Add all parameters with parameter pack expansion [duplicate]

Answer 1

Yes, using a trick I learnt from @Xeo in the Lounge. I originally used it to make a variadic "print" template function.

#include <iostream>

template<int... ints>
int add()
{
  int result = 0;
  using expand_variadic_pack  = int[]; // dirty trick, see below
  (void)expand_variadic_pack{0, ((result += ints), void(), 0)... };
  // first void: silence variable unused warning
  // uses braced-init-list initialization rules, which evaluates
  //  the elements inside a braced-init-list IN ORDER, to repetetively
  //  execute a certain operation
  // second void is to prevent malicious "operator," overloads, which
  //  cannot exist for void types
  // 0 at the end is to handle empty variadic pack (zero-size array initializer is illegal.
  return result;
}

int main()
{
  std::cout << add<1,2,3,4>() << '\n';
}

This works on every compiler that has decent C++11 support (GCC 4.8+, Clang 3.2+, MSVS2013, ...)

Answer 2

One possible variant which uses lambda and std::accumulate:

#include <array>
#include <numeric>

template <int... t>
int add()
{
    return [](const std::array<int, sizeof...(t)>& a)
    {
        return std::accumulate(a.begin(), a.end(), 0);
    }({t...});
}