How to write a variadic template recursive function?

Question

I\'m trying to write a variadic template constexpr function which calculates sum of the template parameters given. Here\'s my code:

template<         


        

Answer 1

template
constexpr int f()
{
    return First + f();
}

template
constexpr int f()
{
    return First;
}

int main()
{
    f<1, 2, 3>();
    return 0;
}

You get this error:

error C2668: 'f': ambiguous call to overloaded function while trying to resolve f<3,>() call.

This is because a variadic parameter pack can be given 0 arguments, so f<3> could work with template by "expanding" to template<3, >. However, you also have the specialization of template, so the compiler does not know which one to choose.

Explicitly stating the first and second template arguments is a completely valid and good solution to this problem.

---

When you try to change the base case to:

template <>
constexpr int f()
{
    return 0;
}

You have a problem because functions cannot be specialized in that way. Classes and structs can be, but not functions.

---

Solution #1: C++17 fold expression with constexpr

template 
constexpr int sum(Is... values) {
    return (0 + ... + values);
}

---

Solution #2: Use a constexpr function

constexpr int sum() {
    return 0;
}

template 
constexpr int sum(I first, Is... rest) {
    return first + sum(rest...);
}

---

Solution #3: Use Template Metaprogramming

template 
struct sum;

template <>
struct sum<>
    : std::integral_constant
{};

template 
struct sum
    : std::integral_constant::value>
{};