C++ variadic template sum

Question

I am trying to understand the C++11 feature called "variadic". Look at this simple code:

#include <iostream>

using namespace std;

template<typename T, typename... Args>
T adder(T first, Args... args) {
  return first + adder(args...);
}

int main() {

  int c = adder(1,8,4);
  cout << c << endl;

  cout << "Hello World!" << endl;
  return 0;

}

Readig the c++ primer book I have understood that they work in recursive way and I also see that in the recursive call the args... part is passed.

I am using MinGW 5 and QtCreator to test this. Look

Ok, to fix the Too few arguments I could call adder(first, args...) but now the recursion is not proper and the program crashes. What to do? I cannot understand how to do this.

---

Looking online I've found an example like this

template <typename T, typename... Rest>
double sum(T t, Rest... rest) {
  return t + sum(rest...);
}

It's basically the same. Do I have to put an explicit (non template) return type?

Answer 1

You need the "stop-recursion-case" (do not know the correct name now; UPDATE: it's called "base-case", thanks Quentin) with just one argument when the template function is unfolding.

#include <iostream>

template<typename T>
T adder(T first) {
  return first;
}

template<typename T, typename... Args>
T adder(T first, Args... args) {
  return first + adder(args...);
}

int main() {
  const int c = adder(1, 8, 4);
  std::cout << c << '\n';
  return 0;
}