The sizeof... Operator

When we need to know how many elements there are in a pack, we can use the sizeof... operator. Like sizeof (§ 4.9, p. 156), sizeof... returns a constant expression (§ 2.4.4, p. 65) and does not evaluate its argument:

template<typename ... Args> void g(Args ... args) {
    cout << sizeof...(Args) << endl;  // number of type parameters
    cout << sizeof...(args) << endl;  // number of function parameters
}