Variadic Template Parameter Packs with Alternating Types

问题

I was wondering if it is possible to capture an alternating parameter pattern using a parameter pack. For example,

template<typename T, size_t U, typename... Args>
class foo<T, U, Args...>
{
   public:
     foo() : my_T(nullptr), my_U(U) {}

   private:
     T* my_T;
     size_t my_U;
     foo<Args...> my_next_foo;
}

So this doesn't work since Args is a parameter pack of only types. Is there a way to modify this so that the typename T, size_t U pattern can be properly captured in a variadic template? Thanks

回答1:

Values as template parameters are second class citizens, in my experience.

Upgrade them to first class with an alias:

template<std::size_t n>
using size = std::integral_constant<std::size_t, n>;

then pattern match:

template<class...>
struct foo;
template<>
struct foo<> {
  // empty
};
template<class T, size_t U, typename... Args>
struct foo<T, size<U>, Args...> {
  foo() : my_T(nullptr), my_U(U) {}

private:
  T* my_T;
  size_t my_U;
  foo<Args...> my_next_foo;
};

and bob is your uncle.

Note, however, that taking U as a template parameter, then storing it as a run time value, is highly questionable.

Users of foo must do:

foo< Chicken, size<3>, Dog, size<17> >

instead of

foo< Chicken, 3, Dog, 17 >

回答2:

Sure, just write your own pair:

template <typename T, size_t U>
struct foo_pair { };

And have a pack of those:

template<typename T, size_t U, typename... Pairs>
class foo<foo_pair<T, U>, Pairs...> {
    ...
};

Which would be instantiated like:

foo<foo_pair<int, 4>, foo_pair<char, 17>, ...> f;

回答3:

You can pass Args to foo as an std::tuple. Use std::integral_constant for U instead of passing an integral constant as a template parameter to the std::tuple. Then the Args parameters pack contains all the pairs of types and sizes.

For example, when instantiating foo you would do so with a type like this:

Foo<std::tuple<int, std::integral_constant<size_t, 5> >,
    std::tuple<char, std::integral_constant<size_t, 3> > >

When implementing foo, you should pass your parameter pack to another std::tuple and use std::tuple_element to select the Nth element of the tuple.

This is just one possible approach, you may find it easier to implement foo if you use type lists.

回答4:

You might create a class representing the data container T* my_T with size_t my_U. Having that, you can implement your template:

#include <array>

using std::size_t;

template<typename... Args>
struct foo;

template<typename T, size_t N>
struct foo<std::array<T, N>>
{
    std::array<T, N> array;
};

template<typename T, size_t N, typename... Args>
struct foo<std::array<T, N>, Args...>
{
    std::array<T, N> array;
    foo<Args...> next_array;
};

int main() {
    foo<std::array<char, 1>, std::array<short, 2>, std::array<int, 4>> foos;
}

Note: I am using std::array as an replacement for the data container.

来源：https://stackoverflow.com/questions/30487378/variadic-template-parameter-packs-with-alternating-types

标签

c++

c++11

variadic-templates