINTERFACE_BODY()

public:
	virtual void Foo() = 0;
};


// .cpp file



#include "Space.h"
#include "Ship_Component_Interface.h"


UShip_Component_Interface::UShip_Component_Interface(const class FPostConstructInitializeProperties& PCIP)
	: Super(PCIP)
{

}

/**void IShip_Component_Interface::Foo()
{

}*/

#### ship comp ###

.h 

#pragma once

#include "GameFramework/Actor.h"
#include "Ship_Component_Interface.h"
#include "ShipComponent.generated.h"

/**
 * 
 */
UCLASS()
class AShipComponent : public AActor, public IShip_Component_Interface
{
	GENERATED_UCLASS_BODY()

	UPROPERTY(VisibleDefaultsOnly, Category = Mesh)
	TSubobjectPtr<class UStaticMeshComponent> Mesh;

	UPROPERTY(VisibleDefaultsOnly, Category = Utility)
	TSubobjectPtr<class USceneComponent> DefaultRoot;

public:
	virtual void Foo() OVERRIDE;
};

.cpp

#include "Space.h"
#include "ShipComponent.h"


AShipComponent::AShipComponent(const class FPostConstructInitializeProperties& PCIP)
	: Super(PCIP)
{
	DefaultRoot = PCIP.CreateDefaultSubobject<USceneComponent>(this, TEXT("Default"));

	Mesh = PCIP.CreateDefaultSubobject<UStaticMeshComponent>(this, TEXT("Mesh"));
	Mesh->RelativeLocation = FVector(0.f, 0.f, 0.f);
}

void AShipComponent::Foo()
{

}
## ??

Error	6	error LNK2001: unresolved external symbol "public: virtual void __cdecl AShipComponent::Foo(void)" (?Foo@AShipComponent@@UEAAXXZ)	C:\Users\Calum\Documents\Unreal Projects\Space\Intermediate\ProjectFiles\ShipComponent.cpp.obj