Question

I’m looking for a way to implement ‘type’ visitor over c++ typelist. Here, I meant type visitor as to execute particular operator (such as sizeof) over types in typelist.

Conceptually what I want to do is:

typedef TYPELIST_3(bool, int, double) tl;
size_t tl_size     = TL_sum_size<tl>();  // 13 = 1+4+8
size_t tl_min_size = TL_min_size<tl>();  // 1
size_t tl_max_size = TL_max_size<tl>();  // 8
vector<size_t> tl_sizes = TL_list_size<tl>();  // {1, 4, 8}
TL_AddCounter<tl>(3); // Call AddCounter(3) for each type in typelist

Of course, each function should be templetized over typelist.
The example uses sizeof, and static void T::addCounter(int x) in the typelist (to track how many time that type is used).
Generically, I want to execute any arbitrary ‘static’ operation about type with arbitrary parameters.

Well, first of all, is any of functions above possible? if yes, how can I do? I am not sure how to iterate through typelist.

Answer 1

Something like this might work:

#include <type_traits>

template <typename T>
struct SizeVisitor : std::integral_constant<unsigned int, sizeof(T)> { };

template <template <typename> class Visitor, typename ...Args> struct Visit;

template <template <typename> class Visitor, typename T, typename ...Rest>
struct Visit<Visitor, T, Rest...> : std::integral_constant<unsigned int,
    Visitor<T>::value + Vist<Visitor, Rest...>::value> { };

template <template <typename> class Visitor>
struct Visit<Visitor, T> : std::integral_constant<unsigned int, 0U> { };

Now you can say Visit<SizeVisitor, double, char, int>::value.

You can modify this approach to take a single (say tuple) class instead of the naked types for the type list, and you can also generalize the SizeVisitor into some arbitrary binary functor, akin to std::accumulate (and instead of 0u you’d have the accumulator’s neutral element).