Do you mean C++ forward declarations? If so, it’s basically an entry in the linker table. A placeholder of sorts. You can’t reference something without it being

Do you mean C++ forward declarations? If so, it’s basically an entry in the linker table. A placeholder of sorts. You can’t reference something without it being known, so you have to make it known first. Where C++ gets weird is that you can’t instance something without it being fully known (the full definition needs to be visible). This is rooted in the .cpp / .h header split, which as far as I know is pretty unique to C.