Filter the types of a parameter pack

问题

I'd like to know if it's possible to filter the types passed to a variadic template (based on a predicate template) to produce another variadic template containing those types which satisfy the predicate:

/** Filter a parameter pack */    
template <template <class> class,
          template <class...> class,
          class...>
struct filter;
template <template <class> class Pred, template <class...> class Variadic>
struct filter<Pred, Variadic> : Variadic<>
{};
template <template <class> class Pred,
          template <class...> class Variadic,
          class T, class... Ts>
struct filter<Pred, Variadic, T, Ts...>
{
    // FIXME: this just stops at first T where Pred<T> is true
    using type = typename std::conditional<
        Pred<T>::value,
        Variadic<T, Ts...>,    // can't do: Variadic<T, filter<...>>
        filter<Pred, Variadic, Ts...> >::type;
};

As you can see, I haven't found a way to "extract" the parameter pack from the rest of the filtered types.

Thanks in advance!

回答1:

That should be fairly straight-forward. At the heart you should have something like this:

template <typename...> struct filter;

template <> struct filter<> { using type = std::tuple<>; };

template <typename Head, typename ...Tail>
struct filter<Head, Tail...>
{
    using type = typename std::conditional<Predicate<Head>::value,
                               typename Cons<Head, typename filter<Tail...>::type>::type,
                               typename filter<Tail...>::type
                          >::type;
};

You just need Cons<T, Tuple>, which turns T, std::tuple<Args...> into std::tuple<T, Args...>, and you need to pass the predicate along (left as an exercise). Cons could look like this:

template <typename, typename> struct Cons;

template <typename  T, typename ...Args>
struct Cons<T, std::tuple<Args...>>
{
    using type = std::tuple<T, Args...>;
};

The result of filter<Args...>::type would be std::tuple<Brgs...>, where Brgs... is a pack consisting of only those types in Args... for which the predicate holds.

来源：https://stackoverflow.com/questions/18366398/filter-the-types-of-a-parameter-pack

标签

c++

c++11

metaprogramming

variadic-templates