Fatal error LNK1120: 2 unresolved externals

Development Programming & Scripting
1

In 4.15 I had this code (which worked) in a utility class (cpp):

template <typename Type>
static Type* ULib::CreateObject(FString Path)
{
	Type* Temp = nullptr;
	Temp = ConstructorHelpers::FObjectFinderOptional<Type>(*Path).Get();

#if WITH_EDITOR
	if (Temp == nullptr) { UE_LOG(LogTemp, Error, TEXT("%s"), *Path) }
#endif

	return Temp;
}

And this in a character class (cpp) in the constructor:

MeshComp->SetSkeletalMesh(ULib::CreateObject<USkeletalMesh>("/Game/AnimationAssets/UE4_Mannequin/Mesh/SK_Mannequin"));
MeshComp->SetMaterial(0, ULib::CreateObject<UMaterialInterface>("/Game/AnimationAssets/UE4_Mannequin/Materials/M_UE4Man_Body"));
MeshComp->SetMaterial(1, ULib::CreateObject<UMaterialInterface>("/Game/AnimationAssets/UE4_Mannequin/Materials/M_UE4Man_ChestLogo"));
MeshComp->SetRelativeLocation(FVector(0.0f, -10.0f, -90.0f));
MeshComp->SetRelativeRotation(FRotator(0.0f, -90.0f, 0.0f)); // Face the X-axis.

But now in 4.18 it no longer works. This is my build file line:

PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "AIModule", "UMG", "Slate", "SlateCore", "RHI" });

Error:

fatal error LNK1120: 2 unresolved externals

Where/What is the problem?

2

Solved it.

C++ template functions need to be placed in the header file OR you have to include the CPP file that contains the templated function. Because the template is a declaration and not a definition and is not compiled until the compiler knows what template-type it receives.

So there are 2 solutions: either include the Lib.cpp file (but likely creates duplicate entries and it’s ugly) or move the functions from the cpp file to the header (best solution).