Question

Say you have a tuple and want to generate a new tuple by applying a metafunction on each type of the first one. What’ the most efficient C++ metafuntion to accomplish to this task? Is it also possible to use C++0x variadic template to provide a better implementation?

Answer 1

How ’bout this one:

template<typename Metafun, typename Tuple>
struct mod;

// using a meta-function class
template<typename Metafun, template<typename...> class Tuple, typename ...Types>
struct mod<Metafun, Tuple<Types...>> {
  typedef Tuple<typename Metafun::template apply<Types>::type...>
    type;
};

Then

typedef std::tuple<int, bool> tuple_foo;

struct add_pointer {
  template<typename T>
  struct apply { typedef T *type; };
};

typedef mod<add_pointer, tuple_foo>::type tuple_ptrfoo;

That’s using a metafunction class by wrapping the apply into a non-template. That allows passing it to C++03 templates (which cannot accept templates with arbitrary parameters by simply doing template<typename...> class X). You may, of course, accept a pure metafunction (not a class) too

template<template<typename...> class Metafun, typename Tuple>
struct mod;

// using a meta-function
template<template<typename...> class Metafun, template<typename...> class Tuple, 
         typename ...Types>
struct mod<Metafun, Tuple<Types...>> {
  typedef Tuple<typename Metafun<Types>::type...>
    type;
};

And use the std::add_pointer template

typedef mod<std::add_pointer, tuple_foo>::type tuple_ptrfoo;

Or you can wrap it into a class so it is compatible with the first version

// transforming a meta function into a meta function class
template<template<typename...> class Metafun>
struct ToClass {
  template<typename ... T>
  struct apply { typedef Metafun<T...> type; };
};

typedef mod<ToClass<std::add_pointer>, tuple_foo>::type tuple_ptrfoo;

Hope it helps.