Question

If I have two variadic template arguments, A and B, how can I ensure at compile-time that the types of all of the members of A are also the types of a subset of B (in the same order)?

Contrived example:

template<typename...A>
struct Foo {
  template<typename...B>
  static void bar()
  {
  }
}

...

Foo<Apple, Orange>:: template bar<Apple, Orange, Grape>(); // this compiles
Foo<Apple, Orange>:: template bar<Orange, Grape>(); // this doesn't

Answer 1

For a general subset I don’t know, but if you can guarantee that B is of the form A..., More..., then this may do:

#include <functional>
#include <tuple>

template <typename ...A>
struct var_equal : std::false_type { };

template <typename A1, typename ...Aother, typename B1, typename ...Bother>
struct var_equal<std::tuple<A1, Aother...>, std::tuple<B1, Bother...>>
{
  static const bool value = std::is_same<A1, B1>::value && var_equal<std::tuple<Aother...>, std::tuple<Bother...>>::value;
};

template <typename ...B>
struct var_equal<std::tuple<>, std::tuple<B...>> : std::true_type { };


template<typename...A>
struct Foo {
  template<typename...B>
  static void bar()
  {
    static_assert(var_equal<std::tuple<A...>, std::tuple<B...>>::value, "Hello");
  }
};

(Sorry, var_equal is a terrible name. It should be called something more appropriate, like initial_equal.)

---

Update: Here is the general solution, worked out in detail by Luc Danton (see here for his beautifully styled code):

#include <type_traits>
#include <tuple>

template <typename Sub, typename Super>
struct subset_of : std::false_type {};

template<typename Same, typename... AOther, typename... BOther>
struct subset_of<std::tuple<Same, AOther...>, std::tuple<Same, BOther...>>
: subset_of<
    std::tuple<AOther...>
    , std::tuple<BOther...>
> {};

template<typename ADifferent, typename BDifferent, typename... AOther, typename... BOther>
struct subset_of<std::tuple<ADifferent, AOther...>, std::tuple<BDifferent, BOther...>>
: subset_of<
    std::tuple<ADifferent, AOther...>
    , std::tuple<BOther...>
> {};

template<typename... B>
struct subset_of<std::tuple<>, std::tuple<B...>>: std::true_type {};

template<typename... A>
struct Foo {
    template<typename... B>
    static void bar()
    {
        static_assert(subset_of<std::tuple<A...>, std::tuple<B...>>::value, "Hello");
    }
};

Test case:

struct Apple{}; struct Orange{}; struct Grape{};

int main()
{
    Foo<Apple, Orange>::bar<Apple, Orange, Grape>();               // this compiles
    Foo<Apple, Orange>::bar<Grape, Apple, Grape, Orange, Grape>(); // this also compiles
    Foo<Apple, Orange>::bar<Orange, Grape>();                      // this doesn't
}