SFINAE with variadic template classes?

Question

I can't seem to find a good solution for using SFINAE with variadic template classes.

Let's say I have a variadic template object which doesn't like references:

template<typename... Args>
class NoRef
{
    //if any of Args... is a reference, this class will break
    //for example:
    std::tuple<std::unique_ptr<Args>...> uptrs;
};

And a class which conveniently checks if an argument pack contains references:

template<typename T, typename... Other>
struct RefCheck
{
    static const bool value = std::is_reference<T>::value || RefCheck<Other...>::value;
};
template<typename T>
struct RefCheck<T>
{
    static const bool value = std::is_reference<T>::value;
};

How do I use this to specialize NoRef for the case where references are present in the arg pack?

Answer 1

This doesn't use SFINAE, but essentially does what you intend:

template<bool Ref, typename... Args>
class NoRef_;

template<typename... Args>
class NoRef_<false, Args...>
{
    std::tuple<std::unique_ptr<Args>...> uptrs;
};
template<typename... Args>
class NoRef_<true, Args...>
{
    // contains reference
};

template<typename... Args>
using NoRef = NoRef_<RefCheck<Args...>::value, Args...>;

// alternative from Nawaz

template<typename... Args> struct NoRef : NoRef_<RefCheck<Args...>::value, Args...> {}

Answer 2

I am afraid this is not possible as-is, because template packs are unwieldy. However, you can pack packs.

// Used to transport a full pack in a single template argument
template <typename... Args> struct Pack {};

We adapt the reference check:

template <typename T, typename... Other>
struct RefCheck
{
    static const bool value = std::is_reference<T>::value
                           || RefCheck<Other...>::value;
};

template <typename T>
struct RefCheck<T>
{
    static const bool value = std::is_reference<T>::value;
};

template <typename... Args>
struct RefCheck<Pack<Args...>>: RefCheck<Args...> {};

And now we can use the Pack:

template <typename P, bool = RefCheck<P>::value> class NoRef;

template <typename... Args>
class NoRef<Pack<Args...>, false> {
    // no reference in Args... here
};