Why am I getting "Undefined reference to vtable"

[AL88]

Why am I getting "undefined reference to vtable enemy when I link the object files for this game engine code. Some guys on an irc channel told me to add a virtual destructor to the base class GameObject but its still giving the same error, why?, what do I need to change to get it to compile?

main.cpp https://pastebin.com/rUc8LUdW
Enemy.h https://pastebin.com/bFz9kjnM
Enemy.cpp https://pastebin.com/W2rh1Nut
Game.h https://pastebin.com/R0hgPApg
Game.cpp https://pastebin.com/WRfsgJHY
GameObject.h https://pastebin.com/7sDcu5Wd
GameObject.cpp https://pastebin.com/drLYNZKy
Player.h https://pastebin.com/f9BixcyB
Player.cpp https://pastebin.com/GNu1PKtq
TextureManager.h https://pastebin.com/9GXWzMsB
TextureManager.cpp https://pastebin.com/703UG3aS

[TheToaster]

You need to make sure that the virtual methods inherited by the Enemy class all have implementations. In Enemy.cpp you only define the update() method.