C ++ Unpacking parameter packages from template arguments

Question

How to achieve the desired I want below? The paramater package that I want to unpack is not in the function argument list, but in the template argument list.

#include <iostream>
#include <array>

const std::size_t SIZE = 10;

template <int...ARGS>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b;
    // I want to set b[n] = true, where n takes on all values from ARGS...
    // what to put in here???
    return b;
}

// Example of what I want to achieve:
int main() {
    const std::array<bool, SIZE> b = func<1,3,7>();
    // I want b[1]==true, b[3]==true, b[7]==true, all others false
    for (int x: b) std::cout << x << std::endl;
}

I have to use this form for func (instead of func (1,3,7)) to make my large program work (I deal with multiple inheritance issues).

Answer 1

Recursive pattern solution:

// recursive helper struct
template <int n, int First, int ...Rest>
struct helper {
  static void funcImpl(std::array<bool, SIZE>& temp) {
    temp[First] = true;
    helper<n - 1, Rest...>::funcImpl(temp);
  }
};

// partial specialization to catch base case
template <int First>
struct helper<0, First> {
  static void funcImpl(std::array<bool, SIZE>& temp) {
    temp[First] = true;
  }
};

template <int ...Args>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b = {}; // 0 inititalize array
    helper<sizeof...(Args) - 1, Args...>::funcImpl(b);
    return b;
}

EDIT: Super simplified version inspired by iavr solution:

template <int... A>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b = {};
    auto values = {A...};
    std::for_each(values.begin(), values.end(), [&](int n){b[n] = true;});
    return b;
}

Answer 2

Here is a much simpler solution that does not require anything extra, just this:

struct _do { template <typename... T> _do(T&&...) { } };

template <int... A>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b = {};
    _do{b[A] = true...};
    return b;
}

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Answer 3

.

func:

template <int... A, int... N>
std::array<bool, sizeof...(N)>
func(sizes <N...>)
{
    return std::array<bool, sizeof...(N)>{{in <N, A...>()...}};
}

template <int... A>
std::array<bool, SIZE>
func() { return func <A...>(range <SIZE>()); }

sizes int, range <S> 0,...,S-1 in<N, A...>() , N A... ( ).

() , N... A.... A..., L... , in(). , .