How to find the length of a parameter pack?

Question

Suppose I have a variadic template function like

template
unsigned length(Args... args);

How do I find the length o

Answer 1

Use sizeof...:

template
constexpr std::size_t length(Args...)
{
    return sizeof...(Args);
}

Note you shouldn't be using unsigned, but std::size_t (defined in ). Also, the function should be a constant expression.

---

Without using sizeof...:

namespace detail
{
    template
    constexpr std::size_t length(void)
    {
        return 1; // length of 1 element
    }

    template
    constexpr std::size_t length(void)
    {
        return 1 + length(); // length of one element + rest
    }
}

template
constexpr std::size_t length(Args...)
{
    return detail::length(); // length of all elements
}

^{Note, everything is completely untested.}