typelist

C++ generic programming technique invented (or at least utilised and popularised) by Andrei Alexandrescu, much visible in his new book Modern C++ Design and in the Loki library which goes with the book.

A typelist is a dummy type that abstracts the notion of a list of types. They're used to perform computation on types (not values!) at compile time. The basic mechanism is quite simple, and is familiar to any Lisp programmer: it's just a car/cdr list terminated by a special sentinel value (the ugly capitalisation is part of Alexandrescu's coding style, unfortunately):

class NullType {};        // Sentinel value
template <typename H, class T>
struct Typelist {
  typedef H Head;
  typedef T Tail;
};

He then provides macros to build typelists with varying numbers of arguments:

#define TYPELIST_1(T1)  Typelist<T1,NullType>
#define TYPELIST_2(T1,T2)  Typelist<T1, TYPELIST_1(T2)>
#define TYPELIST_3(T1,T2,T3)  Typelist<T1, TYPELIST_2(T2,T3)>
// ... goes on to TYPELIST_50 like this, and you could
// add your own if you needed more!

OK, so we can build lists of types.

BUT what can we do with them? Unleash the power of C++ templates, of course. See some examples of computations with typelists.