C++11: Number of Variadic Template Function Parameters?

Question

How can I get a count of the number of arguments to a variadic template function?

ie:

template<typename... T> void f(const T&... t) {     int n = number_of_args(t);      ... } 

What is the best way to implement number_of_args in the above?

Answer 1

Just write this:

const std::size_t n = sizeof...(T); //you may use `constexpr` instead of `const` 

Note that n is a constant expression (i.e known at compile-time), which means you may use it where constant expression is needed, such as:

std::array<int,   n>  a; //array of  n elements std::array<int, 2*n>  b; //array of (2*n) elements  auto middle = std::get<n/2>(tupleInstance); 

---

Note that if you want to compute aggregated size of the packed types (as opposed to number of types in the pack), then you've to do something like this:

template<std::size_t ...> struct add_all : std::integral_constant< std::size_t,0 > {};  template<std::size_t X, std::size_t ... Xs> struct add_all<X,Xs...> :    std::integral_constant< std::size_t, X + add_all<Xs...>::value > {}; 

then do this:

constexpr auto size = add_all< sizeof(T)... >::value; 

In C++17 (and later), computing the sum of size of the types is much simpler using fold expression:

constexpr auto size = (sizeof(T) + ...); 

Hope that helps.