Can I use template aliases as template template parameters?

Question

Can I use template aliases as template template parameters?

template <template <typename...> class> struct foo {};

template <typename T> using simple_ptr = std::unique_ptr<T>;

foo<std::unique_ptr> a; // this doesn't work, std::unique_ptr has two parameters
foo<simple_ptr> b; // does this work?

Answer 1

Yes, it is apparently allowed. According to the latest draft of the upcoming standard I could find, it is stated that

A template-argument for a template template-parameter shall be the name of a class template or an alias template [...].

However, alias templates seems very seldomly supported at the moment, so you might have some trouble making it work with most compilers.

Answer 2

People who read the original question may be writing structs that use template template parameters as meta functions, as demonstrated in the listing below.

template <int T>
struct integer
{
        using value = T;
};

template <class T, class U, template <class...> class Function>
struct binary_op
{
        // Works for add_1, but not add_2
        using type = typename Function<T, U>::type;

        // Works for add_2, but not add_1
        using type = Function<T, U>;
};

template <class T, class U>
struct add_1;

template <int T, int U>
struct add_1<integer<T>, integer<U>>
{
        using type = integer<T + U>;
};

template <class T, class U>
using add_2 = typename add_1<T, U>::type;

add_1 and add_2 are both meta-functions, let's distinguish

• add_1 as an example of nested typedef-style metafunction (which c++03 supported)
• add_2 as an example of template alias-style metafunction (which requires c++11)

The binary_op struct can work either with template alias-style or nested typedef-style metafunctions, but not both. In this answer, I show how such TMP code can be rewritten to avoid this problem.

Suppose that you wish to apply a template template parameter Function to a parameter pack of values Ts.... To apply the metafunction, you need either

using type = Function<Ts...>; // template-alias style

or

using type = typename Function<Ts...>::type; // nested typedef style

It would be useful to have another generic metafunction that detects the kind of metafunction that was passed, and applys it accordingly.

The is_alias_metafunction function, which is implemented below, is a building block for such a facility:

#include <type_traits>

template <class... Ts>
struct sequence;

template <class T>
struct check
{
    static constexpr bool value = true;
};

template <
    template <class...> class Function,
    class                     S,
    class                     Check = void
>
struct is_alias_metafunction
{
    static constexpr bool value = true;
};

template <
    template <class...> class Function,
    class...                  Ts
>
struct is_alias_metafunction<
    Function,
    sequence<Ts...>,
    typename std::enable_if<
        check<typename Function<Ts...>::type>::value
    >::type
>
{
    static constexpr bool value = false;
};

Now, we can write a metafunction apply that applies a template template parameter Function to the parameter pack Ts..., regardless of whether Function is a template alias or a template struct.

template <
    bool                      IsAlias,
    template <class...> class Function,
    class                     S
>
struct apply_impl;

template <template <class...> class Function, class... Ts>
struct apply_impl<true, Function, sequence<Ts...>>
{
    using type = Function<Ts...>;
};

template <template <class...> class Function, class... Ts>
struct apply_impl<false, Function, sequence<Ts...>>
{
    using type = typename Function<Ts...>::type;
};

template <template <class...> class Function, class... Ts>
using apply = typename apply_impl<
    is_alias_metafunction<Function, sequence<Ts...>>::value,
    Function,
    sequence<Ts...>
>::type;

We can now use the apply metafunction as follows:

using type = apply<Function, Ts...>;

and it will abstract away the difference between 'legacy' metafunctions and modern (c++11) metafunctions.