Variadic variable templates

As mentioned before, variable templates may also be variadic. However, variables cannot be defined recursively, nor can they be specialized like class templates. Fold expressions, which simplify generating expressions from a variable number of arguments, are very handy for creating variadic variable templates.

In the following example, we define a variadic variable template called Sum that is initialized at compile-time with the sum of all integers supplied as non-type template arguments:

template <int... R>
constexpr int Sum = (... + R);
int main()
{
    std::cout << Sum<1> << '\n';
    std::cout << Sum<1,2> << '\n';
    std::cout << Sum<1,2,3,4,5> << '\n';
}

This is similar to the sum function written with the help of fold expressions. However, in that case, the numbers to add were provided as function...