How can I take a variadic std::tuple as a template argument?

Question

Suppose I have a struct

template <typename Args>
struct MyStruct
{
};

But I only want to be able to instantiate this class with std::tuple instantiations, eg

Mystruct<std::tuple<>> a; // OK
Mystruct<std::tuple<int, int, double>> a; // OK
Mystruct<double> a; // FAIL

How can I do this?

Answer 1

This is fairly simple. Declare but not define a general template:

template<typename T> struct Mystruct;

then define a specialization:

template<typename... Ts>
struct Mystruct<std::tuple<Ts...>>
{
// stuff
};

Answer 2

In addition to krzaq's answer, to have a better error message, you might want to use static_assert

// The traits:
template <typename T>
struct is_a_tuple : std::false_type {};

template <typename ... Ts>
struct is_a_tuple<std::tuple<Ts...>> : std::true_type {};

// Your structure (for error message)
template <typename T>
struct MyStruct
{
    static_assert(is_a_tuple<T>::value, "T should be a std::tuple");
};

// Your structure (for std::tuple)
template <typename ... Ts>
struct MyStruct<std::tuple<Ts...>>
{
    // Your implementation
};