What is struct NIL { typedef NIL Head; }?

Question

I am reading about template metaprogramming. I could not understand what these lines mean; the following code concerns about doing metaprogramming on a linked list.

struct NIL {
typedef NIL Head;
typedef NIL Tail;
};

template <typename H, typename T=NIL> struct Lst {
    typedef H Head;
    typedef T Tail;
};

template <int N> struct Int{ static const int result = N; };
typedef Lst< Int<1>, Lst< Int<2>, Lst< Int<3> > > > OneTwoThree;

The above is coming from https://monoinfinito.wordpress.com/series/introduction-to-c-template-metaprogramming/ . I would greatly appreciate any guidance as to what it means to have struct NIL, which has typedef NIL Head, and typedef NIL Tail. Nil is a type, sure, but if I have typedef NIL Head, for example, does it mean i have another recursive Head and Tail within each Head?

Answer 1

No, typedef NIL Head; doesn't mean you have a NIL within each NIL .

It simply means NIL::Head is another name for NIL . Ditto for NIL::Tail .

This does mean you can write the types recursively, for example NIL::Head::Tail::Tail::Tail::Head::Head is also a name for NIL . However there's no actual recursive type here.

Answer 2

NIL is just another name for nothing.

It isn't NULL because that was taken.

Here, we are creating a LISP like linked list of "head" and "tail". To represent the end of the list, place a NIL .

NIL satisfies the layout of a list, but one with both head and tail being NIL . Myself I'd be tempted to have written:

struct NIL;

template <typename H, typename T=NIL> struct Lst {
  typedef H Head;
  typedef T Tail;
};

struct NIL:Lst{};

to make NIL as the name of the empty list more clear.

By making NIL have head & tail, some template metaprogramming becomes easier (and some becomes harder). The fact that they are the same type does mean that in one sense the NIL list has an infinite length; but in another sense it has zero length, as we define length to be the distance until we reach NIL .

typedef s are not "within" the type, they are defined inside like how data members are. They are more like pointers than members. In non template metaprogramming:

struct node {
  node* head, *tail;
};
struct NIL:node{
  NIL() : node{this, this} {}
};