3.8 Appending to Typelists

We need a means to append a type or a typelist to a typelist. Because modifying a typelist is not possible, as discussed previously, we will "return by value" by creating a brand new typelist that contains the result.

   Append
   Inputs: Typelist TList, type or typelist T
   Output: Inner type definition Result

   If TList is NullType and T is NullType, then Result is NullType.
   Else
     If TList is NullType and T is a single (nontypelist) type, then Result is a typelist hav-
       ing T as its only element.
     Else
       If TList is NullType and T is a typelist, Result is T itself.
       Else if TList is non-null, then Result is a typelist having TList::Head as its
         head and the result of appending T to TList::Tail as its tail.

This algorithm maps naturally to the following code:

template <class TList, class T> struct Append;





template <> struct Append<NullType, NullType>


{


   typedef NullType Result;


};





template <class T> struct Append<NullType, T>


{


   typedef TYPELIST_1(T) Result;


};





template <class Head, class Tail>


struct Append<NullType, Typelist<Head, Tail> >


{


   typedef Typelist<Head, Tail> Result;


};





template <class Head, class Tail, class T>


struct Append<Typelist<Head, Tail>, T>


{


   typedef Typelist<Head,


         typename Append<Tail, T>::Result>


      Result;


};

Note, again, how the last partially specialized version of Append instantiates the Append template recursively, passing it the tail of the list and the type to append.

Now we have a unified Append operation for single types and typelists. For instance, the statement

typedef Append<SignedIntegrals,


      TYPELIST_3(float, double, long double)>::Result


   SignedTypes;

defines a list containing all signed numeric types in C++.