Atheist… you have right…
but he just put this inside the class “class UBoxComponent;”
without any variable name, without anything… compiler try to understand this as variable…
class UBoxComponent* VariableName; works… of course… in this case no matter it is inside or outside 
i mean inlined forward declaration also works, traditional forward declaration outside of the class also works…
he just tried to do traditional forward declaration inside the class instead of inlined