Member function template with the number of parameters depending on an integral template parameter

Question

I have the following class template:

template
class MyClass;

where T is some type, N - num

Answer 1

Your third variant's second non-type param should have prefix typename not class :

template<class...T2, typename std::enable_if<sizeof...(T2) == N, int>::type = 0>
void foo(T2... args);
..
foo(1,2,3);

Check it

Gcc 4.7.0 snapshots has some bugs with templates I guess, if you try it with gcc 4.6.2/1 it shall work.

Answer 2

Why not use a static_assert?

template <typename T, size_t N>
class MyClass
{
public:
    template <typename... Args>
    void foo(Args&&... args)
    {
        static_assert(sizeof...(Args) == N, "Wrong number of arguments.");
        // Rest of the implementation.
    }
};

Answer 3

Maybe like this:

#include <utility>
#include <type_traits>

template <typename T, unsigned int N>
class MyClass
{
    template <typename ...Args>
    typename std::enable_if<std::is_constructible<MyClass<T, N>, Args...>::value>::type
    foo(Args &&... args)
    {
        // for example:
        MyClass<T, N> m(std::forward<Args>(args)...);
        // ...
    }
};

This will work only if MyClass has a constructor that accepts the relevant arguments directly (like MyClass(A1, A2, A3)), but I don't think it works if MyClass has a constructor that requires an initializer list, nor will it work if MyClass is an aggregate that requires brace-initialization.

That said, it doesn't look like your MyClass could possibly accept an initializer list, since you said that it has to take precisely N arguments, which an IL cannot promise.

Answer 4

May be, based on your 3rd variant, you can extract first argument from initializer list and deduce type by it

template<class U, class...T2, class std::enable_if<sizeof...(T2) == N-1, int>::type = 0>
void foo(U u, T2... args)
{
    MyClass<U, N>({u, args...});
}